Bias problem Marantz PM-94

[freebee]

I could do with some help with fixing my PM-94.

The Marantz has been recapped and all solder joints expected. It appears to function well however there are the following problems:

1. The left heat sink gets really hot while the right one is much cooler. The first step of biasing the left channel to 36 mV is possible, however the second step of biasing to 180mV not (initially it appears to work when turning the trimmer, but then it jumps back and forth in value).
2. Biasing the right channel works in both steps, however the heat sink gets warm but not hot. My feeling is that it's working in class a/b (perhaps this could be related to the quarter A settings?).
3. Have some difficulties to get the DC offset set to 10mV (and it seems to fluctuate).

1. What would be the most likely cause for this erratic behaviour of voltage jumping to different values?
2. Could the quarter A settings cause the right channel to work cooler (in class a/b) while bias settings in both steps are set to resp. 36mV and 180mV?
3. The DC offset changes with the bias and is difficult to keep at 10mV in the right channel. Any idea what might cause this? (am able to set it to around 8 or 14, but then it changes)

All tests were done at least after one hour.

Hopefully someone can point me in the right direction.

 

[huggygood]

have you try to change the bias pot ?

 

[freebee]

Not yet. Though I'm not sure it's the bias pot. Even when I dial it to a different position (let's say 100mV) it jumps back and forth in value. Will investigate non the less.

 

[freebee]

I've just bought some trimpots and was wondering if I could use a 25K ohm instead of a 20K ohm to adjust the output offset voltage. Is there any risk in trying these or should I order the 20K ohms?

 

[Mooly]

The DC offset should be adjusted to zero. The -/+ 10mv is a tolerance and may be an optimistic one in practice. If the offset is below -/+100mv then it will cause no issues however the correct point to aim for is zero when the amp is warmed.

If the amp has had work done to it (particular if transistors have been replaced) then its possible the bias issue may in fact be oscillation rather than a DC problem. That is more likely if you can set both to the correct 36mv point and yet one heatsink is then much hotter than the other. You need to use a scope to look if this is the case.

You also need a scope to check that the adjustable bias circuit is being turned on/off correctly.

 

[Mooly]

I've just bought some trimpots and was wondering if I could use a 25K ohm instead of a 20K ohm to adjust the output offset voltage. Is there any risk in trying these or should I order the 20K ohms?

If the trim pots adjust smoothly then I doubt there is a problem. 25k should work to prove/disprove there is an issue but the value of 20k looks somewhat bizarrely as if it plays a part in the overall gain structure. However the change from 20 to 25k would be minimal