Yamaha M-40

MiamiBoy

Active Member
Greetings fellow AKer's. This is my first post in the Yamaha forum.

Yesterday I purchased an M-40 that was on my local CL. I am thrilled and amazed at the quality of sound produced by this amp. I pulled out my old Infinity RS6b speakers that were stored in a closet, the smallest Infinities I own, and the M-40 made them sound like nothing I have heard before.

The amp has a QC sticker on the back with a date of 9/1984. Everything inside the amp appears to be original so the parts are over 30 years old.

My question is if there are any internal parts or components that are known to fail or tend to degrade with this or similar period Yamaha amps that I might want to replace or upgrade with newer components at equal to or better specs? Or is there something that can be replace that is known to improve the performance because technology has evolved in 30 years?
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The first thing you MUST do is check for and fix DSBG. There are several threads on it here on AK.

In a nutshell, it is corrosion caused by the glue used to support some of the parts on the circuit board. Look around the area of the four medium/small caps near the front, center of the unit.
 
Thank you very much for your response. Even though I now see what a well known problem this is, I would not have noticed unless someone pointed it out.

Good news is that it looks like the glue guy was very sparing with the glue on my M-40. Only two caps of the four at the front with the glue underneath and very little overspill. The glue is the color of peanut butter. I will remove the two caps with the old glue underneath to inspect their connections and replace with new ones if connections are corroded.

Is it recommended to go ahead and replace the old caps with new ones since they are 30 years old or to not replace unless there is a problem?
 
Very nice amplifier :)
I would go ahead and replace the caps while I was in there. From reading through various rebuild threads on this vintage of Yamaha you will probably have a good 25% fall under value.
I've got my A-1020 and M-60 in the queue to refresh and plan on replacing the 'lytics while I'm in there. For signal path I use Nichicon Muse, KZ, Fine Gold or Elna Silmics. For the power supply Nichicon PM or PW.
You also might want to look into replacing the bias trimmers while you're in there. If they go open bad things happen. There are a few different Bournes that can be used. They are sealed and much more reliable.
 
I purchased my M40 new back in the 80's, love it, will never get rid of it.
Do yourself a favor - replace all the caps - the 2 main caps - Michael Percy has 22000uf \ 80V Nichicon Gold Tune that you can use - not a straight drop in fit, but it is just a little extra brain\puzzle work - for the 2d largest - near the front I went all out and got 680uf \ 65V Rubycon Blackgate which Yamaha used but in the 63V flavor, I tried the Elna Duorex 680uf \ 63V, its ok but the BG's are so much more musical.. For the rest of the board I just used what was available for audio purposes - Elna & Nichicon. And yes, for sure, replace the voltage trimmers. You will be better able to set the idle trim. Grab some CRC electronic cleaner and spray the 2 volume pots.
For the future: you may consider upgrading the RCA inputs and the speaker connectors, they are the weak link on this amp and prone to break. In the meantime just be aware that the plastic is brittle after 30 years on these, be gentle.
Also, remove the bottom metal cover, and check the solder side with a magnifying glass \ light, this amp tends to run hot in class and some of the solders develop ring cracks. left uncorrected, you may experience audio fade outs.
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timblade68: Be careful using a fan on top. The bias current on the M80 model (and perhaps on all M series of that year) is very sensitive to external air flow. It drops close to 0 only blowing on the pcb
 
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Ah, ok. To give you an idea how sensitive it is, the current is different with cover than without it...
 
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Again, thanks for all your input as it is a huge help since this is the first time I have even opened an amp much less swapped out parts.

I did open up the M-40 case and inventoried all the lytic caps. I have found sources online for all of the caps. 21 caps to be replaced by my count.

I de-soldered and removed the two medium 680uf caps that had the glue mess on the bottoms. I applied Goo Gone with a Q-Tip and let it soak for an hour on the glue and the old glue lifted right off.

The one thing I do need help with as I have no clue how to spec it in order to find a replacement is the Bias Trimmers. The schematic says "A20k ohms variable resistors". What specific value would I look for as a replacement?
 
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Should I be ordering all "leaded" caps as replacements. None are "snap in" caps, correct? I am assuming I should buy caps with leads then just trim them before installing.

I am used to spending considerably more $ on capacitors when I buy them for restoring speaker crossovers. I am surprised at how inexpensive the M-40 replacement caps are like the Nichicon FG's that are audio quality. Should I not be concerned by the low cost as it does not reflect a low quality?

Black Gate is no longer making caps. Is Nichicon and the other brands mentioned in earlier posts the best caps I can install for improved sound or are there others I should consider that are better?
 
Leaded or snap-in should work. Far as price, the replacement caps are way more affordable then some speaker caps like the poly, etc kind.
As for the BG's, yes, out of production, some are available as 'new old stock', which I purchased, and yes I am very pleased with them. It is a gamble, others here will cringe but the seller below has excellent feedback, and the caps are not cheap! (but worth it)

http://www.ebay.com/itm/BlackGate-Rubycon-Black-Gate-680uF-63v-/261980689765?hash=item3cff441565
 
IMHO it's hard to beat a Panasonic FM in power supplies. In coupling applications, Elna or Muse for example.
 
I'd definitely be careful about ordering NOS stuff off of eBay unless the seller has been vetted here (such as the one noted above). Too easy to wrap any old cap in Blackgate shrink wrap.

The Blackgates in my A-1020 are low but in spec so I'm going to compare them against the Silmic II and see what I think. Good thing about our amps is there is plenty of space to install audio caps, which in the case of the BGs and Silmics are much larger than others of the same uF, since they came standard.
 
Should I be ordering all "leaded" caps as replacements. None are "snap in" caps, correct? I am assuming I should buy caps with leads then just trim them before installing.

I am used to spending considerably more $ on capacitors when I buy them for restoring speaker crossovers. I am surprised at how inexpensive the M-40 replacement caps are like the Nichicon FG's that are audio quality. Should I not be concerned by the low cost as it does not reflect a low quality?

Black Gate is no longer making caps. Is Nichicon and the other brands mentioned in earlier posts the best caps I can install for improved sound or are there others I should consider that are better?

I recapped a M-80 using Elna Silmic II and couple Nichicon "audio grade" bi-polars.

The amp sounds great, but I'm not a firm believer in audio memory for that level of detail and how much any perceived difference might be simply of new caps rather than specific brand/style.

If I were to do it again, and I might (I have two M-40s as well), I'd use Silmic II et al where the BGs were, but use standard caps (like the afore mentioned Panny FC) in other locations.
 
I have no clue how to spec it in order to find a replacement is the Bias Trimmers. The schematic says "A20k ohms variable resistors".
NO no no.. The bias trimmers are 1KΩ. The input attenuator potentiometers are
the 20KΩ variety.
 
NO no no.. The bias trimmers are 1KΩ. The input attenuator potentiometers are
the 20KΩ variety.

Thank you for catching that error! Yes they are both 1KΩ. I mistakenly read the value from the line above in the service manual parts table.
 
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?

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