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Next restoration: Yamaha PC2602M

Thanks for the reply.
Indeed your remark is what you would expect. But the schematic shows that both the base and emitter risistor is 30 ohm of the pnp. This is weird.
The npn has 330 ohm base and 30 ohm emitter resistor which is normal......


The NPN an PNP are complimantary according datasheet. They have exactly the same specs like hfe, etc. I not convinced yet but I think its an error....
When amplifier is working again going to make them equal and measure what the effect is.
In my amp the pnp was broken.

I would like that others check whether these are different in their amps. Maybe it is changend in later productions. I suspect that it has effect on an imbalance in bias current in upper and lowere part of one channel.
What I also see is that several people have to replace transistors in this amp. And it is so powerfull, shorts should be no problem for this tank....the fuse should blow, not a transistor.


Cobra2 said:
Without any expertise, I guess they are current-limiting resistors, and make up for the difference in NPN/PNP ... ?
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bias pc2602.PNG

hello everyone I also take up this old post, for those who are still interested in this discussion regarding the Bias current of the PC2602, in the photo I replicated the operation of the power part with the results reading the voltage between S and +, since I am also working on a restoration.
the generic 4mV voltage that we find refers to the average of the voltage present on each of the power branches, so if the average is 4mV as already read in the various passages of the post means that on each branch there flows a current of 8.5mA that multiplied by 5 we have a quiescent current per section of 43mA that seems in line with amplifiers of this size.

The wrong thing then is to take this voltage to 15mV and above, as already read in some posts, this calibration would lead to a bias current of 160mA, unacceptable.
best regards from italy
 
valter_64 said:
hello amr2 , i don't know if it's a mistake yours or me not understanding, between points S and 0 the voltage is 4mV which corresponds to 8.5mA per transistor branch.
the service bulletin is correct
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Hello. The service bulletin says 4mV between (+) and (-), so it would be 2mV between S and (+)
 
Just to clarify, although it is indicated in the bulletin. 4mV is the initial setting approx. 10 sec after turn on the equipment. Once stabilized the reading should be 10mV.
 
amr2 said:
Just to clarify, although it is indicated in the bulletin. 4mV is the initial setting approx. 10 sec after turn on the equipment. Once stabilized the reading should be 10mV.
Click to expand...
Hello. I see you have done an extensive work on your PC2602M. Could you please help me out with my amp issue? One channel distorts badly and votage between "S" and "+" shows 00.0 mv and can't be changed. As you can see from attache pic, R167 resistor had been burnt and replaced as well as couple more caps that were bad and/or had higher ESR, however it didn't improve a distortion issue. I checked all of the resistors and they seems to be ok. I also checked all of the transistors on that PCB and they are not shorted, I unsoldered output transistors from board and checked them with transistor tester and they appeared to be not shorted and within the specs with respect to Hfe, Veb and Ic. So, at this point I 'm considering to replace all of the transistors and other parts that might cause distortion. Could you please advise me as to which parts should be replaced in a first place as they likely to cause the distortion? BTW, I saw you replaced some of the transistors on the board, but not all of them and also replaced a relay on your amp, should I do the same? Tks.
 

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avionic said:
What does the solder side under Q109 look like?
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It was OK. I already unsoldered those transistors and will be ordering new ones and will be replacing almost all of the transistors including output sc3907 and sa1516. Could you please tell me which ones should I be replacing in the first place because they most likely might cause distortion?
 
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Alex Kruz said:
will be replacing almost all of the transistors including output sc3907 and sa1516. Could you please tell me which ones should I be replacing in the first place because they most likely might cause distortion
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I love the shotgun approach. That gotta' fix it. :rolleyes:

Just make sure you need to send me all the old transistors
 
Can't get more than 0000.5mV between + and - on TP101. Does anybody know what might be causing that? Does that exclude OTP transistors?
 
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Could anyone please tell me how to adjust Bias/DC offset? I can see a test point + S - but what to do with them? Really appreciate. Thanks
 
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