Sansui AU-519 No Bias on right

Fhamre

Super Member
Picked up a 519 from a member knowing the outputs had been shorted on one channel.
Replaced the outputs on that channel and removed the glue,the black flag and replaced the caps and on MV12 diode while I was in there.
Brought it up on my dbt. All good. Can get dc offset to 0V on both but no bias on the repaired channel.
fyi has audio on both channels.
I tested TR10,11,12 And TR13 . They test fine.
Measuring voltage to TR16 b -.023V ,should be -1.2V
E is 0V,should be -.6V
Suggestions?
 
You're busy aren't you!!

I'd say it would be like the 419 I had lately, fusible resistors drifting possibly?

Other than that there could be an intermittent/broken connection somewhere. I'd measure through the current path to make sure everything is connected...

There's a start anyway.

I had this same issue yesterday on a QRX3000 I am rebuilding for a guy, and I could not find one single thing wrong with it, I started to suspect a broken connection, so I pushed on one of the TO220 output transistors and the bias came alive.....It took more digging, but what it turned out to be was the actual leg on the output transistor had come detached internally from the transistor, I could literally gently push this one leg and bring it back. I took the transistor out and you could see the leg was loose.....
Anyway thought that might be useful in your search for the issue, sometimes its simple and right in front of you...

took three hours of head scratching to find it too!!
 
Yes, taking a break from the 1980 to work on something not so scary.Oh I'll eventually find it and yes it will likely be something simple.
I did check the emittors. I have replacements if necessary. I was hoping some one could point me to usual suspects on the board when outputs get shorted. The black flag is gone.
 
TR11 reads on the good channel E -1.87V,C 1.87V, B -1.12V
On the bad channel E -.9V,C -.5V,B -.35V
TR12 Good is E -44V, B -44V,C -1.8V
Bad side E -44V,B -44V,C -1.18V(missing .8V)
 
Thx Lee. I'll look at that. I was distracted today. So I hope to get at it tomorrow.
Will take some more measurements as well. It appears the voltage or lack thereof is affecting the collector.
 
Never mind if it tested good, change TR10.

I am assuming here that you have tested all the resistors, especially the emitter resistors for TR10 and TR12.

If this doesn't fix it let us know if you see any voltage changes.
 
Update.I changed out TR10.Also changed the two emittors. Even though they tested fine.No change in voltages observed so far.
fyi the unit is still stock except for the faulty driver board. Which has been recapped and black flag replaced.
One observation I would make is that it takes close to 10 seconds to come out of protection. A little longer than the norm imo.
 
I replaced the two Styrene caps. No Change
I recall when looking for bias on a 717 John told me how to measure across TR06 C to E and adjust the trimmer to see if bias is present and adjustable.
Is there a similar transistor in this circuit I could try this on?
 
TR11 is the bias transistor, measure across C and E to see if the voltage varies when adjusting VR02. VR02 controls the conduction of TR11, the more TR11 conducts the colder the bias or smaller the voltage drop across TR11 the colder the bias, less idle current. With the voltages you have listed in #5 TR11 can't possibly be on, high idle current, but yet your idle current is still zero across R52/53? Also your TR11 voltages are ALL negative, the collector needs to positive and that depends on TR10 and TR12 conducting. Is R29 good, that's a dreaded fusistor isn't it? Check R33 while you're at it.

Craig
 
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Must have been the idiot before you got it? Does it work now? That's the kind of thing that the armchair troubleshooter's can't help with!
 
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