Sansui AU G33X protector at higher volume

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Alfanijs

Alfanijs

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Joined 2010
#1
Hello,

I have a problem with my Sansui AU-G33X amplifier. It works fine at lower sound levels, but as soon as I crank up the volume the protector circuit kicks in. After the normal blinking the amp starts working again. I tried and it is only related to the right channel. A few months ago I adjusted bias and checked for DC voltage, so all should still be fine.
Of course this unit is from 1983 and I did not yet replace the capacitors.
Any suggestions where to start looking?

Thanks for your advice,
Nijs
 
#2
I would check you speakers and speaker wires, one little stray strand of speaker wire touching from + to - will cause this to happenas soon as a little power is added. Or if your speakers are 4 ohm and when you turn up the music it drops down to less ohm's as the the bass is a kick'n, bam, the speaker protection comes on. I have a Sansui and thats what I found would happen. If you have, or, can try a differance pair of speakers I would do that first before digging in to the amp.
 
#4
Thanks both of you for your replies. You were spot on Rapidroy - it was a stray strand of speaker wire! DC resistance was 1.4 Ohms... Of course I read your messages too late and I discovered this only after replacing all el. caps in the power amp circuitry. Worst thing is I made the speaker cables myself and they had been running without problems on my B2101 power amp, even at higher volume levels - shows again what a beast this is! So I did not suspect the cables or speakers at all.

Everything is fine now. Thanks again!
 
#5
I refresh this old thread with similar problem. My amplifier is AU-G50X with the same F-4702 board. The effect is the same as described above but without any load. The protection switches on when the volume knob is at 13 hour. The amplifier works with headphones. I checked the offset voltages. Initially they are close to 900mV! I reduced offsets in both channels to 1mV or below. Unfortunately the bias in one channel is invariant (kVR2 potentiometer doesn't change the bias). The voltage on proper resistor is equal to 5mV instead recommended 10mV. What to do?
Regards,
f.
 

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#8
Dacz said:
have the same issue on my AU-G33X. Manual said it is 45W per channel at 8 ohms, I was running it at 4 ohms. When i bought 8ohms problem gone 😀
Click to expand...

I'm trying to adjust bias with no speakers attached, so for now I'm ruling that out.
Voltage measurements after kQ8 are off so I'll remove and test it.
Seems weird that both channels have the same defect
 

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#12
gmphadte said:
R50 is a dual resistor and you have to measure the bias from the middle and to either of the other two terminals.

Gajanan Phadte
Click to expand...

Yep, they are so even kind as to provide 2 pins connected to emitter sides of the resistor package.
As per service manual I should be getting around 10mV but I can only adjust between 0 and about 3mV.
 

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#13
JohanH said:
Yep, they are so even kind as to provide 2 pins connected to emitter sides of the resistor package.
As per service manual I should be getting around 10mV but I can only adjust between 0 and about 3mV.
Click to expand...

Try giving the variable resistor a spray of contact cleaner and work it back and forth about ten times,sometimes they get an oxide build up. Or better still replace them with 25 turn sealed plastic Bourns variable resistors. Check other resistors in the circuit haven't gone out of value and check the differential input transistors because if they are failing you'll never get it biased properly.
 
#16
Turned out to be a corroded wirebridge near the psu caps.
Corrosion due to the glue used for the psu caps.
When measured it seemed ok but it fell apart when I resoldered the ends.
After this bias adjustment returned.
(Still goes into protection though)
Next will be replacing other suspect wirebridges
 
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