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Sansui QRX-7500 channel weirdness.

Briant8

Briant8

Active Member
Hi All,
A buddy asked me to look at his QRX-7500, and this is the channel weirdness going on.

When I try 2-channel stereo (front speakers) the left side is strong but the right channel is out.

But when I try the quad settings, the rear right side is good but the left side is out.

All 4 power meters show output when playing, just no sound as described.

All fuses are good, I’ve cleaned all 523 pots and switches, I checked speakers, cables, etc.
I’ve tried multiple inputs with the same results.
Also cleaned the relay.

Quad is a foreign beast to me.

Any ideas?
 
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Almost has to be a switch or relay in the signal path based on the symptoms. The 7500 isn't quite as complicated as the 9001, but still has a LOT of switches in the signal path. The good news is these aren't known for cold solder joint failures as with the 8001/9001, so you can probably rule that out.

You may need to employ a signal tracer to nail it down if it is being that stubborn.
 
Oh, and I did pull the lid off and clean the relay (it actually looked a lot nicer than some others I’ve been into)

One thread I saw mentioned that the level set (4 banger) pot was bad. I did manage to extract it enough to clean it, but it is a mother to get to...

I’ll check on the speaker clips for sure.
Thanks!
 
^ I concur, and should have added that the fact the dropped channel is moving around leads me to suspect a switch more than a relay. I can't remember if the 7500 has a switch that rotates the channels (1/4 turn, 1/2 turn) as on the earlier QR model receivers, but that may factor into this as well, if there is one present.
 
Not the same unit but I was having similar problems with my QRX-7001 and it was a corroded 4 channel mode switch.
 
i sold a different model .drove it maybe 200 miles to be sure it got there in one piece .buyer phoned me half hour after . i said use the switches a few times and its last i heard . it had been in freezing temperature overnight in my car then slowly warmed up on the way .
it was fully tested before it went in the car .
 
Briant8 said:
Yeah, tried that hoping to hear any scratchiness at all in any control.
Wasn’t any evident, but I’m hitting them all again.
Click to expand...
Also realise the meters on this particular model are driven off the Pre-amp, NOT the power-amp, so you may well have power-amp issues.
 
kevzep said:
Also realise the meters on this particular model are driven off the Pre-amp, NOT the power-amp, so you may well have power-amp issues.
Click to expand...

Ah, good to know.
This one doesn’t have the pre-out/ amp in scheme or I would have tried the test the preamp/amp tactic.

Or is there another way to test them independently?
 
Sansuiman said:
^ I concur, and should have added that the fact the dropped channel is moving around leads me to suspect a switch more than a relay. I can't remember if the 7500 has a switch that rotates the channels (1/4 turn, 1/2 turn) as on the earlier QR model receivers, but that may factor into this as well, if there is one present.
Click to expand...
Yes there is a rotating channel selector, as well as many other balance controls bass tremble and so forth for front and rear. The OP wasn't kidding when he said he cleaned all 523 pots:D
 
Briant8 said:
Ah, good to know.
This one doesn’t have the pre-out/ amp in scheme or I would have tried the test the preamp/amp tactic.

Or is there another way to test them independently?
Click to expand...
Easy with an Oscillator and a scope, do you have those items?
Then you can just trace the signal through.

I would be looking at the decoder switches and any other push type switches, as they give a lot of trouble on this model.
I did a big restoration on a QRX7500A last year, there is a lot of disassembly to get to everything properly. I also had to drill holes on the rear of the switches to get contact cleaner.........

Its a big job...
 
Well,
Made a bit of headway.

Cleaned all 523 pots and switches again as well as the speaker clips, and managed to retrieved the rear left channel from the upside down.

However, the right front is still missing in action :(

Interestingly, I’m am now getting bleed-through sound on all 3 good channels (the rears even when in 2-Channel mode) but not a whisper of sound from that front right.

I’m guessing that this pretty well indicates a still dirty contact in one of them, right?
 
Briant8 said:
Well,
Made a bit of headway.

Cleaned all 523 pots and switches again as well as the speaker clips, and managed to retrieved the rear left channel from the upside down.

However, the right front is still missing in action :(

Interestingly, I’m am now getting bleed-through sound on all 3 good channels (the rears even when in 2-Channel mode) but not a whisper of sound from that front right.

I’m guessing that this pretty well indicates a still dirty contact in one of them, right?
Click to expand...
This is where a signal tracer comes in handy, or a scope and signal generator....

I would work out if the signal is getting to all 4 power amp channels with the unit in 4ch Discrete mode and then work from there.
 
kevzep said:
This is where a signal tracer comes in handy, or a scope and signal generator....

I would work out if the signal is getting to all 4 power amp channels with the unit in 4ch Discrete mode and then work from there.
Click to expand...

Unfortunately not pieces of equipment in the arsenal just yet...
 
Well,
My buddy lost interest in deeper diagnostics, so I buttoned her back up, for now anyways.
Thanks all for the help!
 
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