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Sansui B-2101 Calibration Woes

KilgoreCemetery

Apr 12, 2017
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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
 

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Martaine2005

May 12, 2015
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Typing a reply is all too easy from an arm chair!.
I’d make sure all the conductive glue is gone. Check under resistors and all components.
I presume you checked the ESR on all the electros?.

Martin
 

Harald Kapp

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it's an incredibly touchy trim pot
Potentially the wiper of the pot has no longer good contact to the resistive trace. This could also explain the erratic voltage readings.
You can try to clean the potentiometer or replace it by a new one.
 

KilgoreCemetery

Apr 12, 2017
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Martaine2005: All suggestions welcome! I feel like I'm missing something simple and just need someone to point it out. I'll double check for more glue. I haven't checked all the electrolytics, but I did check all of them in the power supply section and driver stage

Harald Kapp: I'll see if I have a replacement. If I need to order one, I think a multi-turn trim pot would do wonders. Do you think it would affect both channels though? That's the part that has me stumped
 

Harald Kapp

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Do you think it would affect both channels though? That's the part that has me stumped
You have a point here. I overlooked that one. On the other hand: a fluctuation in the power supply should not cause DC offset.
Do you have, apart from the calibration instructions, a schematic? One could try to locate the DC correction circuit and look for a possible cause there.
 

KilgoreCemetery

Apr 12, 2017
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You have a point here. I overlooked that one.
Honestly, I wasn't completely sure. This thing has me second-guessing myself

On the other hand: a fluctuation in the power supply should not cause DC offset.
That's good to know!

I do have the full service manual. I'll include a picture of the driver stage and output stage. If you want to see the rest of it, let me know and I'll upload what I can. Otherwise, you can get it directly from HiFiEngine for free
 

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