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Sansui AU-X901 voltage problem.

anderslober

anderslober

Yikes...not again!
Hi forum. Working on a nice au-x901 amp. Protection lights keed blinking. I have taken out all the main outputs and the drivers. Protection still blinks. I am measuring voltages on the power supply. One side…the 35v supply is only half what it should be….18v. See picture of ps below. Would this indicate a bridge diode failure?
The ps is a ‘bee-atch’ to get out on this unit. Before I tear it apart, …….would this be the problem? Or, could there be something else? Thanks….all inputs/ideas welcome.
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I did disconnect both +/- 35v supplies from the amp board. Now the supplies (measured at the ps) are equal at +/-29v……I guess this means it isnt the bridge rectifier….?
 
All testing on dbt. Managed to get the rail voltage about equal…+/-30vdc…..I still have about +/-11vdc on E and B of main outputs right side. All resistors/fusibles test fine…..the drivers/pre-drivers also test fine.
Could q52 cause this? It is a pain to get to….. :(
 
+/-11 vdc there doesn't sound good, and might normally point to blown output transistors.

anderslober said:
Could q52 cause this? It is a pain to get to….. :(
Click to expand...

Re: kQ52 Possibly, that's the bias control/tracking transistor, but anything that can cause the base voltage of kQ54, kQ58 etc to go more positive, and also kQ56, kQ60 bases to go more negative. (In other words no control over bias when kQ52 is not able to pass current), - and thereby pull the bases of the above transistors together to pinch them off, kQ52 has to pass current to limit the bias. But also if you have an original bias trimmer - which will very likely be oxidised and open circuit will very likely cause the same effect. In addition any nearby resistors which have been stressed and have gone high in value (often without showing any signs of distress) will cause this kind of thing.

You can test the operation of kQ52 by monitoring the voltage from collector to emitter - and then adjusting the associated bias trimmer, you should be able to vary the voltage from about 1-4V - the lower the voltage the less will be the bias current. I am betting that kQ52 is dead, and that no adjustment can be made.

Alternatively all this all might be 'smoke and mirrors' - and the voltage you are measuring is actually DC offset, caused by a variety of other problems.
 
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I am a bit stuck…..see picture of amp scat….I have about -10vdc on the bases of q54 and q58…..any ideas? I have checked all resistors in the vicinity…all good. The tr’s are also good. Diodes are also tested good. Let me know what to check…..
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Ok, so this +/-11vdc - what is the voltage as measured across e & b of the output transistors. Is it +11v across the positive transistor and -11v across the negative transistor?.. tell me more...
 
With probes on q51 E and C, I am able to change voltage with trimmer vr51.

Same for q52 with vr52….trimmers work fine. About 2.7vdc each side…..and adjustable..
 
If I measure main output q61 ACROSS e+b, I get 0.14vdc….
Q63 ACROSS e+b…0.45vdc

All outputs/drivers are tested and confirmed good.
 
Well, the amp is in protection, so I guess protection is working.…..I will leave that alone.

I was to chicken……..bias DOES adjust! Both trimmers work…….should i set it to 16mV? I set to 12mV both sides for demo…
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Are you using a dim bulb tester while doing this work?
At times a bulb too low of wattage can cause some of the problems you are having.
 
I am on dbt 180watts (3x60watt bulbs.) I just adjusted the other side trimmers too…..dialled straight in. All 4 trimmers set at 14mv across their respective resistor. Is this ok for further testing?….I hate to connect to normal power and then fry something… :oops:
 
My dbt is showing very dim, 216vac and 0.23A draw. To low? Amp has been on a while now…..both channels bias adjusted and holding at 14mv.

More power?? I live in a 230vac country. Unit voltage selector is set at 240v. Only other possibilty is 220v.
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