Sansui G22000 Basketcase

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Hi guys, I have a G22000 under restoration that I feel like simply does not want to live. Little background, when it came in right channel of the amp out, left "worked". Found the power supply for the right channel had out of spec resistors that looked really crusty, along with bad regulators. Specifically R08 and R16 which are in series with the driver board rails. Replacement brought the channel back, recapped the driver board and the power supply and sounded great.

Both channels worked now so recapped the left channel and left channel power supply. After the left channel was recapped noticed what sounded like low frequency oscillation. Ran function generator into channel and noticed under 100hz or so the distortion was terrible, then...smoke, R33 and the resistor on the back of the driver board smoked and burned taking out the outputs. Both amps were biased fine. So now I am thinking it is something what was replaced so have gone back through and checked every part replaced and not finding anything wrong. When I take voltages on the left channel power supply I get a higher then normal positive rail. Looks like normal should be around 82vDC, I am getting 94-95vDC. This amp is rather confusing in how the rail comes off the power supply to the driver board, back to the power supply. If I power the unit with the driver board in, the resistor R33 on the driver board starts to smoke.

The B+ rail of 94-95vDC I am reading with the driver board out. I am comparing it to the working side and it is higher and never reduces to a little over 80v which it seems it should. Any tricks to pin point the short or part that is drawing excessive current? I am thinking the driver board is a victim of the power supply having an issue down stream. Something notorious for failure on the board I am missing?

Thank you in advance for any help!
 
In addition blew F01 and F04 when the short occured. Both are in the positive rail circuit which leads me to think there is a dead short somewhere.
 
Might pm Paul (ManyMoons Audio), think he has one on the bench at the moment and might provide some voltages to compare, the service manual is not all that great for armchair assisting as the voltages are almost impossible to make out.
Not having worked on the big G but several others in the G series not sure sure this will help but its my day off so---
Pull both driver cards, place unit on a variac or dbt, pull F01 and hook up a dmm on amp scale and see what she draws comparing left to right. Then install left card and measure, then install right and compare. Have you verified all the output transistors, emitters and predrivers for integrity already?
Finally, any chance you have a failed diode bridge?
-Lee
 
maybe you put transistors in not compatible with the circuits and maybe need to add some capacitors to stop oscillations ? just a thought ... :idea: ..
 
Overundr1 : thank you, if I spend another day on this will give Paul a PM. Yes, I agree the SM is terrible for this unit. I think the issue is in the power supply circuit after the rail returns from the driver board.

petehall347 : Used the same in the other channel and no problems. More of a distortion then oscillation but see where your thinking. Thank you! I think somewhere the power supply was leaking and it just bit the dust.
 
That unit has 2 sets of rail voltages. I can't make out any voltage readings on the 22000, but can see the 33000 very clear. The B+ and B- voltage going to the collector of the outputs and the collectors of the drivers, comes directly from the Big Filter Caps. In the 33000 it shows +95, and -95. The other B+ and B- is the regulated voltage for the first section of the driver board, up to the R33 resistor feeding the predriver TR10. That B+ and B- Rail for the 33000 shows to be about +85 and -85v. Assuming the 33000 runs higher than the 22000 I make the assumption that in the G22000 the output/driver rail is probably around +90 and -90v and the regulated rail should be around +80 and -80V. If the regulated rail voltage feeding R33 is up at 94v, then you have probably a shorted or leaky regulator transistor (TR1, TR2) causing the high rail voltage, or you have a shorted/leaky TR10, and or D5 allowing the higher DC to come through the predriver transistor.
 
To quote Tim Conway in McHale's Navy: "Gee, I love that kind of talk."

- Pete
 
Then what you need to realize is there is a different supply for the front end of the driver board. Everything from the FET and up to TR10 and TR11 is separate from the drivers and outputs.
So the current mirror and bias circuit are off this different supply, you measure this at terminal 14/15 on the driver board.
14 is supposed to read +80.8VDC, and 15 is supposed to be -80.8VDC.
Make sure ZD01 and ZD02 are working correctly.
 
Pretty sucky oops there...

Check your work, check your work, check your work.
 
Follow up on this, yes the rails should be 82v.

Thanks Paul, done three times nothing is incorrect. Other channel is rebuilt exactly the same and is fine.

TR1 which was replaced with an ON Semi replacement is shorted. Same sub was done in other channel and is fine. Thankfully I have the other channel to reference.

I'm thinking the issue (short) is downstream in the regulated circuit causing the driver board to cook. Replaced TR1 and checked other components in the area. Fired the amp and voltages are good. Installed the driver board and the negative side of the driver board burns the same resistors!!! Removed driver board and the negative voltage is high now! Enough Sansui nonsense for the day, will dig back in tomorrow evening. Might remove the series resistors to the regulated circuits and see if the unit can be powered with the driver board in place. That will isolate where the problem is at least.

Units like this are prime examples of why restoration should be done before failure. This amp had many resistors out of spec and caps that were cooked.
 
What did you use as a sub for TR01?
Did you check the ZD02 is not shorted?
Have you checked D01 and D02?
What resistors burned up? R33 (if so indicative of the bias running away) Have you replaced the bias trimmers? Checked the STV-3H diodes and the MV103 diodes?
Did you check the Current Mirror transistors? TR02-09?

Lets have a look at these things and see where we are at.
 
What did you use as a sub for TR01?
Did you check the ZD02 is not shorted?
Have you checked D01 and D02?
What resistors burned up? R33 (if so indicative of the bias running away) Have you replaced the bias trimmers? Checked the STV-3H diodes and the MV103 diodes?
Did you check the Current Mirror transistors? TR02-09?

Lets have a look at these things and see where we are at.

Back on this...

TR01 was replaced with MJE15032
ZD02 is not shorted
D01 and D02 are good
STV-3H show up as an LED on my Peak Atlas but showing as good
All of the MV103 were replaced with 3 1N4148 in series which was also done to the other driver board operating perfectly
TR02 and TR03 replaced with 2SC3503, TR04 replaced with MJE15033, TR05 and TR06 replaced with 2SA1220
TR07 replaced with MJE15032, TR08 and TR09 test good

R33 and R34 are the resistors that are burning up so yeah seems to be a bias runaway. I do not think it's the bias trimmer since the negative rail voltage on the power supply board is not high, up are -95v. This was the same as before with the positive rail up around 95v and after finding TR01 shorted brought the rail back in spec.
 
What way are you turning the bias trimmer to set min bias before you fire it up?


Also we need to talk about the subs you have fitted, that all needs tidying up, your substitutes are well off the mark, so I will look 'em up so you can fine tune that.
But it should still work
 
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Also could you take a picture of the driver board, nice and clear so I can see your work...? Thanks.
 
On right driver card, possible issues---
C10, C11 failure
Verify D03,D04 have not gone open, I might suggest removing those stv-3H and measure their total diode drop if your dmm can handle them, Should be around 1.8v
Verify D05, D06 orientated correctly
Verify C05, C06 oriented correctly
your initial post about observing low frequency distortion is interesting, check for any missing or broken ground traces on that card or its connector since without a solid ground reference all sorts of nasty things happen.
-Lee
 
You haven possibly got the triple 1n4148 arrangements wrong with one reversed or something like that?
 
If there was glue on any of the components on the driver board, carefully inspect the leads on any component touched by the glue. Years ago I was chasing a distortion in a G22 and it turned out to be a lead that was completely eaten through by the corrosive glue.

- Pete
 
Ah, I have a G22000 on the bench right now, I am just setting the Bias and DC offset, Clockwise with the trimmer increases bias.

Its not the same as the G8000/9000 where Clockwise decreases bias....

Always have to check for that damned glue in any Sansui of that era...
 
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