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Mschn
Member
Well, this amp is some 20 years old. Is it still that good? Aftar all, I have had to repair it after it exhibited rather unsuttle distortion.
/Martin
AnalogSteph
Master Contributor
Fixed a ca. late 1985 Kenwood KT-880L tuner over the weekend... not really anything special, but it happened to include an original AM loop antenna matching my KT-1100 (which looks comically large on this slimline unit, I guess they had to order a bunch more than they had KT-1100s to sell) and was cheap enough, so I saw the broken tuner as a bonus challenge. Dead silence on any band, no S-meter action on FM, let alone a stereo light.
Ultimately I found not one, not two, but three dead-short electrolytics in this unit.
C110 and C95 (470µ/10V) kept the stereo decoder and preceding buffer from working.
C30 (47µ/16V) was shorting the FM frontend supply, and it's underneath the LO shield, because of course it is.
Fortunately removing the back proved more manageable than I was afraid it would be, even the solder joint very close to chassis was just about reachable (barely).
I suspect that the unit may have sat for an extended period of time. Capacitor voltage ratings in this model tend to be on the tight side in general.
The culprits were replaced by 470µ/16V and 47µ/25V types, respectively.
The quadrature discriminator coil needed a major adjustment as well (+4 V at nominal center instead of ~0 mV), I suspect that the capacitor in the can may have started to corrode as these seemingly can do in yet older Kenwoods. Thankfully this is only relevant to center tuning, but that's still important as it was the last thing keeping stereo from coming on. Effectively that's 4 fixes needed to restore stereo operation!
The PLL discriminator only needed a minuscule tweak. My multimeter's frequency counter let me down when it came to VCO adjustment, so I went oldschool - ballpark center of adjustment range with working stereo (at least I had a DC reading) and observe how quick the stereo light comes on.
Misc:
Filters in this Euro model (I don't think longwave coverage was a thing anywhere else) are 2x SFE10.7MS3G (180 kHz), slightly deviating from the MJG + MS3G listed in the service manual. Decent 200 kHz separation but not exactly pure canned DX. (Looking at the layout, coupling cap C36 is virtually begging to be replaced by a 3rd filter. You've got some leeway for increasing gain via R39 if need be, since it has been turned down in the Euro model because its filters sport lower insertion loss than the GDT types used elsewhere.) The frontend seems to perform similarly to other basic 4-gangers without AGC, there are some mixing products from the local flamethrowers.
The AM filter is a basic 2-element job, I think, SFZ series or something.
At least the unit is kind of a power miser when on, about 6.2 W on AM and around 7.5 W on FM (at least once fixed... it was several watts more initially). The same can't be said about the off state though, the power switch is secondary-side so things don't go below 1.6 W and change.
Sound quality strikes me as good, as one would hope (that's the one thing they arguably didn't skimp out on, with a PLL discriminator plus a distortion cancellation circuit).
This model is quite similar to the preceding KT-770(L) (whose schematic with more numerous indicated voltages helped me pin things down around the MPX) but I would rather consider it a slight step down in several respects, e.g. somewhat worse selectivity and dropping the pilot leakage adjustment.
Ultimately I found not one, not two, but three dead-short electrolytics in this unit.
C110 and C95 (470µ/10V) kept the stereo decoder and preceding buffer from working.
C30 (47µ/16V) was shorting the FM frontend supply, and it's underneath the LO shield, because of course it is.
Fortunately removing the back proved more manageable than I was afraid it would be, even the solder joint very close to chassis was just about reachable (barely).I suspect that the unit may have sat for an extended period of time. Capacitor voltage ratings in this model tend to be on the tight side in general.
The culprits were replaced by 470µ/16V and 47µ/25V types, respectively.
The quadrature discriminator coil needed a major adjustment as well (+4 V at nominal center instead of ~0 mV), I suspect that the capacitor in the can may have started to corrode as these seemingly can do in yet older Kenwoods. Thankfully this is only relevant to center tuning, but that's still important as it was the last thing keeping stereo from coming on. Effectively that's 4 fixes needed to restore stereo operation!
The PLL discriminator only needed a minuscule tweak. My multimeter's frequency counter let me down when it came to VCO adjustment, so I went oldschool - ballpark center of adjustment range with working stereo (at least I had a DC reading) and observe how quick the stereo light comes on.
Misc:
Filters in this Euro model (I don't think longwave coverage was a thing anywhere else) are 2x SFE10.7MS3G (180 kHz), slightly deviating from the MJG + MS3G listed in the service manual. Decent 200 kHz separation but not exactly pure canned DX. (Looking at the layout, coupling cap C36 is virtually begging to be replaced by a 3rd filter. You've got some leeway for increasing gain via R39 if need be, since it has been turned down in the Euro model because its filters sport lower insertion loss than the GDT types used elsewhere.) The frontend seems to perform similarly to other basic 4-gangers without AGC, there are some mixing products from the local flamethrowers.
The AM filter is a basic 2-element job, I think, SFZ series or something.
At least the unit is kind of a power miser when on, about 6.2 W on AM and around 7.5 W on FM (at least once fixed... it was several watts more initially). The same can't be said about the off state though, the power switch is secondary-side so things don't go below 1.6 W and change.
Sound quality strikes me as good, as one would hope (that's the one thing they arguably didn't skimp out on, with a PLL discriminator plus a distortion cancellation circuit).
This model is quite similar to the preceding KT-770(L) (whose schematic with more numerous indicated voltages helped me pin things down around the MPX) but I would rather consider it a slight step down in several respects, e.g. somewhat worse selectivity and dropping the pilot leakage adjustment.
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AnalogSteph
Master Contributor
I may or may not have a problem with tuners following me home. (Mind you, I've had the T-904, the KT-80, the KT-1100 and an Onkyo T-4650 and Grundig T 7500 - not seen here - for many years.)
Fixed the midi sized Onkyo T-422 (second from top) today. Fished it out of the bay "for spares or repair" with a cut cord and a bit scuffed up, so presumably a recycling special. Ca. 1997. Primed by my kitchen tuner, the slightly newer T-405X (top), I fitted a cord equipped with a switch, and the 8 W measured in standby proved me right there. I didn't fancy depinning the connector, so cut off some of the original mains cable and soldered to the rest, plus some heatshrink (which fought me a bit) and electrical tape.
Turns out there's nothing truly majorly wrong with the unit. Now I have never seen the inside of a top cover being quite that dark, suggesting that it must have been running (at least in standby) for much of its life. So I'd be willing to bet that there are some tired electrolytics in the power supply section, Nichicon or not. Anything at this level of quality I'd have to order, the local store carries tinkerer grade parts only. They may have a matching heatsink for the 5.6 V regulator though, which is approaching 70°C Tcase and only sports the PCB footprint for one.
Unlike the newer T-405X, you can only set the RDS Accuclock station frequency (for automatic day / time adjustment) using the remote on the accompanying A-922 with a RI system connection, which is a bit of a bummer. It's working here anyway, so perhaps I got lucky or the unit automatically searches for one in the background. There were no presets stored or anything, so I'm pretty sure the goldcap was empty as received.
Circuitry wise, I kind of knew what to expect circuitry wise from the Japanese market T-422M:
Which is good because I could not find a service manual anywhere. And by that I mean you can't even buy one.
The frontend I think is pretty much the same compact 4-gang Mitsumi job as in the T-405X (and would actually be fully accessible for alignment), as is the filter complement (2x MA5 + 1 MZ2K). That being said, the first IF amp uses 2 transistors instead of one, and there's an LA1266 IF IC (which also handled the AM section) and an LA3401 stereo decoder instead of the combo LA1837. The RDS decoder is a µPC1346CS, as in my old TX-SV636; the T-405X sports a BU1922 instead.
The page mentions these alignment items:
Armed with an antenna plus the ELV SUP2 test RF generator kit that I assembled last weekend (spoiler alert: the 3/4h build time seems wildly optimistic), I checked over the alignment. Discriminator center tuning was about 0.37 V off (P121), and adjusting L101 very gingerly (it's a very delicate adjustment) brought it back within 10 mV. Maybe 10 mV of difference between top and bottom band edge, so as usual the reference xtal is fine. (I'm always doing this using OTA stations as they have very tight standards for frequency accuracy.)
Not yet being set up for loopback testing, I skipped L102 for now.
Instead, I went to stereo distortion and separation (frontend IFT, R201), feeding L-only / R-only 440 Hz sine into the SUP2 and listening on the opposite channel plugged into my Atom Amp+ with HD600. Best separation was again obtained in the position I found R201 in, which is to say the far left end. (Something must be off with the circuit design there, I'll have to investigate. Separation achieved at this point strikes me as very good though. EDIT: Found it. From pin 4 of LA3401, there's R203 (47k) - R201 - C206 to ground. Total resistance is adjustable from 47k to 157k. Basically I could just install a jumper wire across R203.... so that's what I ended up doing. Afterwards, I found my optimum at about 59k total, so technically that would have been just within range even before.) If separation is well-adjusted, distortion on the opposite channel becomes easily audible. The IFT ultimately ended up about 30° further to the left to minimize distortion.
R801 ended up pretty much exactly where it was to begin with. RDS sensitivity still seems a bit meh. The TX-SV636 never was the hottest in that regard either, so I guess the NEC IC just isn't the most sensitive. The T-405X strikes me as better.
Frontend IM3 performance is, much like the T-405X's, a bit naff. While they would be accessible in this model, I'm not touching any varnished air coils, no thanks.
I am still not quite trusting the Si4711-based kit generator (the chip has a specified separation of 35 dB typ), not to mention its analog inputs aren't the cleanest due to board layout issues. Only after finding those did I notice that there's actually a completely revamped SUP3 available, still Si4711 based but with a radically different layout that should be much cleaner plus USB audio input and an OLED display. Twice the price, but what's 100€ in relation to actual test equipment?
The uncertainty still got the better of me... a "legit" FM stereo RF generator is on the way. I wasn't planning on taking any real vacation anyway, and let's just say the tunerage isn't getting any less...
Fixed the midi sized Onkyo T-422 (second from top) today. Fished it out of the bay "for spares or repair" with a cut cord and a bit scuffed up, so presumably a recycling special. Ca. 1997. Primed by my kitchen tuner, the slightly newer T-405X (top), I fitted a cord equipped with a switch, and the 8 W measured in standby proved me right there. I didn't fancy depinning the connector, so cut off some of the original mains cable and soldered to the rest, plus some heatshrink (which fought me a bit) and electrical tape.
Turns out there's nothing truly majorly wrong with the unit. Now I have never seen the inside of a top cover being quite that dark, suggesting that it must have been running (at least in standby) for much of its life. So I'd be willing to bet that there are some tired electrolytics in the power supply section, Nichicon or not. Anything at this level of quality I'd have to order, the local store carries tinkerer grade parts only. They may have a matching heatsink for the 5.6 V regulator though, which is approaching 70°C Tcase and only sports the PCB footprint for one.
Unlike the newer T-405X, you can only set the RDS Accuclock station frequency (for automatic day / time adjustment) using the remote on the accompanying A-922 with a RI system connection, which is a bit of a bummer. It's working here anyway, so perhaps I got lucky or the unit automatically searches for one in the background. There were no presets stored or anything, so I'm pretty sure the goldcap was empty as received.
Circuitry wise, I kind of knew what to expect circuitry wise from the Japanese market T-422M:
ONKYO T-422M - BLUESS Laboratory
・2014年9月、実は T-405X と一緒に買ってきました。 ・FM108M...
bluess.cocolog-nifty.com
The frontend I think is pretty much the same compact 4-gang Mitsumi job as in the T-405X (and would actually be fully accessible for alignment), as is the filter complement (2x MA5 + 1 MZ2K). That being said, the first IF amp uses 2 transistors instead of one, and there's an LA1266 IF IC (which also handled the AM section) and an LA3401 stereo decoder instead of the combo LA1837. The RDS decoder is a µPC1346CS, as in my old TX-SV636; the T-405X sports a BU1922 instead.
The page mentions these alignment items:
+ there's R801: RDS 57 kHz filter alignment
Armed with an antenna plus the ELV SUP2 test RF generator kit that I assembled last weekend (spoiler alert: the 3/4h build time seems wildly optimistic), I checked over the alignment. Discriminator center tuning was about 0.37 V off (P121), and adjusting L101 very gingerly (it's a very delicate adjustment) brought it back within 10 mV. Maybe 10 mV of difference between top and bottom band edge, so as usual the reference xtal is fine. (I'm always doing this using OTA stations as they have very tight standards for frequency accuracy.)
Not yet being set up for loopback testing, I skipped L102 for now.
Instead, I went to stereo distortion and separation (frontend IFT, R201), feeding L-only / R-only 440 Hz sine into the SUP2 and listening on the opposite channel plugged into my Atom Amp+ with HD600. Best separation was again obtained in the position I found R201 in, which is to say the far left end. (Something must be off with the circuit design there, I'll have to investigate. Separation achieved at this point strikes me as very good though. EDIT: Found it. From pin 4 of LA3401, there's R203 (47k) - R201 - C206 to ground. Total resistance is adjustable from 47k to 157k. Basically I could just install a jumper wire across R203.... so that's what I ended up doing. Afterwards, I found my optimum at about 59k total, so technically that would have been just within range even before.) If separation is well-adjusted, distortion on the opposite channel becomes easily audible. The IFT ultimately ended up about 30° further to the left to minimize distortion.
R801 ended up pretty much exactly where it was to begin with. RDS sensitivity still seems a bit meh. The TX-SV636 never was the hottest in that regard either, so I guess the NEC IC just isn't the most sensitive. The T-405X strikes me as better.
Frontend IM3 performance is, much like the T-405X's, a bit naff. While they would be accessible in this model, I'm not touching any varnished air coils, no thanks.
I am still not quite trusting the Si4711-based kit generator (the chip has a specified separation of 35 dB typ), not to mention its analog inputs aren't the cleanest due to board layout issues. Only after finding those did I notice that there's actually a completely revamped SUP3 available, still Si4711 based but with a radically different layout that should be much cleaner plus USB audio input and an OLED display. Twice the price, but what's 100€ in relation to actual test equipment?
The uncertainty still got the better of me... a "legit" FM stereo RF generator is on the way. I wasn't planning on taking any real vacation anyway, and let's just say the tunerage isn't getting any less...
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AnalogSteph
Master Contributor
Next patient: A rather dead Kenwood KT-1100D. (Not explicitly sold as such, actually.) A few scratches but came with AM loop (and the original price tag: DM 995,-). Same as KT-880L, case construction seems similar as well.
Powers up (sticky power switch) and seems to operate, emits some low-level hiss on FM and AM, but no reception and no trace of LOs on either band. 21 W from the wall, spec is 18 W on 220 V.
Checked the VT/GND test points - 1.5 mV. Well, that's not gonna work. Not shorted though. I'll go poke my nose into the service manual to check where the tuning voltage supply comes from...
Found it. Tested 100R dropper resistor R266, measures 650ish kOhms and looks a bit toasty as well (no doubt a fusible, too). Sure enough, there's a short of 2.2 ohms on one side (should be towards the PLL LPF amp).
Who would have guessed, it's another dang shorted capacitor.
C161, 33µ/35V. Measured straight across it, yep, as shorted as can be.
Hmm, I don't have any 33µ here but I do have 47µ/63V. A bit big but should still fit there...
I tested the 28 V supply at R266. Goes to about 15.5 V and slowly keeps climbing. You've gotta be effing kidding me, there's more? Fine, I'll check the regulator area next...
EDIT: IT'S ALIVE!
Turns out R266 actually was not open at all, so the short was dragging down the regulator. Oops.
And the center tuning discriminator was roughly 100 kHz off.
The LO trimmer cap could use the usual slight tweak as well.
MPX VCO was just barely out of spec.
Power consumption now: Standby 1.76 W, FM 16-17.5 W, AM 16 W.
I may have to modify the tuning speed à la KT-880L, while it has a tuning knob the actual tuning IC (TC9157AP) does not accept input very rapidly, and turning the knob quickly makes things go slower if anything.
EDIT 2: I should have stopped while I was ahead. Cracked the core in L1. *insert string of expletives here* At least it seems to be stuck / tuned where it's supposed to be, so I guess that counts as escaping with a black eye. I am convinced that it must be possible to glue the things (despite reports of little success), but in this case it may best be left alone. Why does it always have to happen on fancy tuners... (This type of coil only seems to be shared with other relatively high-end models either - 990D, 7020, 6040. 5020 too. A single one remains in the 3050.)
I must say I'm not a fan of this signal-dependent variable high blend / stereo blend feature (slider above memory buttons). You seemingly cannot fully disable it on weaker signals. Something doesn't sound quite right either way.
The S-meter is a bit of a joke, too, it's supposed to illuminate the 7th segment by 40dBµV and seems limited on the bottom end as well. That's even worse than older Kenwoods.
The variable AM bandwidth feature is neat but also very necessary, as filtering is not great (one SFU450E + IFT again). You can just about get rid of 9 kHz hets but things get rather muffled then. Despite the lack of an additional RF amp, sensitivity still seems good.
In order to fully disassemble the front, you'd need to remove some push rivets - not a fan. IFT alignment also calls for a very obscure instrument, a genescope (combo sweep generator and scope).
I believe this model basically is an evolution of the preceding KT-990SD. The mainboard has gained several subassemblies. This construction is trying to be a bit too clever for its own good for my tastes while attempting to hit a certain price point, and I am not a fan of the tradeoffs this generates. That's certainly a downside to the tuner wars.
Powers up (sticky power switch) and seems to operate, emits some low-level hiss on FM and AM, but no reception and no trace of LOs on either band. 21 W from the wall, spec is 18 W on 220 V.
Checked the VT/GND test points - 1.5 mV. Well, that's not gonna work. Not shorted though. I'll go poke my nose into the service manual to check where the tuning voltage supply comes from...
Found it. Tested 100R dropper resistor R266, measures 650ish kOhms and looks a bit toasty as well (no doubt a fusible, too). Sure enough, there's a short of 2.2 ohms on one side (should be towards the PLL LPF amp).
Who would have guessed, it's another dang shorted capacitor.
C161, 33µ/35V. Measured straight across it, yep, as shorted as can be.Hmm, I don't have any 33µ here but I do have 47µ/63V. A bit big but should still fit there...
I tested the 28 V supply at R266. Goes to about 15.5 V and slowly keeps climbing. You've gotta be effing kidding me, there's more? Fine, I'll check the regulator area next...
EDIT: IT'S ALIVE!
Turns out R266 actually was not open at all, so the short was dragging down the regulator. Oops.
And the center tuning discriminator was roughly 100 kHz off.
The LO trimmer cap could use the usual slight tweak as well.
MPX VCO was just barely out of spec.
Power consumption now: Standby 1.76 W, FM 16-17.5 W, AM 16 W.
I may have to modify the tuning speed à la KT-880L, while it has a tuning knob the actual tuning IC (TC9157AP) does not accept input very rapidly, and turning the knob quickly makes things go slower if anything.
EDIT 2: I should have stopped while I was ahead. Cracked the core in L1. *insert string of expletives here* At least it seems to be stuck / tuned where it's supposed to be, so I guess that counts as escaping with a black eye. I am convinced that it must be possible to glue the things (despite reports of little success), but in this case it may best be left alone. Why does it always have to happen on fancy tuners... (This type of coil only seems to be shared with other relatively high-end models either - 990D, 7020, 6040. 5020 too. A single one remains in the 3050.)
I must say I'm not a fan of this signal-dependent variable high blend / stereo blend feature (slider above memory buttons). You seemingly cannot fully disable it on weaker signals. Something doesn't sound quite right either way.
The S-meter is a bit of a joke, too, it's supposed to illuminate the 7th segment by 40dBµV and seems limited on the bottom end as well. That's even worse than older Kenwoods.
The variable AM bandwidth feature is neat but also very necessary, as filtering is not great (one SFU450E + IFT again). You can just about get rid of 9 kHz hets but things get rather muffled then. Despite the lack of an additional RF amp, sensitivity still seems good.
In order to fully disassemble the front, you'd need to remove some push rivets - not a fan. IFT alignment also calls for a very obscure instrument, a genescope (combo sweep generator and scope).
I believe this model basically is an evolution of the preceding KT-990SD. The mainboard has gained several subassemblies. This construction is trying to be a bit too clever for its own good for my tastes while attempting to hit a certain price point, and I am not a fan of the tradeoffs this generates. That's certainly a downside to the tuner wars.
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AnalogSteph
Master Contributor
My "legit" FM RF signal generator (YOLO edition) came in yesterday, a Levear (ex Panasonic) VP-8193D. These seem to be much newer than you'd think from their looks, I found 2007/8 dates in the service documentation. This one does AM and FM from 100 kHz to 140 MHz, no DDS option. This model does not sport RDS but nobody seems to have the software for that anyway. Specs for FM are distortion <=0.05% at 75 kHz and channel separation 55 dB. That's about the minimum you need for decent tuners. It'll do mono, L=R, L only, R only, L=-R, with selectable pre-emphasis for 25, 50, 75 µs, using either an internal 1 kHz / 400 Hz source or external input (which I suspect may further improve distortion performance but we'll see).
Luckily I was able to borrow a BNC to IEC adapter cable locally from a repair café acquaintance so I could start right away.
As you might be able to guess, I have been busy tweaking the stereo separation and distortion alignments on a whole bunch of my tuners (again).
The Onkyo T-422's FM IFT ended up pretty much back where it had originally been, which I'm counting as a good sign. Separation now is at a total of 37 kOhms, so unless the little 10µ is super-turbo-dead in mine, you could never get to this level stock with a 47k in series. I was amazed by the image rejection and IF rejection levels of the little 4-gang Mitsumi frontend; it just seems to completely freak out any time any signal of 107 dBµV or greater is being presented anywhere on the spectrum, including 10.7 MHz. (???)
(EDIT: The T-405X has the same quirk at the same level. Image rejection is about 87-88 dB.)
(EDIT 2: Looks like it's actually my generator that's messed up, as the switching from 106 to 107 is accompanied by relay noise. It seems to be losing about 20 dB of output amplitude at this point.)
On the KT-900 I had to find a compromise between L->R and R->L crosstalk, looks like I may have to install some hand-matched metal film resistors in the output stage for optimum channel balance. It also seems a few dB less sensitive than some of my other tuners, slightly improving towards the top end which I had previously realigned, and neither image rejection (about 76 dB by 108 MHz) nor IF rejection (about 87 dB, way short of the 110 claimed) are blowing my socks off. Something is definitely fishy there. The KT-80 does noticeably better. Not looking forward to tweaking air coils. Oh well, it remains a project.
Next patient: A Pioneer F-757 Mark II tuner (ca. 1992, quite fancy - balanced mixer, 3 / 5 filters in "wide" and "super narrow" respectively). Pretty much the very last generation not yet equipped with RDS decoding, which by 1993 had become impossible to resist. Seems to receive (even if the S-meter is suspiciously flickery) but completely silent on FM, with only the merest high-pitched hint of signal when cranked. Does have output on AM.
Power consumption: Approaching 5 W in standby, about 14 W in operation. Inside, the power transformer looks awfully similar to what the KT-1100D uses, same OEM I bet.
This model seems a minor revision of the first F-757, for which a service manual is thankfully available online.
I pulled out all the stops for this one, even the scope got used for tracing the signal. Ultimately it gets lost in the very last buffer stage with muting on IC601, and sure enough, the MUT input (pin 17) sits at 0 V rather than the 5.3 V indicated in the schematic. And that in turn goes right back to the microprofessor IC801, which obviously has to be unhappy with something. Not the muting input from the detector IC, that much I checked (quadrature coil alignment has also been tweaked but was only marginally off to begin with). I hope it's not damaged or something.
What I did notice is that the +26V tuning voltage supply is pretty much cooked. It's among those that keep running in standby 24/7, and since it's zener-regulated the current consumption never reduces much. I'm measuring like 5-8 V of AC ripple at C703, and if you tune into the LO signal there's rather pronounced hum on it, not at all worthy of a tuner like that. Some other parts may not have fared much better... I noticed a particularly warm little cap on the thermal camera, tried to bend it a bit, and with a crunch one of its solder joints gave way. It's C717, presumably being heated by its parallel reference zener for the -8.5V supply, another part that's active 24/7.
The power supply section is rather cramped, warm-running and thermally not all that great anyway. There's zeners half-hidden underneath electrolytics and fun stuff like that. I won't be one bit surprised if bad solder joints and dead electrolytics abound. The 'lytics aren't really any bigger than modern parts either, the contrary at times (my 220µ/63s are a bit more short and squat than C703, for example).
The unit is not super easy to work on either. There is no removable bottom so you'd have to remove the entire main PCB, which is doable but a bit involved. I can't really figure out how the front panel ribbon cables ought to be removed. Also, several of the points I wanted to measure at ended up buried underneath parts or cables. As neat and quality as the thing looks inside, it sure is trying to make your life hard.
All of the above means that this isn't just a quick service job and more of a full-blown project. This one goes back in the box for now.
The '90s and their standby shenanigans...
To think that earlier today, I had been annoyed by the IF programming on my banged-up Grundig FineArts T-904 (real power switch, thank goodness)... this had been done at the factory by a series of 4 perforated tabs on the PCB edge, to be broken off to indicate a 0. Using the nifty test mode, I found that optimum distortion would be achieved in 1100 (10.7500) instead of 1011 (10.7375 MHz), so I'd have to pull the board to restore one somehow - not a super hard job on this model. Filters are two orange dot MS3Gs (which seem to have drifted closer to white dot territory) and a pair of red dot MA8s. You can probably imagine that the passband is a bit of a ripply mess, with side peaks at +/- 100 kHz. At least LO realignment seems to have improved sensitivity by a few dB, it's one of my more sensitive tuners on the DISTANT input. RF tracking proved fine as found once that was done. (Image rejection is not super great though. Maybe the 70s?) It's still more overload-prone than e.g. the KT-880L (no surprise given the BJT mixer), but still has the less sensitive LOCAL input if you need to contend with cable levels.
This tuner is quite the contrast to some of the others I have. Instead of employing a voltage regulator work of art, this one is like 7812 and be done with it. That then also serves as the voltage source for tuning voltage generation with some RC filtering - the range is 2.5 to 8.5 V only, nothing in the 20s in sight. I think it's making use of the fact that both LA1266 and LA3401 seem to have internal regulators, so ultimately it's just mono SNR that's being limited by ground pollution (and that tends to be more than high enough anyway). You can probably tell that it was Grundig's base model in this series at the time, ca. 1991 (ones with "real" GDT filters and RDS decoding were also made). The quadrature coil alignment calls for alignment by maximum / minimum noise (first / second coil) on a small signal modulated input, not sure I get that.
I actually quite like the operation and such of this model, it feels fairly modern (quick search, direct frequency entry, system remote control etc.)... my main gripe is the low-contrast front panel lettering that would no doubt fail WCAG guidelines and is giving me a bit of a hard time. The top cover is also badly scratched and in need of a respray, no idea how that happened (I bought it like that in maybe 2007). One thing that is decidedly oldschool is the 2x AAA memory battery backup... but I sure can't complain about its longevity, the heavy duty cells in there were anything but new when the unit was put away almost a decade ago and they still get the job done to this day.
Luckily I was able to borrow a BNC to IEC adapter cable locally from a repair café acquaintance so I could start right away.
As you might be able to guess, I have been busy tweaking the stereo separation and distortion alignments on a whole bunch of my tuners (again).
The Onkyo T-422's FM IFT ended up pretty much back where it had originally been, which I'm counting as a good sign. Separation now is at a total of 37 kOhms, so unless the little 10µ is super-turbo-dead in mine, you could never get to this level stock with a 47k in series. I was amazed by the image rejection and IF rejection levels of the little 4-gang Mitsumi frontend; it just seems to completely freak out any time any signal of 107 dBµV or greater is being presented anywhere on the spectrum, including 10.7 MHz. (???)
(EDIT: The T-405X has the same quirk at the same level. Image rejection is about 87-88 dB.)
(EDIT 2: Looks like it's actually my generator that's messed up, as the switching from 106 to 107 is accompanied by relay noise. It seems to be losing about 20 dB of output amplitude at this point.)
On the KT-900 I had to find a compromise between L->R and R->L crosstalk, looks like I may have to install some hand-matched metal film resistors in the output stage for optimum channel balance. It also seems a few dB less sensitive than some of my other tuners, slightly improving towards the top end which I had previously realigned, and neither image rejection (about 76 dB by 108 MHz) nor IF rejection (about 87 dB, way short of the 110 claimed) are blowing my socks off. Something is definitely fishy there. The KT-80 does noticeably better. Not looking forward to tweaking air coils. Oh well, it remains a project.
Next patient: A Pioneer F-757 Mark II tuner (ca. 1992, quite fancy - balanced mixer, 3 / 5 filters in "wide" and "super narrow" respectively). Pretty much the very last generation not yet equipped with RDS decoding, which by 1993 had become impossible to resist. Seems to receive (even if the S-meter is suspiciously flickery) but completely silent on FM, with only the merest high-pitched hint of signal when cranked. Does have output on AM.
Power consumption: Approaching 5 W in standby, about 14 W in operation. Inside, the power transformer looks awfully similar to what the KT-1100D uses, same OEM I bet.
This model seems a minor revision of the first F-757, for which a service manual is thankfully available online.
I pulled out all the stops for this one, even the scope got used for tracing the signal. Ultimately it gets lost in the very last buffer stage with muting on IC601, and sure enough, the MUT input (pin 17) sits at 0 V rather than the 5.3 V indicated in the schematic. And that in turn goes right back to the microprofessor IC801, which obviously has to be unhappy with something. Not the muting input from the detector IC, that much I checked (quadrature coil alignment has also been tweaked but was only marginally off to begin with). I hope it's not damaged or something.
What I did notice is that the +26V tuning voltage supply is pretty much cooked. It's among those that keep running in standby 24/7, and since it's zener-regulated the current consumption never reduces much. I'm measuring like 5-8 V of AC ripple at C703, and if you tune into the LO signal there's rather pronounced hum on it, not at all worthy of a tuner like that. Some other parts may not have fared much better... I noticed a particularly warm little cap on the thermal camera, tried to bend it a bit, and with a crunch one of its solder joints gave way. It's C717, presumably being heated by its parallel reference zener for the -8.5V supply, another part that's active 24/7.
The power supply section is rather cramped, warm-running and thermally not all that great anyway. There's zeners half-hidden underneath electrolytics and fun stuff like that. I won't be one bit surprised if bad solder joints and dead electrolytics abound. The 'lytics aren't really any bigger than modern parts either, the contrary at times (my 220µ/63s are a bit more short and squat than C703, for example).
The unit is not super easy to work on either. There is no removable bottom so you'd have to remove the entire main PCB, which is doable but a bit involved. I can't really figure out how the front panel ribbon cables ought to be removed. Also, several of the points I wanted to measure at ended up buried underneath parts or cables. As neat and quality as the thing looks inside, it sure is trying to make your life hard.
All of the above means that this isn't just a quick service job and more of a full-blown project. This one goes back in the box for now.
The '90s and their standby shenanigans...
To think that earlier today, I had been annoyed by the IF programming on my banged-up Grundig FineArts T-904 (real power switch, thank goodness)... this had been done at the factory by a series of 4 perforated tabs on the PCB edge, to be broken off to indicate a 0. Using the nifty test mode, I found that optimum distortion would be achieved in 1100 (10.7500) instead of 1011 (10.7375 MHz), so I'd have to pull the board to restore one somehow - not a super hard job on this model. Filters are two orange dot MS3Gs (which seem to have drifted closer to white dot territory) and a pair of red dot MA8s. You can probably imagine that the passband is a bit of a ripply mess, with side peaks at +/- 100 kHz. At least LO realignment seems to have improved sensitivity by a few dB, it's one of my more sensitive tuners on the DISTANT input. RF tracking proved fine as found once that was done. (Image rejection is not super great though. Maybe the 70s?) It's still more overload-prone than e.g. the KT-880L (no surprise given the BJT mixer), but still has the less sensitive LOCAL input if you need to contend with cable levels.
This tuner is quite the contrast to some of the others I have. Instead of employing a voltage regulator work of art, this one is like 7812 and be done with it. That then also serves as the voltage source for tuning voltage generation with some RC filtering - the range is 2.5 to 8.5 V only, nothing in the 20s in sight. I think it's making use of the fact that both LA1266 and LA3401 seem to have internal regulators, so ultimately it's just mono SNR that's being limited by ground pollution (and that tends to be more than high enough anyway). You can probably tell that it was Grundig's base model in this series at the time, ca. 1991 (ones with "real" GDT filters and RDS decoding were also made). The quadrature coil alignment calls for alignment by maximum / minimum noise (first / second coil) on a small signal modulated input, not sure I get that.
I actually quite like the operation and such of this model, it feels fairly modern (quick search, direct frequency entry, system remote control etc.)... my main gripe is the low-contrast front panel lettering that would no doubt fail WCAG guidelines and is giving me a bit of a hard time. The top cover is also badly scratched and in need of a respray, no idea how that happened (I bought it like that in maybe 2007). One thing that is decidedly oldschool is the 2x AAA memory battery backup... but I sure can't complain about its longevity, the heavy duty cells in there were anything but new when the unit was put away almost a decade ago and they still get the job done to this day.
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Philbo King
Major Contributor
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- May 30, 2022
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I have a 3 string cigar-box guitar waiting for me to install a stratocaster pickup into it.
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AnalogSteph
Master Contributor
Next patient: A "spares or repair" Kenwood KT-3050L (ca. 1993), primarily obtained as a parts donor but I wanted to see whether I could the thing going again as it's actually a fairly nice tuner with two 110 kHz and two 230 kHz GDT filters and RDS (only the discriminator is merely of the basic quadrature type) and not in half-bad shape cosmetically. It takes a while to "wake up", and will run in standby absolutely forever from that point on, but if you turn it on it starts to suffer from bizarre freakouts after several minutes, with the display going blank and coming back, which gets worse and worse until the unit eventually settles back into standby.
When it did work, it seemed to operate fine, minus the discriminator being about +50 kHz off. That was triggering my OCD, so I ballparked L20 using the tuning meter, which turned out to be surprisingly accurate (just a few tens of mV off when actually using the assigned test points).
The power supply area is a bit toasty around the zeners for the -33 V supply but generally all the voltages are there and seem stable. Power ca. 3.5 W standby, 7.5 W on. Doesn't actually look super high hours, just normal for the age I guess. I can't really approve of a design that even uses the VFD for standby indication, mind you, no wonder the display isn't exactly super bright.
I noticed some voltage fluctuation on the 5 V on the end of R238 during the "freakouts".
That's a line marked "CE", and it goes here on the control PCB, the input of IC11:
Further measurement yielded 3.3 V on the output of IC11. ?! That's a PST529D reset IC with an open collector output that's supposed to be either pulled up via R298 or pulled down to near 0 V, not a 3.3 V LDO.
Then I noticed something.
Do you see what I see?
The designation C197 belongs to the footprint of of an electrolytic. Not at all the 0.1 µF (100n) ceramic/film shown in the schematic. Sure enough, there's 3.3 V on the + end and 0.8 V on the other, no wonder the darn thing won't come out of reset. Stinkin' electrolytics.
The parts list pretty clearly says "C197 - CF92FV1H104J - MF 0.10UF J" (MF = metallized film?). Possibly this was a last-minute change because the reset may not have been working reliably or the beancounters were at it again. (This model was, after all, dirt cheap for what you got, with a DM 449,- MSRP.)
Great, now what do I put there and how do I even get the board out? the big flat flex connector on the main PCB does not appear to be a locking type, am I just supposed to yank on it or what?
When it did work, it seemed to operate fine, minus the discriminator being about +50 kHz off. That was triggering my OCD, so I ballparked L20 using the tuning meter, which turned out to be surprisingly accurate (just a few tens of mV off when actually using the assigned test points).
The power supply area is a bit toasty around the zeners for the -33 V supply but generally all the voltages are there and seem stable. Power ca. 3.5 W standby, 7.5 W on. Doesn't actually look super high hours, just normal for the age I guess. I can't really approve of a design that even uses the VFD for standby indication, mind you, no wonder the display isn't exactly super bright.
I noticed some voltage fluctuation on the 5 V on the end of R238 during the "freakouts".
That's a line marked "CE", and it goes here on the control PCB, the input of IC11:
Further measurement yielded 3.3 V on the output of IC11. ?! That's a PST529D reset IC with an open collector output that's supposed to be either pulled up via R298 or pulled down to near 0 V, not a 3.3 V LDO.
Then I noticed something.
Do you see what I see?
The designation C197 belongs to the footprint of of an electrolytic. Not at all the 0.1 µF (100n) ceramic/film shown in the schematic. Sure enough, there's 3.3 V on the + end and 0.8 V on the other, no wonder the darn thing won't come out of reset. Stinkin' electrolytics.
The parts list pretty clearly says "C197 - CF92FV1H104J - MF 0.10UF J" (MF = metallized film?). Possibly this was a last-minute change because the reset may not have been working reliably or the beancounters were at it again. (This model was, after all, dirt cheap for what you got, with a DM 449,- MSRP.)
Great, now what do I put there and how do I even get the board out? the big flat flex connector on the main PCB does not appear to be a locking type, am I just supposed to yank on it or what?
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AnalogSteph
Master Contributor
You could say I am fully capacitated now - my ELV ESR 1 arrived, and I finally bought a lab power supply (PeakTech 6227 - 60 V, 6 A, 150 W). Using the PSU limited to 0.002 A (which really is more like 0.8 mA and 0.001 A is virtually nil, but I mean it's a 6 A supply so what do you expect) and my multimeter's µA range, I conducted some experiments to see how far I could get electrolytic cap leakage down.
Put a few caps old and new to the test:
I always thought the old Elna 1000µ/25 out of my Kenwood KT-80 was in better shape than its counterpart out of the KT-900, but nope. ESR 0.04-0.05 ohms vs. 0.03 ohms, higher leakage (about 3 µA vs. 0.5ish µA at 25 V, things get a bit noisy sub-1 µA) and lower sustained voltage (31 V is really pushing it, the other one does 35 easily). With capacitance measuring about 0.98 mF as well, the KT-900 part could basically pass for a new cap. I did notice that pushing the voltage could worsen leakage once you returned to a lower one, so these old caps may not have much in terms of self-repair abilities left in them and may be living on borrowed time (electrolyte pH gone off - oh well, that sort of thing can happen after 44-45 years I guess).
(EDIT: With the 100k current shunt, leakage of the KT-900 specimen settles at 0.55-0.59 µA at 25 V. The next day, it's at 0.45 µA and still slowly falling... 0.41 µA... 0.335 µA... 0.28 µA... 0.26 µA... seemed pretty stable there. OK, that'll do. The KT-80 specimen has made it to 2.44 µA, but obviously that's still not nearly in the same ballpark.)
I was delighted to find some Nichicon UPW 1200µ/35V (16x20mm) at the local parts store. I was initially disappointed with the first specimen as leakage started out relatively high, however it only seems to have needed some "waking up" as I was able to get it into the 0.5 µA range at 35 V eventually (EDIT: 0.53 µA with a 10k shunt), and the second one behaved more like a new cap. These guys will also sustain 50 V fine. ESR measures 0.03 ohms, in line with the 0.029 ohm spec.
(EDIT 2: Leakage is down to about 0.15 µA after accidentally sitting for a few hours. Superb. Another 9 hours or so later, it has settled at around 0.10 µA. Swapping back in the 100k shunt, things settle to about 0.099 µA. Still seems to be going down veeery slowly, it's around 0.090 µA a good hour later, though at this point changes in ambient temperature seem to have a greater effect than anything else.)
A Jamicon TK 1000µ/63 that's been rolling around in my parts jar for the last 15 years seems like it might be a tad overrated in both capacitance and voltage handling from the way it behaves. It's more like an 820µ/50. ESR is fine at 0.03 ohms though.
Nichicon PL 2200µ/16 also from the parts jar, nothing unusual to note. ESR 0.02 ohms, in line with the <0.028 ohm spec.
NOS 4.7µ/25V Philips axial also from the parts jar (probably old even when I bought them), now that one needed some thorough reforming. These things had always struck me as quite leaky, and that's because they were. (These have a reputation of being crap anyway.) I managed to get this one to calm down to the point of 0.00µA with occasional blips of 0.02 and 0.04 µA on my meter at 38 V (clearly I need a more resolving test setup), things go downhill quickly around 40 V. I cobbled something together using a 100k resistor as a current shunt, that gives me 6.3ish mV or about 0.063 µA at 38 V, dropping to 0.030 µA by 35 V, 0.014 µA by 30 V, 0.006 µA by 25 V and 0.002 µA by 20 V. ESR 2.5 ohms, #2 is a bit better at 1.9 ohms.
Oh no, I just read the print on the cap... these are supposed to be 100V types.
OK, let's try 60 V then.... The cap has slowly made it to sub-0.5 µA and is inching towards 0.3 µA, but that's still hardly exciting. At least it seems to have helped leakage further down... 38 V now shows about 0.012 µA.
Cap #2 is, if anything, even worse. It has now reached8.0 5.0 2.5 1.5 µA at 60 V, still going down...
You can readily observe the effect of dielectric absorption when going back down with your voltage.
If you are in a bit of a hurry getting the leakage current down, going somewhat above rated voltage can speed this process up. Most caps seem fine at the next higher rating or at least +20%. I would wait with going higher until leakage is firmly in double-digit µA territory though.
Does anyone know what sort of function describes leakage current vs. applied voltage? It seems to be quite extreme when approaching the voltage limit. At times I would go up a volt and it would virtually double. My guess is perhaps something like a*V + b*e^(V-Vth), but what do I know. I guess it's some sort of avalanche breakdown?
Either way, these experiments have confirmed my hunch about why the KT-880L uses a 2200µ/16V cap for +5 V memory backup instead of the 6.3V type found on the older KT-770(L) or misc. low-end models, it must bring leakage down substantially. With electronics obviously in the sub-µA range, capacitor self-discharging may otherwise have a substantial effect on backup times. Plus that cap, even if a bit bulky, still seems to work just fine, which is more than you can say about many of the double layer capacitors (aka goldcaps) that came later. (I still have to replace the one in my Onkyo T-4650 that is conveniently located underneath the VFD, plus rework the reset/sleep circuit on the main PCB which is known marginal and not helping matters.)
Put a few caps old and new to the test:
I always thought the old Elna 1000µ/25 out of my Kenwood KT-80 was in better shape than its counterpart out of the KT-900, but nope. ESR 0.04-0.05 ohms vs. 0.03 ohms, higher leakage (about 3 µA vs. 0.5ish µA at 25 V, things get a bit noisy sub-1 µA) and lower sustained voltage (31 V is really pushing it, the other one does 35 easily). With capacitance measuring about 0.98 mF as well, the KT-900 part could basically pass for a new cap. I did notice that pushing the voltage could worsen leakage once you returned to a lower one, so these old caps may not have much in terms of self-repair abilities left in them and may be living on borrowed time (electrolyte pH gone off - oh well, that sort of thing can happen after 44-45 years I guess).
(EDIT: With the 100k current shunt, leakage of the KT-900 specimen settles at 0.55-0.59 µA at 25 V. The next day, it's at 0.45 µA and still slowly falling... 0.41 µA... 0.335 µA... 0.28 µA... 0.26 µA... seemed pretty stable there. OK, that'll do. The KT-80 specimen has made it to 2.44 µA, but obviously that's still not nearly in the same ballpark.)
I was delighted to find some Nichicon UPW 1200µ/35V (16x20mm) at the local parts store. I was initially disappointed with the first specimen as leakage started out relatively high, however it only seems to have needed some "waking up" as I was able to get it into the 0.5 µA range at 35 V eventually (EDIT: 0.53 µA with a 10k shunt), and the second one behaved more like a new cap. These guys will also sustain 50 V fine. ESR measures 0.03 ohms, in line with the 0.029 ohm spec.
(EDIT 2: Leakage is down to about 0.15 µA after accidentally sitting for a few hours. Superb. Another 9 hours or so later, it has settled at around 0.10 µA. Swapping back in the 100k shunt, things settle to about 0.099 µA. Still seems to be going down veeery slowly, it's around 0.090 µA a good hour later, though at this point changes in ambient temperature seem to have a greater effect than anything else.)
A Jamicon TK 1000µ/63 that's been rolling around in my parts jar for the last 15 years seems like it might be a tad overrated in both capacitance and voltage handling from the way it behaves. It's more like an 820µ/50. ESR is fine at 0.03 ohms though.
Nichicon PL 2200µ/16 also from the parts jar, nothing unusual to note. ESR 0.02 ohms, in line with the <0.028 ohm spec.
NOS 4.7µ/25V Philips axial also from the parts jar (probably old even when I bought them), now that one needed some thorough reforming. These things had always struck me as quite leaky, and that's because they were. (These have a reputation of being crap anyway.) I managed to get this one to calm down to the point of 0.00µA with occasional blips of 0.02 and 0.04 µA on my meter at 38 V (clearly I need a more resolving test setup), things go downhill quickly around 40 V. I cobbled something together using a 100k resistor as a current shunt, that gives me 6.3ish mV or about 0.063 µA at 38 V, dropping to 0.030 µA by 35 V, 0.014 µA by 30 V, 0.006 µA by 25 V and 0.002 µA by 20 V. ESR 2.5 ohms, #2 is a bit better at 1.9 ohms.
Oh no, I just read the print on the cap... these are supposed to be 100V types.
OK, let's try 60 V then.... The cap has slowly made it to sub-0.5 µA and is inching towards 0.3 µA, but that's still hardly exciting. At least it seems to have helped leakage further down... 38 V now shows about 0.012 µA.Cap #2 is, if anything, even worse. It has now reached
You can readily observe the effect of dielectric absorption when going back down with your voltage.
If you are in a bit of a hurry getting the leakage current down, going somewhat above rated voltage can speed this process up. Most caps seem fine at the next higher rating or at least +20%. I would wait with going higher until leakage is firmly in double-digit µA territory though.
Does anyone know what sort of function describes leakage current vs. applied voltage? It seems to be quite extreme when approaching the voltage limit. At times I would go up a volt and it would virtually double. My guess is perhaps something like a*V + b*e^(V-Vth), but what do I know. I guess it's some sort of avalanche breakdown?
Either way, these experiments have confirmed my hunch about why the KT-880L uses a 2200µ/16V cap for +5 V memory backup instead of the 6.3V type found on the older KT-770(L) or misc. low-end models, it must bring leakage down substantially. With electronics obviously in the sub-µA range, capacitor self-discharging may otherwise have a substantial effect on backup times. Plus that cap, even if a bit bulky, still seems to work just fine, which is more than you can say about many of the double layer capacitors (aka goldcaps) that came later. (I still have to replace the one in my Onkyo T-4650 that is conveniently located underneath the VFD, plus rework the reset/sleep circuit on the main PCB which is known marginal and not helping matters.)
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solderdude
Grand Contributor
Yep a common failure.
When I worked for the Technics service dept. (30 years ago) this was already happening with similar chassis.
The original parts aren't available any more.
Very nice solution fits nicely in the rectangular gaps in the plastic !
When I worked for the Technics service dept. (30 years ago) this was already happening with similar chassis.
The original parts aren't available any more.
Very nice solution fits nicely in the rectangular gaps in the plastic !
I think most likely the rubber broke because off the missing transport screws when shipping.
Also possible the rubber looses most of his elasticity because of ozon.
solderdude
Grand Contributor
The rubber just starts to crack and fall apart over time (many years)
The spring does not seem to be carrying the weight of the rather heavy sub chassis + platter.
As soon as the rubber tears the plateau starts to hang a little lower and is very close or even touching the plinth when all the rubbers are torn.
I suppose the intention was for the springs to absorb shocks/vibrations and the rubber to damp it. In practice the rubber also carries a lot of the weight until it starts to decay and tear.
When transporting it is enough to just remove the platter.
The spring does not seem to be carrying the weight of the rather heavy sub chassis + platter.
As soon as the rubber tears the plateau starts to hang a little lower and is very close or even touching the plinth when all the rubbers are torn.
I suppose the intention was for the springs to absorb shocks/vibrations and the rubber to damp it. In practice the rubber also carries a lot of the weight until it starts to decay and tear.
When transporting it is enough to just remove the platter.
AnalogSteph
Master Contributor
My "spares or repair" KT-3050L lives again.
Turns out I was wrong, C197 very much is the 100n film shown in the schematic, and it appears it was just doing its job. Oh well, having the front off was no loss in terms of accessibility, and taking it all apart meant I could remove a bunch of old dust (of which there has been no shortage in this unit). And yes, the big ribbon cable connector is a friction fit affair. Was a bit crunchy at first.
Hiro-kun (the legend) put me onto the right track. C140, eh? Sure enough, the ESR 1 attested 10.3 ohms.
Going through the power supply section yielded these measurements:
So only two obviously bad caps, a bunch more questionable ones (which you will find are associated with the always-on parts, no surprise) and a few which seem OK.
I had to remove the main PCB to access the bottom side, but overall it wasn't a terrible job, 4 screws at the back side and half a dozen on the board. I left the power cable connected, it wasn't too much of an issue.
Ultimately I only replaced the two obviously bad caps, using CapXon GL 10µ/63 for both that were briefly formed at 60 V just in case. That did the trick. (The antenna connectors also seemed a bit wobbly, so I gave some attention to their solder joints as well.)
Old C140 (4.7µ/50V, power-off detection) still measures 1.66 µF, that's definitely toast.
Old C145 (10µ/35V, +5.6V reg output) still measures 9.66 µF, but the comparison with C146's ESR shows that it is quite tired. That wouldn't have been helping matters here.
Wide separation and distortion adjusted fine and weren't too far off to begin with, narrow distortion is a bit "eh" not the least due to a ca. -15 to 20 kHz passband offset that you can't correct (there are zero distortion adjustments for narrow, period). When you've got 110 kHz filters going, that sort of stuff obviously matters. I don't think the LM7001 PLL has any facilities for IF fine-tuning, it seems to be running in basic "f_ref = 50 kHz for FM, 9 kHz in AM" mode here. They didn't even bother to implement 1 kHz for longwave even though the chip could do it and the band was notoriously plagued by off-grid stations, shows you how much of a priority it was.
I had one hell of a time trying to get the FM frontend to track properly, if tuned as described the high end would always be worse. Ultimately I had to deviate from the specified tuning voltage range (2.0 to 11.5 V). I lined up the tuned circuits for good sensitivity on the high end at 11.5 V and then retuned the LO inductor until things were tracking on the low end, went back to the high end to retune the LO trimmer (a bit scratchy but not unworkable, btw), rinse and repeat. Final result was about 2.37 V to 11.435 V. So in a way this unit is the opposite to the Grundig FineArts T-904, which picked up sensitivity just from readjusting the tuning voltage range to spec.
It's a nice-sounding tuner in Wide with sensitive and quick RDS decoding, and the "Active Reception" logic seems to be doing a good job picking the right bandwidth. Narrow yields somewhat quieter output due to auto-adjusted separation. Looong muting interval. I'll have to check memory retention times now that the "hiccups" are gone. Power consumption is about 3.7 W in standby, 7.9 W on FM and 7.1 W on AM.
These don't seem to be selling at all right now. I mean, it's vacation time and the tuner market is in the toilet in general, but still. I paid more for this grubby spares unit than people are willing to pay for a working one in good condition.
Now that this one is also working, I guess I'll have to find another parts donor for my RF coil. And buy another switched power cord.
Turns out I was wrong, C197 very much is the 100n film shown in the schematic, and it appears it was just doing its job. Oh well, having the front off was no loss in terms of accessibility, and taking it all apart meant I could remove a bunch of old dust (of which there has been no shortage in this unit). And yes, the big ribbon cable connector is a friction fit affair. Was a bit crunchy at first.
Hiro-kun (the legend) put me onto the right track. C140, eh? Sure enough, the ESR 1 attested 10.3 ohms.
Going through the power supply section yielded these measurements:
Code:
C137 330µ/25 0,20
C139 330µ/50 0,11
C140 4,7µ/50 10,30 X
C141 33µ/50 1,0 ?
C142 47µ/50 0,53 ?
C145 10µ/35 2,38 X
C146 10µ/35 0,68
C147 2200µ/25 0,04 ?
C148 47µ/10 0,73 ?
C149 1µ/50 4,12
C150 47µ/16 0,23I had to remove the main PCB to access the bottom side, but overall it wasn't a terrible job, 4 screws at the back side and half a dozen on the board. I left the power cable connected, it wasn't too much of an issue.
Ultimately I only replaced the two obviously bad caps, using CapXon GL 10µ/63 for both that were briefly formed at 60 V just in case. That did the trick. (The antenna connectors also seemed a bit wobbly, so I gave some attention to their solder joints as well.)
Old C140 (4.7µ/50V, power-off detection) still measures 1.66 µF, that's definitely toast.
Old C145 (10µ/35V, +5.6V reg output) still measures 9.66 µF, but the comparison with C146's ESR shows that it is quite tired. That wouldn't have been helping matters here.
Wide separation and distortion adjusted fine and weren't too far off to begin with, narrow distortion is a bit "eh" not the least due to a ca. -15 to 20 kHz passband offset that you can't correct (there are zero distortion adjustments for narrow, period). When you've got 110 kHz filters going, that sort of stuff obviously matters. I don't think the LM7001 PLL has any facilities for IF fine-tuning, it seems to be running in basic "f_ref = 50 kHz for FM, 9 kHz in AM" mode here. They didn't even bother to implement 1 kHz for longwave even though the chip could do it and the band was notoriously plagued by off-grid stations, shows you how much of a priority it was.
I had one hell of a time trying to get the FM frontend to track properly, if tuned as described the high end would always be worse. Ultimately I had to deviate from the specified tuning voltage range (2.0 to 11.5 V). I lined up the tuned circuits for good sensitivity on the high end at 11.5 V and then retuned the LO inductor until things were tracking on the low end, went back to the high end to retune the LO trimmer (a bit scratchy but not unworkable, btw), rinse and repeat. Final result was about 2.37 V to 11.435 V. So in a way this unit is the opposite to the Grundig FineArts T-904, which picked up sensitivity just from readjusting the tuning voltage range to spec.
It's a nice-sounding tuner in Wide with sensitive and quick RDS decoding, and the "Active Reception" logic seems to be doing a good job picking the right bandwidth. Narrow yields somewhat quieter output due to auto-adjusted separation. Looong muting interval. I'll have to check memory retention times now that the "hiccups" are gone. Power consumption is about 3.7 W in standby, 7.9 W on FM and 7.1 W on AM.
These don't seem to be selling at all right now. I mean, it's vacation time and the tuner market is in the toilet in general, but still. I paid more for this grubby spares unit than people are willing to pay for a working one in good condition.
Now that this one is also working, I guess I'll have to find another parts donor for my RF coil. And buy another switched power cord.
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Sokel
Grand Contributor
- Joined
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Did a silly unbalanced to balanced converter with stuff from the drawer (OPA2134, etc)
Schematic taken from here:
www.radioworld.com
And despite the mess and the almost point to point soldering at the wrong side of the PCB (I wanted to check first without wasting it) :
That's up to E-MU's loopback with couple of dB penalty despite the +6dB gain.
It also can do some 30dB gain relatively clean.
Any suggestions about improvement to the schematic are welcome!
(decoupling caps didn't make much difference, I added them later)
Schematic taken from here:
Build an Unbalanced-to-Balanced Adaptor - Radio World
Buc Fitch provides an easy solution to an everyday engineering problem.
www.radioworld.com
And despite the mess and the almost point to point soldering at the wrong side of the PCB (I wanted to check first without wasting it) :
That's up to E-MU's loopback with couple of dB penalty despite the +6dB gain.
It also can do some 30dB gain relatively clean.
Any suggestions about improvement to the schematic are welcome!
(decoupling caps didn't make much difference, I added them later)
Sokel
Grand Contributor
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Stereo version is just as nice even with different opamps for comparison (OPA2134 vs LM49720) :
This time I changed 3.3kOhm with 4.7kOhm so I can have negative gain too if I like.
Pots are life saviors, can adjust gain to the last mV.
(ignore the thingy with the resistors on top, it's a salvaged Sony TRS connector with the resistors bypassed)
This time I changed 3.3kOhm with 4.7kOhm so I can have negative gain too if I like.
Pots are life saviors, can adjust gain to the last mV.
(ignore the thingy with the resistors on top, it's a salvaged Sony TRS connector with the resistors bypassed)
- Thread Starter
- #720
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