Sansui Au-7900 cuts out then comes on again

vic

Active Member
Sansui Au-7900 been sitting in storage a long time the previous owner was a smoker, takes about 30 seconds to click then power turns on, It stays on for while then sound cuts out for a few seconds clicks back on and sound comes back. the power light always stays on while this is happening.
thank you for your help
 
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While the unit is clicked "on" measure the DC offset of reach channel at the speaker terminals for each side
 
sorry don't know about schematics It takes about 30 seconds to click and turn on and now playing for an hour and still good, the controls a yellow from tobacco stains? anything else I can try
 
As above, you need to check the DC offset as your first port of call (and adjust if necessary). If you’re not confident in doing so, find a local tech.
 
As above, you need to check the DC offset as your first port of call (and adjust if necessary). If you’re not confident in doing so, find a local tech.
so do you think its an easy fix? I haven't cleaned the amp yet would that help?
 
ok an update amp was on for about half hour then I heard a popping noise. sound went off for about 10 seconds then back on, keeps repeating this pattern
PS the heat sink gets hot as well
 
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You obviously have no electronics knowledge or tools. Take it to a qualified tech. It's an inexpensive repair now but if you continue to operate it, it may get damaged and turn into an expensive repair. If you tell us what city you live in we may be able to refer you to a tech.
 
You obviously have no electronics knowledge or tools. Take it to a qualified tech. It's an inexpensive repair now but if you continue to operate it, it may get damaged and turn into an expensive repair. If you tell us what city you live in we may be able to refer you to a tech.
ok I know about taking it to a tech but no one is answering my questions like if I clean all ports and switches would that help? and now its staying on much longer an hour and still good
 
ok I know about taking it to a tech but no one is answering my questions like if I clean all ports and switches would that help? and now its staying on much longer an hour and still good

I doubt that the switches/pots are the issue here, particularly given that you mentioned the popping noise and hot heatsink. The longer you leave it running without being able to monitor/adjust/diagnose the amp, the longer you risk damage IMHO.
 
I doubt that the switches/pots are the issue here, particularly given that you mentioned the popping noise and hot heatsink. The longer you leave it running without being able to monitor/adjust/diagnose the amp, the longer you risk damage IMHO.
so do u think it jus needs adjustments? Just before it pops it makes like hissing noise then stops for 10sec. and sound comes on again
 
Most likely it is not an adjustment. Adjustments don't make popping and hissing noises. It sounds like a transistor is going bad. If you keep running it, the transistor will fail and probably take other transistors with it.
 
Most likely it is not an adjustment. Adjustments don't make popping and hissing noises. It sounds like a transistor is going bad. If you keep running it, the transistor will fail and probably take other transistors with it.
 
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so do u think it jus needs adjustments? Just before it pops it makes like hissing noise then stops for 10sec. and sound comes on again

No, as dr*audio mentions, it's likely to be a transistor making that noise.

It's impossible to diagnose this remotely - you need to get a tech involved, or learn how to diagnose faulty transistors. Otherwise, we're playing an electronic version of 'pin the tail on the donkey'. :)
 
just cleaned all ports and boards with electric spray and now powering up as normal clicks in about 3 sec. and turns on I will leave it on for a few hours to see if it turns off again, So what do you think did it just need a clean?? Like I said it was sitting in storage for a long time.
 
just cleaned all ports and boards with electric spray and now powering up as normal clicks in about 3 sec. and turns on I will leave it on for a few hours to see if it turns off again, So what do you think did it just need a clean?? Like I said it was sitting in storage for a long time.
ok after 30 minutes I lost sound for a few seconds then came back on but this time no pop only sound lost?
 
Vic, you've gotten the advice of some very knowledgeable members here. Your choices are either learn enough to do your part in a remote diagnosis project (assuming one of these members or another one is willing to do it with you), or take it in for repair.

If the latter, tell us where you are so that someone can possibly recommend a remote tech.

If the former, you will need to get a decent multimeter, learn the principles of using it to measure live voltages and test transistors, diodes and transistors, and learn how to do small scale electronics soldering and desoldering.

Something more than bias, offset or dirty/oxidized contacts is most likely amiss here, By the way, I would recommend, strongly, against playing with bias and offset adjustments.

If you have a multimeter, post your DC offset measurements. With the amp off, put the black probe in the the black terminal for Speaker A, right channel. Put the red probe in the red terminal for Speaker A, right channel. Select AUX on the front panel, turn volume to zero, balance to middle, and tone controls to center. No filters on.

Now turn on the amp (make sure Speaker A is selected), and measure the DC voltage. Turn off the amp, remove the probes and re-insert them into the Speaker A left channel speaker connectors (again, red to red, black to black. Turn on the the amp and read the DC voltage on that channel.

Report back.
 
ok will do in the meantime this is what I found when sound lost I tapped the amp sound came back on and now 1 hour still on so thinking a solder joint or loose connection somewhere?
 
ok will do in the meantime this is what I found when sound lost I tapped the amp sound came back on and now 1 hour still on so thinking a solder joint or loose connection somewhere?
OR - you could always follow the advice that everyone has given above, and take things step by step and report back?

Just a suggestion.
 
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