Yamaha M-40

I decided I am going to split this restoration into two phases. First phase will be replacing the two 22,000uf caps, the two 680uf caps and the two bias trimmers. I have already ordered these replacement parts and should be getting them next week. I will put everything back together and listen for a while. Phase II will be all the rest of the caps.

I feel more comfortable doing this in two phases since it is my first amp restore and I will have less to back track diagnose if I did something wrong.

Also, on top of everything else, yesterday I purchased an M-85. But that will be another thread :yes: and having learned so much from the M-40 I feel more confident now to do the M-85. I also have a pair of Infinity Kappa 9's that I am in the process of doing a crossover restore/upgrade on. I have gotten this crazy idea to try to reconfigure the Kappa 9's crossover network so that I can tri-amp them using the M-40 on the highs, M-85 on the mids and another amp I have for the base. But first I will try just bi-amping with the M-85 on top and if that works than that's how I will leave it.

BTW, when I measured the Nichicon 22,000uf caps they both measured right at 20% variance from spec. Both of the BG 680uf caps were within 5%. Does that mean I should put the BG caps back in? Their leads were clean and not corroded.

Thank you all for your help and input.
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It's really up to you with the BG caps. They are old and can crap out whenever they want but they won't harm anything so you could always go back and change them later if you feel the sound is deteriorating. The biggest thing is removing the glue and getting reliable trimmers in there. After that you're safe and can go at it any way you like.

I've been on the hunt for an M-80/85 for quite a while now and none ever seem to pop up locally. I've got the T-80 and the C-80 I just need the amp to complete the set.
 
If those are the OEM 680´s then I was wrong about the Nichicons. Those sure look like BG´s. I´d still replace them with the Nichicon KG´s if I was doing the refurb. The M45,60,65,80 and 85 used the KS´s in those two positions.
 
but they won't harm anything
If one shorted it would not be a very good thing. Burn some fusible resistors for sure.And slam a rail.
 
Do these brown resin deposits on the larger solder connections indicate a possible contact / corrosion problem that should be resoldered? Or are they just old resin residue and can be ignored as long as there are no cracks in the solder and I am getting good continuity?

View attachment 617546

Should clean up with IPA and a old tooth brush or acid brush. Its just solder resin as stated above.
 
if those are the oem 680´s then i was wrong about the nichicons. Those sure look like bg´s. I´d still replace them with the nichicon kg´s if i was doing the refurb. The m45,60,65,80 and 85 used the ks´s in those two positions.

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Question about bias trimmer values in Class A.

I replaced all the capacitors and the two bias trimmers. Amp sounds great and all seems to be performing well.

I do have a question/concern about the bias trimmers because the Service Manual says that when in Class A mode they both should read "70mV +-8mV (Idling Current 0.35A)". After I installed the new trimmers and adjusted them to their target spec of 10mV and then switched over to Class A, the highest reading I could get was 19mV on RCH and 23mV on the LCH. These tests were performed with the amp not under load and having warmed up for a few minutes. Am I doing something wrong or is there another problem somewhere with Class A?

I measured the old trimmers after I put the new caps in and the old trimmers measured very close to the new trimmers' mV readings in Class A.
 
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I replaced all the capacitors and the two bias trimmers. Amp sounds great and all seems to be performing well.

I do have a question/concern about the bias trimmers because the Service Manual says that when in Class A mode they both should read "70mV +-8mV (Idling Current 0.35A)". After I installed the new trimmers and adjusted them to their target spec of 10mV and then switched over to Class A, the highest reading I could get was 19mV on RCH and 23mV on the LCH. These tests were performed with the amp not under load and having warmed up for a few minutes. Am I doing something wrong or is there another problem somewhere with Class A?

I measured the old trimmers before I removed them and they were also at the same mV reading in Class A.

Thats probably as high as it going to get. Let is set "idle" for 1/2 hour and see if the heatsinks get noticeably warmer.
 
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It definitely runs hotter in class A than in class A/B. Class A/B no load feels like room temp. Class A runs about 15F degrees hotter no load than A/B (class A off). The left side is always a little warmer but that is where the power transformer is. These temps are based on my digital laser thermometer. (No cook should be without one.)

The only DSBG was on two capacitors and nothing else. No corrosion anywhere. See prior pics.

I am happy with the sound. My question was not about temp but why the specifications were so far off from the readings I was getting?
 
I've never seen it go much higher than 20mv. Funny thing is that the M-45 basically the same amp design with slightly increased rail voltage . Same reading says 60mv +/- 30mv...:scratch2: I've gone as far as replacing all the class A circuitry with new components. Nada damn difference.
The M-65 circuit - the same design higher rail voltage is just looking for 20mv or greater.. in Class A... And the M-60 says 110mv +/- 10mv. I'm thinking the early models M-40/60/80 have issues with that class A measurement which were corrected in the M-45/65/85 manuals to more realistic values. I have no problems with the 45/65/85 but the 40/60/80 never come close to printed values.
 
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I've never seen it go much higher than 20mv. ..... the 40/60/80 never come close to printed values.

Avionic, that's good enough for me. Case closed.

I am happy with the way the amp turned out and I learned a lot on this restore. I have started digging into my M-85 and have a few questions on that so I will be starting a new M-85 thread shortly.

Thanks everyone for your help on this project.

Here is one final picture of what I did to install the new 22,000uf caps. I installed Mundorf MLytic AG caps (22,000uf 63V) but they were so much smaller than the BG's I had to hard wire them to the pcb, soldering both ends of the connections. Mundorf also sells clamps for these caps that I used.

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I find the left side of the M40 warmer also, and ditto for the class A reading.....no biggie, it sounds super.
 
I started this thread a number of months ago when I purchased an M-40 and ended up refurbishing it. Amongst other things I replaced all the lytic caps including the two largest caps. I replaced them with Mundorf MLytic AG caps 22,000uf 63v. I have sinced purchased two additional Yamaha amps, an M-85 and an MX-1000U. The M-85 I also cleaned and refurbished but did not replace the large four caps in the M-85. I have done nothing so far with the MX-1000U.

Over the past month I have been rotating the M-40, M-85 and MX-1000U doing some A/B listening tests. I have come to the conclusion that the M-40 is the best sounding of the three amps. Yes the other two amps have much higher wattage and can play louder but even at louder listening levels the M-40 sounds better. The only reasonable conclusion to account for this is that the M-40 has the new Mundorf MLytic caps and the other two amps still have their original large caps.

What do you think? Could the Mundorf MLytic caps account for the sound difference or might there be something else I am not considering? I would like to know if I am right because if so I likely will replace the large caps in the other two amps. The Mundorfs are much smaller and they come in values that I could parallel wire them to equal the original values.
 
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