Yamaha M45 Help Needed

Hi folks, I am usually ok with repairing stuff but just don't have enough understanding of circuits. I hope one of you guys can help me by pointing me in the right direction?

Bought a Yamaha M45, the right channel was gone. All bar one of the outputs were fried, plus a load of other stuff.

These included
Q114, Q116, Q117, Q120, R150, R146 (driver board?).
Q132, Q134, R190 (auto class A circuit?)

I have completely recapped the amp with Panasonic FM/FC series, therefore that side of things is now ok (there were some bulgers, especially on the right channel). Didn't change the main filter caps.

I have also changed the bias pots for decent 20 turn ones.

It still ain't right, I have measured through every transitor on those god damn pcb's and they all read good (with diode test on multimeter). left channel is sweet but i measure over 60V (the supply voltage?) all over the place on the right channel, where the left channel reads mV.

This includes the emittor resistors against ground.

Are there any of you bright guys out there who can take a look at the service manual and point me in the right direction, please?
Service manual free to download at HiFi Engine (thanks HFE!).

Any help would be much appreciated.


Paid Member
2004-06-06 8:31 pm
Georgetown, On
Hi mercedesjenz,
Well, your first mistake was changing a bunch of stuff before you had it working. My strong advise to you would be to put the same bias controls back into the amplifier. Don't add anything else except for replacements for the dead parts. Have a good close look at your solder connections after you have cleaned the flux off the PCB.

You will need to replace all the outputs with new, and the driver transistors. I normally replace everything back to the earliest blown part, then one stage before that to get into healthy "flesh". From there you can start to rebuild it. Check all resistors because they can go open and look perfectly fine. Do this after you pull all the blown transistors. Makes checking easier.

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Thanks for replying, Chris. I am a bit in awe as I have been following your posts on here and elsewhere over the years and you are clearly one of the most knowledgeable guys out there. Especially your explanations around how Carver amps work helped me understand my PM1.5a's when I did some work on them.

So I don't take your advice lightly. I have replaced all outputs, all drivers and everything which was blown (that I could find). This includes all the components around the blown bits, ie the transistors, diodes, resistors. This is how I found all the other broken bits.
Yes, you are absolutely right in saying that I should have done the recap and the bias pot replacements after fixing it. I kind of stand by my work, and the fact that the left channel is absolutely fine gives me confidence that my work on the right channel is also good. Besides, I would have had to replace some of those caps anyway.

I will definitely go through my solder joints with a fine tooth comb, so thanks for that advice, too.

Any chance you could have a look at that schematic and give me an idea where the "healthy flesh" starts, judging from the list of blown components in my first post?

I would even more apprecaite if you could hazard a guess as to what may actually be at fault here. It is the circuit theory that has me stumped. I just don't know enough to look at the schematic and say "this circuit does that so if this happens then the fault must lie there".

I guess i still have a lot to learn. So far I have always come up trumps. Eventually.


Not necessarily directly related to your problem, but two things that you should look out for when servicing mid-'80s Yamahas:
1. Bad solder joints
2. The "fixing glue of death" around big electrolytics and stuff, which tends to absorb moisture and become acidic, corroding components it happens to cover

Also note that multiturn pots aren't necessarily always a good idea; look up the datasheet specs for wiper current and power dissipation if in doubt and compare to circuit requirements. A competent amp designer would have chosen series resistor values such that a good-quality single turn pot gets the job done just fine.

In case of such a catastrophic fault like the damage seems to indicate (BTW, some of these parts are on the left channel?!), it generally is a good idea to check every single component. Look for resistors that have gone high, diodes that have shorted out (both generally doable in-circuit), smoked traces, stuff like that. Sometimes you might even find a dead film cap. Thought right now my bets would be on a duff Q104. BTW, if there's +/-60V on the output, C108 is most likely toast.
Thanks guys, all help appreciated.
The "fixing glue of death" was removed during recap, shouldn't therefore be a problem now. C108 was replaced with brand new Panasonic FM from Farnell (they're only down the road, wahey!), just like all the others. I will unsolder, check and replace...
Q104 reads fine with diode test, but will investigate further.

Re some of the components being on the left channel: Yamaha seems to have been quite leisurely in their labeling. I have come across a number of components which were marked incorrectly in their diagrams. In fact, the quality of the whole things seems a bit flimsy. The pcb isn't great quality, with the tracks and solder pads easily lifting off and the outputs just being soldered onto their pads under tension, it all seems a bit half-arsed.

I'll have a look at the p/s, although it appears to be in order. I will also look at those other components mentioned and report back. Btw, I did measure just about every single component on those pcb's but will endeavour to persevere...any more suggestions welcome.
Any explanations of circuits even more welcome>

New developments. I have carried on measuring and it appears that Q144, Q145 and Q146 have also had it. Everything else seems to be reading alright in circuit.

Could the failure of these 3 transistors cause the symptoms described (ie 61V just about everywhere (bias pot, emitter resistors, drivers) in the right channel?

Also, does anybody know drop in replacements for 2sa970 and 2sc2240? My stocks are exhausted...



Paid Member
2004-06-06 8:31 pm
Georgetown, On
Hi mercedesjenz,
Look for KSA970 and KSC2240. Both On Semi and Fairchild have licenses to produce some of the Japanese parts. As for other replacements, it depends on the circuit they are in. Normally there are a few other numbers that are suitable. I'm actually extremely busy at the moment. Try to deal with what you have on your plate and come up with the current status of your repair. The faults must certainly have changed with all the part replacements you have before you.

Do you have a "variac", a variable AC transformer? If not, since you are doing other things it would greatly benefit your efforts to have one. You can find them surplus. Try to get voltage and current meters. I got a B&K 1655 "AC Power Supply" from Ebay that is just great. Well worth getting.

Keep on repairing, Chris