Sansui 8080 Driver Board F-2436-1

[JJC]

Hey everyone, new to the forum though I've been watching Dave's videos for awhile.

I have a strange problem where my Sansui 8080 powers up and plays fine for a random period, then the left channel power meter bumps a third of the way to the right, and the volume increases. Right channel is fine. If I shut the power off and then restart it, it plays fine again until the left power meter bumps and the volume increases. When i plugged the board in reversed the problem switched to the right channel.

This state is actually an improvement. Previously the meter had jumped and stayed there, and the DC offset for the left side was over 700mV instead of the 0mV it had been set to. The best I could adjust it to was over 200mv. All the trimmers, transistors, electrolytic caps and fusible resistors have been replaced on the board. I've checked for bad solder joints, measured voltages and tested other resistors to make sure they're in spec. Can't find a problem. Yesterday, I was able to actually adjust the left channel offset to the proscribed 0mV, and now the offset resets when powered down, so sort of an improvement.

Has anyone seen anything like this, something disrupting the DC offset and actually causing the volume to increase? Then returning to normal after restarting, at least until the cycle repeats.

I thought about a part warming up and causing the problem, but wouldn't it still be hot when I powered up again right away? Any thoughts?

 

[akimpowerscr]

All the trimmers, transistors, electrolytic caps and fusible resistors have been replaced on the board.

But you seems to have forgotten to replace the two diodes D01 and D03 ......!!!!

This is the first thing I would have changed.

Your repair strategy is not good ....

You must first take measurements and find the cause of the failure before changing any components.

By replacing all the components, as you do, you risk introducing new defects, it is a complete mistake.

NB: I forgot to welcome you to this forum, my apologies for this shortcoming

[KevinA]

The fault will be in the differential amplifier.

Careful measurements will point you to the culprit.

[JJC]

Hey guys thanks for your response.

I didn't want to make it a long story, so here's some context. This came to me doing nothing other than coming out of protect. It was so dirty the first step was to brush with one hand and vacuum with the other. Bugs, dirt, you name it, needed to be cleaned out. I recapped while I went through it cleaning. First issue was a fluctuating power supply, measured, tested and replaced the 2 transistors on the power board. Power went to the 36VDC it was supposed to, and the unit began playing semi-reliably.  AKIM, I tested those transistors with a Peak DCA55, and it called them good. I had the replacements and put them in even though the test said I didn't need to, and it solved the problem. So, testing is great, but if you're not testing under load... So my strategy wasn't to replace everything in sight.

I've been testing/listening to this for just about 2 hours and have had one self-correcting hickup, where the left side bumped but immediately dropped back to normal and kept playing. This after I re-cleaned the push-button switches with DeOxit. While doing that I noticed the metal shield over the Volume/Balance board (F-2543?) was not sitting correctly over the board, but had slipped over to one side. Straightened it and started to test.

Do you know what the metal shield is supposed to keep off of the Volume/Balance board? Do you think the shield being out of place could have allowed whatever it was meant to shield to interfere with one side via the balance?

[floobydust]

The shield-can shorting to something would cause volume problems but not affect the F-2436 amp driver board. Otherwise, the shield does not make a huge difference, they can be to keep channel separation high between L+R or shield the pre-amp for better S/N. You can only hear it (crosstalk) at very low signals like -50dB or less.

From your symptoms (volume drop, DC offset) check/compare diodes D05, D06 VD1212 "varistor" they are notorious for being intermittent. They seem to be from an era where the Japanese engineers were conquering thermal drift and came out with a line of these oddball semiconductors with the poor translation as "varistor" for the English name. Consensus is they are diode-arrays, but may have a integral thermistor in series as well.

These "varistors" were commonly used for bias thermal tracking - but not on the 8080 instead it is using a Vbe-multiplier (TR07, TR08).
https://www.eevblog.com/forum/beginners/identification-of-diode-in-a-form-ive-never-seen-before/msg2866286/#msg2866286