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2235B Bias Trim Pot

VintageMad89

VintageMad89

Active Member
Hi all, one of the bias current trimming resistors (R767) on the power amp board has gone bad and I am looking for a suitable replacement.

The service manual says it is 200Ohms but I'm at a bit of a loss as to what I'm looking for:idea:

Any pointers would be most welcome!

Cheers guys
 
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Does the manual call for a (B) trimmer?

how did it fail? Is it burnt? tried cleaning it?
 
Yeah sorry, (B) trimmer.

Well it won't let me adjust to 8mV at the test points- sporadic- like the windings inside are bad. It feels gritty turning it. Have given it a good workout with deoxit to no improvement :(
The only measurement I can get from it is anywhere from 0mV to off the scale with only 150mV+ in between.
 
I had the same problem with my 2235B on both channels. They are wirewound resistors that can fail open and cause the idle current to go high. I cleaned mine but that only worked for a short time.
My idle current was to spec when the resistance was around 160 ohms so I installed a 200 ohm 0.25 watt fixed resistor and put a 1k ohm multi-turn trim pot in parallel with it. Those values gave me an adjustment range wide enough to finely adjust the bias. Has worked great ever since. The values to get yours in the range may differ.

If you look at other models of Marantz they had a similar setup with 2 paralled resistors. That way if the trimmer fails open there is still a resistor in the circuit so the idle current doesn't damage the outputs. Not sure why they changed in the 2235B.
 
Is there a particular kind of trimmer I should be looking at as a replacement?

I have access to Mouser and have seen the bourns multi turn trimmers- 200 ohms.

Cheers
 
Hi folks, sorry to dig up an old thread, BUT...

I ended up ordering the bourns multi-turn 200 ohm trimmers. Now, the old trim pot has four posts (one just for support on the board I think, as it has no trace coming from it).
The issue I have is that the little legs on the new trim pots are far too short to reach the original holes on the board. Is there a different part I could try or does anyone know of a "DIY" fix that would make it fit?

Thanks in advance!
 

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