Marantz 2230. Bias too low.

[misterguitar]

I have a Marantz 2230 I just finished recapping the Amp boards for a friend of a friend. (once people hear you work on this stuff it's amazing how many new "friends" you make. lol) So far so good. I can't seem to get the bias up high enough on both boards. One seems worse than the other. I can get one to about 5.7 and the other to 7.6mv but nowhere close to 10mv, What would cause this. I'm about halfway through reading the Doug Self book on power amplifiers. Mainly because I really want to understand how this stuff works so I don't keep asking stupid questions. I did not replace either trimpot on either of the amps. Could that be the problem? I rechecked all my work on the boards and its beautiful and everything is correct. Unless there is a silkscreen error on this board as far as cap polarity I'm not aware of. Do any of you know what would cause this to happen? Thanks guys!

 

[rBuckner]

Check the manual errors thread to be sure the procedure is done correctly. Also you shouldn't be on a dim bulb tester. There, that's the easy possibilities!

 

[misterguitar]

haha I was worried about that! No DBT. I checked because of the polarity error on some silkscreens for one cap. That wasn't the issue either. What is the model# Bourne trimpot most folks are ordering for these now days if any of you guys know? I'm going to try replacing those just as preventative medicine. the old ones are kind of crusty looking. I notice there is a service bulletin regarding bias current stability for these. It appears this one has the modification. A thermistor and resistor. Do these go bad? I would not even begin to know how to figure out what to replace that with. My other thought was maybe there is a problem with H752? But would they go on both sides? Would seem kind of odd. Already checked my power supply, which was recapped and I have the rails to 35.6. no fluctuations.

 

[catrafter]

DC Balance and bias interact with each other., Try setting the DC Balance for about 29VDC at J758 (black meter lead to chassis), volume all the way down. Then set the bias for about 10mVDC, red meter lead to to J759, black meter lead to J757 (polarity is not important for this reading).

Tom

 

[misterguitar]

DC-Balance, is that R760?

 

[misterguitar]

Hey it worked! I had a ton of adjustment range after that.

 

[catrafter]

Tom