Sansui AU-X501, protection problem

[jimakos]

Hi to all.
After a repair in my Sansui AU-X501, I try to adjust the Bias and DC 0V.
During this adjustment the protection activated and form then is always on.
I check everything in the protection circuit and I can't find the problem.
The amplifier works fine. I bridge the protection relay contacts and sound is clear and loud.

 

[patrick101]

Maybe your output DC is a little bit high and trigger the protection circuit.

You can check the output DC and power transistor bias current before the protection circuit ok or not.

 

[redrooster]

Could be your Bias and/or Offset Variable Resistors have gone flaky and need changing? Ive had the same problem a few times now so I always change the Variable Resistors to sealed 25 turn types.

 

[jimakos]

patrick101
Output Offset is 0.05 ~ 0.10 mV
I have check it and before the relay. I think it is OK.

redrooster
Do you suggest me to replace the variable resistors;
The service manual marked as "semi-variable resistor".
It is a problem? Can I use a trimmer?

 

[sgrossklass]

Could be your Bias and/or Offset Variable Resistors have gone flaky and need changing? Ive had the same problem a few times now so I always change the Variable Resistors to sealed 25 turn types.

Good point, old pots can be a bit crusty and may appreciate a bit of isopropyl alcohol at least (or suitable contact cleaner, like DeOxit D5 or similar). Swapping them out is not usually necessary in a well-designed circuit, unless they are literally falling apart (the Pihers in old Grundig amps are doing that at times).

My prime candidate would be the offset pot. Maybe do a sanity check of its operation with the multimeter.

EDIT:

patrick101
Output Offset is 0.05 ~ 0.10 mV
I have check it and before the relay. I think it is OK.

Hmm. Please measure the voltage across each of the 0.22 ohm resistors. Anything more than 10-15 mV to be seen here?

If so, check resistor values to the best of your abilities (measuring sub-1 ohm resistors can be tricky on a basic multimeter, but compare a few to be sure).

If not, something in the protection or relay driver circuit may have chosen an unlucky time to pack it in.

 

[jimakos]

++++ PROBLEM SLOVED +++++++
Was a open resistor in protection circuit.

Thanks ALL for help !!!!!

 

[Minemas]

Hey, I´m new to the forum and have the same issue.

In the moment I moved the DC-Offset poti the protection went on and is now in this state permanently.
I checked all the resistors in the protection section off the board, on the basis of jimakos problem, but they all seem ok.






Any other ideas what the problem might be?