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Marantz 2230 Output Transistors overheating

B

bobd127

New Member
Hello, recently I finished a full recap of both the left and right p750 amp boards on my marantz 2230 as well as replaced the three small transistors on each board (not the snowflake heatsinked ones or output transistors.) This was about three days ago and I have used it a fair amount since with no issues. Everything seemed to be fine until today when I went to turn it off I noticed that the output transistors that control the right speaker were very very hot while on the left was completely fine. After removing the top I determined it probably isnt a single bad transistor, as they both get equally hot on the heatsink. The varistor wires were not broken during the recap, and it is making solid contact with the heatsink. The speakers themselves are fine and aren't drawing excess current. Right is connected to remote right and left is connected to main left. The amp had only been on for about 7 minutes and both channels still sound equally nice even after the ordeal.

The weird part is that I have been unable to replicate the issue. I ran it for about 20 minutes at a fair intensity after the fact checking back every few minutes to make sure it wasn't getting hot and it stayed completely cool this time. Could it be the bias is set improperly for this board? I don't feel great running if its being temperamental like this. Does anyone have any ideas of what it could be or where to start? Thanks for your help!
 
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bobd127 said:
Hello, recently I finished a full recap of both the left and right p750 amp boards on my marantz 2230 as well as replaced the three small transistors on each board (not the snowflake heatsinked ones or output transistors.) This was about three days ago and I have used it a fair amount since with no issues. Everything seemed to be fine until today when I went to turn it off I noticed that the output transistors that control the right speaker were very very hot while on the left was completely fine. After removing the top I determined it probably isnt a single bad transistor, as they both get equally hot on the heatsink. The varistor wires were not broken during the recap, and it is making solid contact with the heatsink. The speakers themselves are fine and aren't drawing excess current. Right is connected to remote right and left is connected to main left. The amp had only been on for about 7 minutes and both channels still sound equally nice even after the ordeal.

The weird part is that I have been unable to replicate the issue. I ran it for about 20 minutes at a fair intensity after the fact checking back every few minutes to make sure it wasn't getting hot and it stayed completely cool this time. Could it be the bias is set improperly for this board? I don't feel great running if its being temperamental like this. Does anyone have any ideas of what it could be or where to start? Thanks for your help!
Click to expand...
edit: I think I caught it in the process of thermal runaway. Does this imply an issue with that varistor on the heatsink?
 
Yes both bias and voltage has been set but I feel like Im making a mistake somewhere. I adjusted R765 to 10mV between J757 and J759 and set R760 to 30V DC. However, I noticed that 10 mv value never really stays the same. Itll change about 5mv in either direction. I originally set the bias about 15 minutes after letting it warm up at volume 0 with no speakers attached on aux. I reset the bias again last night and it played fine and cool, but I tried it this morning and this time the other amp board was warming up so I feel like its less likely the thermistor (thank god) and just that i'm setting bias wrong.

Additionally, I have not changed out the filter capacitors and big main capacitor yet. Would that have something to do with it?
 
I doubt it on the big caps. A noisy / flaky bias pot would make it do this though! Mark the adjustment position and try (with power off), getting some DeOxit or Fader F5 into the pot and work it back and forth. Return it to the spot that was the place it was adjusted to and redo the adjustment. Ideally working the pot at different time intervals and waiting at least overnight before powering on.
 
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