Sansui AU-D11ii

[TravelITC]

Happy Holidays everyone;

I rarely reach out for help however, I have reached a point where I need some input from the experts.
This is my 1st time working on an X balanced circuit and I am very perplexed about this situation.

The amp is in protection, all outputs are good and I am able to adjust the 0vdc for the Equalizer Amp and Tone Control Amp, no problem.
I am able to adjust the Hot/Cold Balance but when I try to adjust Center DC 0v, the adjustment pot seems to have no effect...symptom is the same on both channels...I am getting -26v.
I am able to adjust bias ok.

Here is the weird thing and where this design has stumped me...When I measure the main rails they are not balanced despite the delta being about 70vdc, I measure -30v and +41v. They should be equal at about +-36vdc.
This is the same on both sides of the amp.
All 4 main rail caps are new/good.

Looking at one side for instance, circuit bd 4267, my voltages on the front side seem to be correct but on the back end I am reading the following incorrect values...pin18 = -28v, pin 16 = -24v, pin 13 and 14 = -26v, pin17 = -28v, pin 15 = -24v.

I am stuck with the Chicken or the egg...are these values off because the main rail or is the circuit causing the imbalance on the main rails?

I also measured the resistors that create the artificial ground and they are ok.

I know there are a few people here like Leesonic, Hyperion and Bunpalump that probably have the experience to help point me in the right direction and hoping they may be able to chime in.

Thanks to anyone that can offer insight!
Paul

 

[Hyperion]

I measure -30v and +41v. They should be equal at about +-36vdc.

This is what you should work on, it is the first principle of fault finding - check all supply voltages - you shouldn't do anything else until this is resolved.

Check for cracked solder joints (with a magnifying glass), where the upright driver boards are attached to the horizontal power amp board, if it isn't that you may have some blown transistors. You could start by checking kQ10-13 and kQ14-17, best to remove them and test each one - be sure to note well their positions & pinouts before re-installing them.

 

[TravelITC]

This is what you should work on, it is the first principle of fault finding - check all supply voltages - you shouldn't do anything else until this is resolved.

Check for cracked solder joints (with a magnifying glass), where the upright driver boards are attached to the horizontal power amp board, if it isn't that you may have some blown transistors. You could start by checking kQ10-13 and kQ14-17, best to remove them and test each one - be sure to note well their positions & pinouts before re-installing them.

John, thank you sooo much for responding. I agree with you regarding supply voltages. I have removed for inspection and reflowed the daughter boards and then verified continuity of every pin to the mother board and have done very close inspection of all the solder points and verified with continuity. I never usually have to ask for help and have a lot of years doing conventional amp troubleshooting and repair but this topology had me questioning myself. I am going to remove the daughter boards again and double check every component on them at this point. It's just odd that the delta voltage exists but the balance is off...this is where the floating ground concept has me wondering WTF is causing the shift. If I isolate the main rail it's balanced correctly.

 

[TravelITC]

When adjusting the "Center DC" do you see the balance of the main rail change as indicated in my description where the positive side is 11v higher than the negative side or is it unrelated?

 

[TravelITC]

UPDATE!!!!!
Problem Resolved!
Someone had been in there and used the wrong value resistor in ckt!

Thanks for your help...now time to do a complete refurb!
Merry Christmas to me!!!
Paul

 

[leesonic]

This thread might be useful :

Sansui AU-D11 II trimmer replacement WITH PICS