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Marantz 2270 Power Amp prblem

AngelAT

AngelAT

Well-Known Member
Dear all,

I have been working on the power amps of my 2270.
Both power amps were taken out of the unit. Replaced the output transistors h001-h004, h758, h759, h751 and the two electrolytic caps - c759 and c760.
Idle current and dc-offset adjusted.
Then both power amps returned into the unit and the right one has a problem - severe distortion. Again taken out for tests and perfect sound. The only difference being that the power amp is not grounded to the chassis (just connected via the two big filter caps).
When I placed it back I found that if I completely isolate the right power amp from the chassis the sound is again ok.
Left power amp is grounded to chassis and sounds perfect.

Did you guys ever had such a problem and can you suggest a remedy?

Thanks in advance!

BR
Angel
 
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Make sure the small screws/nuts holding the drivers are tight as they are part of the collector circuit to the board traces.
Had a loose one once that caused a similar issue.

Bob
 
wlhd1610 said:
Make sure the small screws/nuts holding the drivers are tight as they are part of the collector circuit to the board traces.
Had a loose one once that caused a similar issue.

Bob
Click to expand...
Bob, thanks for the prompt reply!
Do you mean the screws tat hold the board+heat sink to the chassis?
These were tight and there was connection b-n chassis and board...
But still I will check again.
 
No , there are screws that hold the U shaped TO66 driver heat sinks to the circuit board .


Bob
 
Thanks Bob for clarifying!
But what is the logic behind this- would the problem be available when the amp is out of the unit?
Regards
Angel
 
H758 and H759 are the driver transistors. They each have two screws that secure them and their heat sinks to the board. The case of the transistor is the collector, so there is no solder joint for that connection. The only electrical connection to the board is via the pressure from those screws forcing contact with the board. Since you changed those transistors, he just wants you to verify tightness of the nuts to confirm a good collector connection.
 
In my case just the vibration/handling of the amp module would cause it to come and go.
Just throwing the possibility out there.

There are normally two short ground straps from the chassis to the bottom of each heatsink.
Not connecting them may make a difference.


Bob
 
Problem silved!
Thanks Bob and Steven!
I retightened the transistors and heatsink. After that I isolated the leg of the temperature detecting diode - I think it might have been shorting.
So now sound is perfect.

Thank you for your support!
Cheers,
Angel
 
Thanks Bob!

But still this right power amp needs work, because it is getting hot...
So I will continue to dig...
 
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