Marantz pm 94 + Bias adjusting

[CederPM94]

Last autumn I bought an old none working Marantz PM 94 from a young guy who had given up on it. He had changed the FET’s but didn’t know how to adjust the bias. Well, I’m far from a pro my self regarding electronics, but I have always wanted this amp, so I thought I would give it a try. I started replacing all the electrolytes on the driver boards. On New Year’s Day, with just a little hangover, I finally got to fire it up. It all seemed nice until the phone rang. Unfortunately I did not turn off the amp while answering the phone. Suddenly fireworks began all over again behind my bag, but this time on my living room table…. Damn. That meant 6 new pairs of SK1529/SJ1200. So I started to look for errors on the two driver boards. Amazing how many things I found. Wrong values of caps, resistors... Well, I have finally managed to get it back together. To start with I only put in one pair of FET’s in each channel. Worked out fine. Adjusted the Bias (+/- 90mV) and DC with a 20 ohms resistor on the output. And it sounded really nice. Then I installed the rest of the FET’s. But now I can’t get the bias and DC correct. I’ve hooked up my speakers to the amp, while adjusting. On the right channel I get something like +90/-80 mV and 0 DC. On the left channel I get -90/+60 and -20mV DC. Then the DC- trimmer reaches its stop. But maybe I’m doing something wrong. I don’t know if you are supposed to make adjustments with the speakers hooked up or if there should be no resistance on the output while adjusting. Or maybe I’m being tricked by the Quarter A design. I would very much appreciate if someone could tell me if I’m during something wrong or if any one knows the standard procedure for adjusting a Marantz PM 94. Or could it be that I’m having trouble because the original SK405/SJ115 is substituted with SK1529/SJ1200???

Thanks in advance

Carsten