Bought a Marantz PM7200 - dead channel

[Homer4beer]

Well guys this amp is done. I also modified the phone stage to make it sound better as per below.

My question is that I read that the marantz amp had too much capacitance installed on the phone stage to stop EMI

Page 9 C2553/4 should not be fitted and C2555/6 should be 150pF, not 680pF
That will give you ~200pF plus the cable capacitance

The class A bias adjustment did vary quite a bit as the heat cycled on and off. I set both channels as close together as I could and then waited 15 mins, checked again, realigned, checked again in 15 mins, adjusted again as necessary then left then for an hour. During the time they did vary above and below the 90mV I set but never more the -+5mV as per specification so I'm happy.

I also set the speaker DC offset voltage again and managed to get them pretty much the same at +1mV stable, minor fluctuations only.


Left class A adjustment

Right class A adjustment

Thanks again to everybody for all the AMAZING help I've received.
Cheers!

 

[Rich Collins]

Well guys this amp is done. I also modified the phone stage to make it sound better as per below.





The class A bias adjustment did vary quite a bit as the heat cycled on and off. I set both channels as close together as I could and then waited 15 mins, checked again, realigned, checked again in 15 mins, adjusted again as necessary then left then for an hour. During the time they did vary above and below the 90mV I set but never more the -+5mV as per specification so I'm happy.

I also set the speaker DC offset voltage again and managed to get them pretty much the same at +1mV stable, minor fluctuations only.


Left class A adjustment

Right class A adjustment

Thanks again to everybody for all the AMAZING help I've received.
Cheers!

 

[Rich Collins]

Hi, I also had a PM7200 with dead channel, and this thread was helpful, but not the fault with mine.
So I will offer my findings for others.
When I originally had this amp, there was intermittent output form one channel, crackling, but after a few minutes warming up, the amp was ok. This was due to the bias transistors 7263 - 7266 having their soldering cracked due to temperature I guess. They all run quite warm. Small clip on head sinks might help.
After a couple of months, the right channel died.
The collectors on these transistors were showing 0v and the head sink on their side was cold. The other side showed a small voltage, and plenty of audio signal.
The culprit was resistor 3276, 47k ohm open circuit. This has the full VB+ to VB- voltage, less the drop across the 4 diodes 1n4148s, a little over 100v. So its got about 0.2 watts to get rid of. Its rated at .5 watts, but still will be running a little warm all the time.

Amp now working well.
Rich

 

[Erik Jensen]

It's the best thread I've read in a long time, even if a lot is incomprehensible .. Thanks for showing us how you think about troubleshooting.


I want to try my luck and buy an old Marantz PM7200 that has problems with one channel. I really hope that it is just a cold transistor soldering that is mentioned as a fault at the end of the thread. If not, its just bad luck for me.. I will never could do the same as your guys do.