Vintage Yamaha A760 - Restore and upgrade

[BogdanSava]

Hello. This is my first post here and my first amp. I am just starting in this hobby and I need some guidance
I got a Yamaha A760 in pretty good shape, it does smell like tobacco but I hear that is a standard feature of any amp from the 80s.
I do a lot of automotive electronics repair and some gpu/motherboard repair/bga reballing so soldering anything is not a problem.
This Amp has a infamous Rifa 0.47uF 250VAC RFI capacitor that exploded, I ordered a new part Kemet P409 to replace it.
So I was thinking, since I am opening it up to replace the cap and clean everything, I could do some upgrades.
Maybe upgrade the big 6800uF 63V capacitors with 10000uF 63V ones, or more importantly upgrade the opamps.
It has JRC NJM4558D/NJM4559D and I was reading about OPA2134, OPA1642 or OPA1656. I don't mind soldering a socket and using an adapter for smd opamps.
The thing is I have no idea if that would improve or lower the sound quality. I tried reading about opamp upgrades and people say that a certain opamp might sound good on one amp and not as good on another. So if anyone here has some experience with upgrading vintage amps or general knowledge about the subject I would appreciate the help.
I have a pair of Sony SS-CS5 as speakers. I don't mind getting better speakers down the line, but I want to gain a little knowledge before I start buying more expensive stuff.
Thank you

 

[DanielT]

Hello, welcome to ASR!

You said general knowledge. Here are some threads that touch on what you are thinking about:

What is it in old amplifiers, receives that smells?

Have you experienced that otherwise functioning old amplifiers and or receives can emit a special smell? I can't put my finger, or nose, on exactly describe that type of smell. Kind of smelling warm old electronics.It sort of smells of its age and I know vaguely described.It's not a smell that...

audiosciencereview.com

Maybe upgrade the big 6800uF 63V capacitors with 10000uF 63V ones, or more importantly upgrade the opamps.

Considering it's over four decades old:

https://www.hifiengine.com/manual_library/yamaha/a-760.shtml

...and especially if it has been played, used a lot so then it might make sense to change the caps in the power supply, but on the other hand:

Importance of replacing the electrolytic capacitors in any vintage gears

Quote from this part of the article below. In that link you can see how the values of the capacitors were measured and how much they deviate from spec/tolerance. The question is whether it is possible to generalize and say this: As one can see from the test results about 30% of all original...

audiosciencereview.com

Otherwise, what kind of service, cleaning, check DC offset and so on for your Yamaha A760 that can be considered sensible to do, I leave to the more technically amp knowledgeable to answer.

Much of the thread below is about modern stuff plus the classic op amp NE5532 and if replacing it adds anything positive:

Replace OP amps. Completely pointless, or not?

Why do so many OPs change amps in, for example, the amplifier AIYIMA A07 TPA3255? What's the point of that? Does not modern OPs amps amplify the signal completely linear straight(if the construction they sit in is well constructed, and made to fit OP amp X)? Do not modern OP amps have a...

audiosciencereview.com

Edit:
A quick look at Hifishark, Yamaha A760 seems to be sold for around $90, so if I were you, I'd think about how much money I'd put down on a big service, re cap. How much is it worth? If you do it because you think it's fun to do, as a fun hobby project, it's another matter though.

Used Yamaha A-760 Integrated amplifiers for Sale | HifiShark.com

Used Yamaha A-760 Integrated amplifiers for sale on 400+ second hand hifi sites & shops. Use Hifi Shark to monitor pricing and global availability

www.hifishark.com

 

[BogdanSava]

Thank you Daniel, I will give all those links a read, I will start with the opamp discussion.
It is not necessarily about value, I mean I wouldn't like to buy and potentially ruin an iconic vintage amp. I was looking for a Yamaha A1 but quickly reconsidered. I don't need a great amp, just a good one. That is why I got this one, I paid about 120dollars for it here in Romania. I was also looking for a A960 but couldn't find one close by.
It is more about getting used to the technology, I do a lot of bga replacements, read and write egine control units, fix vrm issues on gpus, etc. that are pretty simple if you have the right tools. I have basic to moderate electronics understanding and great soldering skills.
The whole music amplifier world is new to me and I am excited to try stuff out. I love to tinker with things also, I have modded my kids toys even
I think I will get all those opamps and try them out to see if I can hear a difference between them.

 

[DanielT]

I, or rather my friend who is more technically savvy, have replaced capacitors in my vintage amps. I have him help drive trash to the dump with my trailer for those jobs. The capacitors themselves I paid for. Did it need to be done? Perhaps? Maybe not, but thats his hobby, tinkering with old receivers/amplifiers, so he gave it a shot. It wasn't about that much money (he buys a lot of electronics through Mouser at a good price)

Fostex 600 power amp, before and after:


___
Sela power amp before and after, slightly larger (in value, from 2x2 4700uF to new 2x2 5600uF) capacitors in the power supply:


The last pages of the thread are about and if so how much larger capacitors you can pop in during a change:

Why 12 power transistors in a class AB (?)120 W (?) amplifier?

Why 12 power transistors in a class AB (?)120 W (?) amplifier in .. 4-8 Ohm (?) What is the point of 6 power transistors per channel in such an amplifier? Information and pictures below . I would be very grateful for clarification. The stuff works, but I'm just curious. :) Some facts. Sela, a...

audiosciencereview.com

_____
Speaking of fixing vintage amps. I was annoyed by a humming transformer in my Luxor 7082 amp. A mechanical hum due to vibrations so it was heard from the amplifier not in the speakers. I put a cut out piece of rubber between the chassis and the transformer and tightened the screws damn tight. The annoying humming sound disappeared.

 

[Doodski]

I think I will get all those opamps and try them out to see if I can hear a difference between them.

Replacing OP amps with a blind/guess choice of OP amps may cause oscillation of the circuitry and poor sound. Just because you replace them does not mean better. More important things are controls/switches cleaning and lubrication, go over all the PCBs and re-solder cracked and burnt joints. Calibrate the DC offset and idle current/bias to specification. If the amp smells like cigarette smoke you can clean/spray it with isopropyl alcohol but keeping in mind you do not want that in the controls and switches. Otherwise OP amp rolling is a blind guess and you may get bad results even if you can't hear the oscillating.

 

[BogdanSava]

Thank you Doodski for the advice. I will take it apart and clean everything, the smell is in all my house at the moment and I don't like it. I think it is also coupled with the smell from the exploded capacitor oil. I cut off the cap but I didn't disassemble the board to clean the residue. I am waiting for the replacement cap to do that.
I also noticed that after the amp gets cold when I first start to play music at low volumes the right speaker crackles, like a bad connection. If I turn it up loud for a second it goes back to normal. I will see if I can see any problems with the soldering. Thanks for the adjustment guide.
I think I will add a grounded lead like I read somewhere where the chassis had 90v to earth.

"While I do consider the addition of a 3-wire mains cord a good idea in principle, I would not just blindly slap PE onto chassis - this is just inviting ground loops. Instead, connect PE to the midpoint of C504, 505 and 506 on Control Circuit 2. (There is an inviting jumper wire next to C506.)
This will not only eliminate previous mains leakage via C504||505 - C506 but also give transformer leakage currents a path to earth (the xfmr does not appear to feature a dedicated shield winding so would be likely to have several hundred pF worth of pri-sec coupling, hardly ideal when you've also got this triac voltage regulator thingy generating who knows how much RF noise; it also is where the difference in mains plug polarity comes from, as coupling from each end of the primary coil tends to be different)."

I did not measure for mains leakage on my amp but it seems a good idea to add a power cord with grounding.
So apart from basic maintenance there is nothing interesting I can do to it?

 

[Doodski]

Thank you Doodski for the advice. I will take it apart and clean everything, the smell is in all my house at the moment and I don't like it. I think it is also coupled with the smell from the exploded capacitor oil. I cut off the cap but I didn't disassemble the board to clean the residue. I am waiting for the replacement cap to do that.
I also noticed that after the amp gets cold when I first start to play music at low volumes the right speaker crackles, like a bad connection. If I turn it up loud for a second it goes back to normal. I will see if I can see any problems with the soldering. Thanks for the adjustment guide.
I think I will add a grounded lead like I read somewhere where the chassis had 90v to earth.

"While I do consider the addition of a 3-wire mains cord a good idea in principle, I would not just blindly slap PE onto chassis - this is just inviting ground loops. Instead, connect PE to the midpoint of C504, 505 and 506 on Control Circuit 2. (There is an inviting jumper wire next to C506.)
This will not only eliminate previous mains leakage via C504||505 - C506 but also give transformer leakage currents a path to earth (the xfmr does not appear to feature a dedicated shield winding so would be likely to have several hundred pF worth of pri-sec coupling, hardly ideal when you've also got this triac voltage regulator thingy generating who knows how much RF noise; it also is where the difference in mains plug polarity comes from, as coupling from each end of the primary coil tends to be different)."

I did not measure for mains leakage on my amp but it seems a good idea to add a power cord with grounding.
So apart from basic maintenance there is nothing interesting I can do to it?

You can also use ammonia/Windex to melt off the smoking residue. It will melt off that stuff and run off like water. But as I said don't get that stuff into the switches and controls. The right speaker is crackling because of cracked solder or dirty switches and controls. You need to inspect every PCB for bad solder. Most likely the amp PCBs have overheated cracked joints.

 

[BogdanSava]

I will prepare everything I need and do it as a weekend project, if the cap arrives in time. I will take pictures and post here.

 

[DanielT]

Good luck!

Have you seen this page? Apparently more than you have experienced an unpleasant smell. It doesn't have to happen to yours, but still. :

The "burned" part: a Rifa 0.47uF 250VAC RFI capacitor​

Upon opening the cover, the "burned" part was pretty obvious. The Rifa RFI capacitor had overheated, shorted and exploded, spilling oil over the surrounding components and printed circuit board. The smell of burned oil was very strong.

Restoration of a Yamaha A-760 vintage integrated amplifier

A vintage Yamaha A-760 audio amplifier and its matching T-760 tuner

gigapod.free.fr

 

[BogdanSava]

I know that page. At the end it says:

"The A-760 uses 4558/4559 op-amps for the tone controls and the RIAA preamp. The JRC NJM4558D/NJM4559D was a decent part in the early 80's, but modern op-amps perform a lot better. Just compare the datasheets:

• JRC NJM4558D/NJM4559D dual low-noise op-amp. Note the 4559's slightly better guaranteed frequency response characteristics.
• Burr-Brown OPA2604 dual high-performance op-amp.
• NE5532 dual low-noise op-amp.
• MC33078 dual low-noise op-amp.
• Burr-Brown OPA2134 dual high-performance op-amp."

That is where I got the idea to experiment with op amps.

 

[Doodski]

I know that page. At the end it says:

"The A-760 uses 4558/4559 op-amps for the tone controls and the RIAA preamp. The JRC NJM4558D/NJM4559D was a decent part in the early 80's, but modern op-amps perform a lot better. Just compare the datasheets:

• JRC NJM4558D/NJM4559D dual low-noise op-amp. Note the 4559's slightly better guaranteed frequency response characteristics.
• Burr-Brown OPA2604 dual high-performance op-amp.
• NE5532 dual low-noise op-amp.
• MC33078 dual low-noise op-amp.
• Burr-Brown OPA2134 dual high-performance op-amp."

That is where I got the idea to experiment with op amps.

Even a dated OP amp like those you have is sufficient for audio use. Rolling OP amps is a guessing game.

 

[AnalogSteph]

Speaking of basic maintenance, don't forget the speaker protection relay contacts. It probably wouldn't hurt to disassemble and clean every switch in the signal path. (Just contact cleaner and paper, NO SANDING. Relays may need to be replaced if contacts are severed pitted / charred.)

NJM4558/9 are relatively slow, low current parts. Replacing them with much faster ones can really backfire if the layout isn't up to par, and on devices of this vintage it generally isn't... not to mention adding bypass caps may introduce noise as these are single layer boards with shared signal and power ground returns, so there's a real risk of ground pollution. You may need to reinforce the existing ground traces or even introduce dedicated power ground returns travelling back to the power supply alongside the supplies.

The phonopre uses a discrete FET input amp and supplies approaching +/-18 V - proceed with utmost caution here. (Anyone got a clue what the purpose of zeners D105/106 in the FET input tail is? I do know what the pompously named "Pure-current servo amplifier" is, it's a parallel voltage regulator - being fed by a constant current source, it should offer excellent power supply rejection, not so sure about noise levels though.)

An area that may be interesting is the tone control amplifier. It already features local rail bypassing (note the use of RC filtering to minimize ground pollution) and supply rails of little more than +/-15 V. An OPA1642 may well be worth a shot. OPA1656 may be a bit too hot-headed, I would consider the more conservatively compensated headphone driver variant OPA1688 or the lower-cost OPA1678 instead.

Having some means to check for signs of oscillation is very much advised when performing such mods.

 

[BogdanSava]

Finally found some spare time to play with the Yamaha. I removed the control board 1 and 2 pretty easy, just one screw.
Once it was off I was amazed at the mess the blown capacitor left behind. I also ordered some 6800pf capacitors from KEMET - PME271MA4680MR30. That is the only X2 paper capacitor I could find. I think the caps on the board are ok but are kind of falling apart. I also ordered a Peak ESR70 Gold, I've been wanting one for a while and this was a good excuse

 

[BogdanSava]

I cleaned everything, changed the paste on the NEC AC16D triac and also soldered a lead for ground on the midpoint of C504 C505 and C506. I soldered it on the jumper lead suggested in the post I mentioned before. I had an extension PC power cord and used the female end of it. I can unplug the power cord now and that is nice. I will however change the cord because the leads were kind of thin.
After doing this I turned on the amp and to my surprise the humm that I had even with volume to a minimum is gone. I can turn the volume to the max with no input and I have to put my ear near the speaker to hear a small hiss. Before it made quite a bit of noise. Also I want to mention, RCA cables matter. I had a pair of OFC RCA cables laying around, and when I plug them into the in the amp with the other end unconnected the amp humms loud. I bought a pair of Roland Black Series Shielded RCA cables. They were about 15 dollars. Absolutely no hum with these. I don't know if the capacitor or the grounding made the difference. I will test it without the grounding when I change the cable.
After this I made all the adjustments. I had pretty bad values, 70mv instead of 10mv, 30mv instead of 0 and 1v instead of 1.2v. I got them as close as I could and closed the amp. I didn't notice any change in the noise or sound BUT the amp is cold now. Before this, after 15 20 minutes of playing the amp would get hot in the region of the power button. Also the smell is gone. That muck smelled horribly, and when it got hot the whole room smelled. Now it is cleaned and cold so yey The rest of the amp is pretty clean, just dusty.
I will get an osciloscope, I keep postponing getting one because of limited space on my desk. After I do that I will try and play around with OPAMPS.
I will update here after I do the rest of the maintenance. Relays, pots, etc.

 

[BogdanSava]

Also I forgot to mention. I would like to upgrade the spring clip terminals, and make a cover for the little board near the power button.
It has big connections from the mains right on top, you could easily touch them with your hand when doing the adjustments. I used a plastic cover to be sure I don't die )
And something else I did not understand. I saw that C506 is floating. It goes to the chassis with a screw, the trace is a dead end. Why is that?

 

[Doodski]

Also I want to mention, RCA cables matter. I had a pair of OFC RCA cables laying around, and when I plug them into the in the amp with the other end unconnected the amp humms loud.

When the end is not connected the amp is supposed to hum. It is picking up 60 Hz from the air. This is normal.

 

[BogdanSava]

When the end is not connected the amp is supposed to hum. It is picking up 60 Hz from the air. This is normal.

Well it doesn't humm with the shielded cable. I don't think the other cable i had was shielded. I will test again with both cables connected on both ends and the Dac turned off, out of curriosity.

 

[AnalogSteph]

I also ordered some 6800pf capacitors from KEMET - PME271MA4680MR30. That is the only X2 paper capacitor I could find.

It doesn't have to be a MP cap. These days you can easily get metallized films as Y2.

After doing this I turned on the amp and to my surprise the humm that I had even with volume to a minimum is gone.

It shouldn't be that surprising. This model has a bunch of mains leakage when stock and will float at close to half the mains voltage, so any open input tends to hum like nobody's business just from capacitive coupling to the outside - unless the unit is grounded via a signal source. Back in the day that role would have fallen to the tuner with its external antenna, today it could be a PC. Still annoying though. Hence the mod to divert the mains leakage.

Also I forgot to mention. I would like to upgrade the spring clip terminals, and make a cover for the little board near the power button.
It has big connections from the mains right on top, you could easily touch them with your hand when doing the adjustments. I used a plastic cover to be sure I don't die )

Yeah, Japanese gear from the time tends to be a bit... rustic in that regard. Designers these days are generally taking the double insulation bit rather more seriously.

And something else I did not understand. I saw that C506 is floating. It goes to the chassis with a screw, the trace is a dead end. Why is that?

Didn't you write earlier that you soldered your PE wire to the midpoint of C504-505-506? And the other end of C506 goes to chassis. So how exactly is it floating?

 

[SuperDan2u]

Hello. This is my first post here and my first amp. I am just starting in this hobby and I need some guidance
I got a Yamaha A760 in pretty good shape, it does smell like tobacco but I hear that is a standard feature of any amp from the 80s.
I do a lot of automotive electronics repair and some gpu/motherboard repair/bga reballing so soldering anything is not a problem.
This Amp has a infamous Rifa 0.47uF 250VAC RFI capacitor that exploded, I ordered a new part Kemet P409 to replace it.
So I was thinking, since I am opening it up to replace the cap and clean everything, I could do some upgrades.
Maybe upgrade the big 6800uF 63V capacitors with 10000uF 63V ones, or more importantly upgrade the opamps.
It has JRC NJM4558D/NJM4559D and I was reading about OPA2134, OPA1642 or OPA1656. I don't mind soldering a socket and using an adapter for smd opamps.
The thing is I have no idea if that would improve or lower the sound quality. I tried reading about opamp upgrades and people say that a certain opamp might sound good on one amp and not as good on another. So if anyone here has some experience with upgrading vintage amps or general knowledge about the subject I would appreciate the help.
I have a pair of Sony SS-CS5 as speakers. I don't mind getting better speakers down the line, but I want to gain a little knowledge before I start buying more expensive stuff.
Thank you

Hello, I can't think if a better deal for efficient, very affordable, 2 way, bookshelf speakers than the NHT brand. Used prices are cheap enough to try and if not your preference put in the garage. The Zero is what I bought for $50. I was very impressed and when I inherited a Polk 10" sub I was blown away. The configuration I set with crossovers made it sound as good as the Klipsch critical listening in the living room.

Like you, I have a collection of vintage audio receivers. Harmon Kardon twin power 730, 330. They are in need of restoration. I had to leave them in storage too long and temperature(-15F to 110F) have savaged them. Most things work but intermittent issues abound. Are you or anyone you know doing this work? TIA