Marantz PM-80mkII repair (DC at one output))

Hi everyone and merry christmas :)

I am currently trying to repair an old Marantz PM-80mkII.
The schematics are in attachment.


The thing outputs around 150mV DC at the right channel. The other one is fine (~1.5 mV DC). It is more or less the same in AB class and in A class.

As far as I can tell, nothing is obviously dead inside. I switched almost all transistors between channels (except Q704, Q706 and Q708), the issue is still at the right channel.

However, when measuring DC bias points as indicated in the schematics, I have 0V DC everywhere after Q716/Q718 (no input, no load).
The other channel is completely fine, and all DC measurements are correct.

Trouble is, I still cannot find any dead component...

Any ideas?
 

Attachments

  • PM80mkII.jpg
    PM80mkII.jpg
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You were 100% right, the fault was due to bad solder joints.


Although I checked many times, I did not see the broken solder joints on Q716 and Q718 until I tried to move them.
Now, I have around 5mV DC at the output, which is perfectly OK.



Only thing left is to adjust the bias currents and I will be able to enjoy this nice amp.


Thank you very much for your help! :)
 
Hi,
Q701 and Q702 are the most likely cause. ( Differential pair ).
Any variation in these transistors can cause voltage offset problems.
While amplifier is on touch the transistors with a plastic pen. Moving them will sometimes
cause changes. Do this with the voltage being monitored. De-solder them and look for a green color between the pins. If there is color, they are are definitely faulty.