I've been having some trouble getting this power amp finished up and I'm hoping someone can help me out. Initially, all I found wrong with it was a broken ground trace on the power board and some nasty, corrosive/conductive brown glue, which is common for this era of Sansui equipment. I ran a jumper wire to fix the trace, removed the brown glue everywhere I found it and replaced all the components that were corroded by the glue.
Everything seems to work fine, but the DC offset at the speaker terminals wanders all over the place. It's a steady increase/decrease of about 120mV and it does it on both channels. I've tried going through the calibration procedure, but the readings for steps 3 and 4 basically match the speaker terminal readings. It wanders out of the +-5mV tolerance within a second or so and it's an incredibly touchy trim pot. The slightest turn can jump it from -400mV to +900mV. I tried working it back and forth a bit in case it had oxidized, but no change.
Also, since it affects both channels, I've been taking a hard look at the main power supply. There were a few questionable solder joints, but reflowing them didn't help. However, I do notice some voltage fluctuations at the main filter capacitors. My multimeter only shows one decimal place here, but it drifts between 41.2V and 42.0V. I'm not sure if I'm just chasing ghosts here, though, since it's basically a split power supply.
From there, I backed up and checked the switches on the back. There's a voltage selector switch and speaker impedance selector. They both ohmed out ok. I also pulled the back panel and cleaned the contacts on all of the relays since one of them acts as like a soft start. The voltage drop across the contacts is very low.
Unfortunately, this kinda puts me back to square one. Is the output stage causing the power supply to fluctuate, or vice versa? Can anyone give me an idea of what to check next? I'm pretty much out of ideas
Everything seems to work fine, but the DC offset at the speaker terminals wanders all over the place. It's a steady increase/decrease of about 120mV and it does it on both channels. I've tried going through the calibration procedure, but the readings for steps 3 and 4 basically match the speaker terminal readings. It wanders out of the +-5mV tolerance within a second or so and it's an incredibly touchy trim pot. The slightest turn can jump it from -400mV to +900mV. I tried working it back and forth a bit in case it had oxidized, but no change.
Also, since it affects both channels, I've been taking a hard look at the main power supply. There were a few questionable solder joints, but reflowing them didn't help. However, I do notice some voltage fluctuations at the main filter capacitors. My multimeter only shows one decimal place here, but it drifts between 41.2V and 42.0V. I'm not sure if I'm just chasing ghosts here, though, since it's basically a split power supply.
From there, I backed up and checked the switches on the back. There's a voltage selector switch and speaker impedance selector. They both ohmed out ok. I also pulled the back panel and cleaned the contacts on all of the relays since one of them acts as like a soft start. The voltage drop across the contacts is very low.
Unfortunately, this kinda puts me back to square one. Is the output stage causing the power supply to fluctuate, or vice versa? Can anyone give me an idea of what to check next? I'm pretty much out of ideas
