Technics Linear Phase SB-R3 (1979) rebuild

[yewneek]

Refurbishment over the last 5 years.
Woofers refoamed. Midrange drivers refoamed. Ferrofluid replaced in HF tweeters.
File Image

Crossover capacitors replaced like for like, bipolar electrolytics from HifiCollective, back in 2018. Mundorfs and some others which are a nice green colour. Speaker terminals replaced with gold plated terminals.

Measurements but sadly photos seem to have disappered. Bit peaky, maybe the MF needs to be replaced, but I'm damned if I can find anything that fits! Very odd size, oh well.
Measured using calibrated Sonarworks measurement microphone at 1m distance with Focusrite Scarlett Solo Gen.3

The MF and HF adjustment potentiometers don't make much more than a 1dB change, so maybe I need to look at the crossover, seeing as the parts are 45 years old and non-standard sizes.

C&C welcome.

GB

 

[thewas]

The MF and HF adjustment potentiometers don't make much more than a 1dB change, so maybe I need to look at the crossover, seeing as the parts are 45 years old and non-standard sizes.

Usually the adjustment range is quite larger on such designs, so it seems there really is something wrong with them.

 

[yewneek]

While I think that could be true for one, there are four total, all performing the same, without having changed properties audibly since the refurb, Sttistically, I'm sure you'd agree it's unlikely for four wirewound resistors to 'fail' the same way, linearly, simultaneously.
Empirically, I will check anyway, by taking the speaker terminal plate off and checking them with two multimeters.

Cheers!


GB

 

[Penelinfi]

While I think that could be true for one, there are four total, all performing the same, without having changed properties audibly since the refurb, Sttistically, I'm sure you'd agree it's unlikely for four wirewound resistors to 'fail' the same way, linearly, simultaneously.
Empirically, I will check anyway, by taking the speaker terminal plate off and checking them with two multimeters.

Cheers!


GB

Maybe they've all got corrosion the same way after years?
But yes usually these things at least had a drastic effect right at the end of the range , usually at the negative value. At least on speakers I've had with such controls .

 

[tomtoo]

That tweeter realy enjoys his job.

 

[yewneek]

I cleaned them but I'll clean and check again. That will be pretty straightforward to check/rectify with a DMM and contact cleaner.

The ferrofluid may be responsible for the HF uplift, as it increases the efficiency of the driver I believe. However, I don't see that as a detraction as it can be dialled back and isn't really perceptible.

I'm going to try and reduce the bumpy LF resonances with some extra wadding, as there was only one sheet in each cabinet.

I found a schematic of the SB-R2 crossover network, so I'd imagine it's very similar with one more ponentiometer added. When I've got the backplate off to check the pots, I'll note down the values of the components too.

 

[AnalogSteph]

The pot may have lost its ground connection or something, that would certainly do it. Or the wiper is badly oxidized. Should be easy enough to check. It's an L-pad circuit.

 

[yewneek]

That's relative. Easy if you know how. I don't. It's really annoying when you get someone saying 'it's easy' It's not.

I've been twatting around for the last two hours trying to work out why the MF driver doesn't want to work. I bypassed the MF pot, connecting the centre tap to the + rail of the pot and it still doesn't work. I swapped over the MF drivers and it's the same issue. It's not the MF driver. There's some voltage getting to the driver terminals, bypassed or not, and it's not giving any output. Thr driver shows impedance and resistance. The connections on the drivers are all good as demonstrated.

I'm sweating more than Rodney King walking past a LAPD patrol car and I'm annoyed and hungry.

The board inside the cabinet with the fuses on is impossible to unmount because the pots have nuts on them, recessed and very tight, impossible to undo and dismount. It's clearly going to be a fix which is glaring me in the face, but I cannot see,nor test for it nor can I access where I think it is.

 

[yewneek]

OK it's unmounted.

Trip PCB, and pots. One with longer shaft is OK, shorter shaft is not adjusting. Compared like for like on the other speaker the short-shaft pot is OK on working speaker, NOT ok on this speaker.

Going to test it now with VTVM

 

[yewneek]

Pots tested on VTVM: Both show identical properties and attenuation around the wiper: from 8ohms to 26 ohms at 95% or so, then infinity at full deflection.

The pots are NOT misaligned or failing.
So, it's the crossover network or the fuse board.

Goddamn this is a ballache!

 

[yewneek]

I'm going to bridge out the protection circuitry. It's pointless. Now to work out how and where to jump or bridge....

 

[yewneek]

I'm going to bridge the breakers, lift/desolder one end of all of the diodes. This should leave in place the required connections but remove the reisistive divider and permit the MF to work again. If not, then it's one of the components in the crossover that acts for the MF driver.

 

[yewneek]

Breakers have been bridged, diodes lifted, driver protection circuits now inoperative.
Speakers reassembled, same problem. Protection bypass shown to be good, however same problems, MF/attenuator pot not working. MF drivers swapped, drivers working on other speaker.

Crossover now tested, components not showing any signs of failure. All joints on crossover PCB reflowed with PB/Sn solder.

What am I missing? Every single component, joint, has been tested and shown good where present in circuit.

Fresh set of eyes in the morning, I give up, for now.

 

[AnalogSteph]

So to summarize - driver OK, pot measures OK, no obvious bad connection. I would give everything in series with the midrange a good hard look and ohm out anything from terminals to series capacitors on both sides (you should have continuity from the driver negative terminal all the way back, as well as from driver positive terminal to one capacitor leg). Unless it's a very sneaky bad joint (or crimp) or cracked PCB, my bets would be on either a series resistor that's gone high or a dried-out series electrolytic with sky-high ESR.

 

[yewneek]

Just done:
driver impedance 6.1ohms, driver +to pot: good,
driver- to middle pot: good,
pot testing identical impedance to other pot both wiper sides,
pot to crossover interconnect: good,
crossover interconnect/jumper: resoldered good,
board continuity: good, no solder bridge mistakes
crossover capacitor 4.7uF tested to just ovr 5uF, low ESR: good, checked with multimeter: good,
capacitor resoldered/checked: good
board to - terminal via interconnect: good

To be checked like-for-like simultaneously with working speaker.

Will update

 

[yewneek]

Nothing to see or wrong after two complete rebuilds!

I'm going to recap both crossovers again, even if the one cap for the MF isn't bad, I just can't be arsed any more.

I removed the safety trips for the second speaker, and re-tested, and it works just fine.

Reassembled both speakers, now going back in the attic! I'm done for now. Three days to no benefit.