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Bias and offset on Proton 550/950

Nat

Nat

AK Subscriber
Subscriber
Anyone know how to set bias and offset on the Proton 550 and 950? There is a service manual at HiFi Engine, but the copy is defective and this information didn't get scanned - one edge of it is visible, but none of the actual content.
 
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I'll have to find it and report back. I'm having breakfast, not looking at the thing.
I'm a capable solderer, know some theory and when the manual has clear instructions, I can do both bias and offset. B I am neither experienced or confident with electronics, so forgive my basic questions. Am I measuring the voltage drop or to ground?
 
stoN cold - so far not able to identify R648. While looking, I did find R676 which looks like it is overheating, so guess I will replace it.
 
I think I've found 648 - it is one of the pair in the sandcast block. 6.3 millivolts across it, and 6 across the corresponding resistor on the other channel.
I have gotten the offset down to close to 0 - pots are fidgety so not a smooth procedure. But haven't done idle current/bias because I have yet to find the test points that both Proton and NAD blithely refer to as if obvious. The test points may be identified on the foil side - there is some confusing reference to the foil. The location in the diagram may coincide with one of the three pins on the sandcast thing.
 
Nat said:
I think I've found 648 - it is one of the pair in the sandcast block. 6.3 millivolts across it, and 6 across the corresponding resistor on the other channel.
I have gotten the offset down to close to 0 - pots are fidgety so not a smooth procedure. But haven't done idle current/bias because I have yet to find the test points that both Proton and NAD blithely refer to as if obvious. The test points may be identified on the foil side - there is some confusing reference to the foil. The location in the diagram may coincide with one of the three pins on the sandcast thing.
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I think you can adjust the pots from the foil side, trough small holes in the circuit board. There should be a page in the service manual that points out the access holes.
 
Interesting - I'd noticed holes but had not connected them with the test points. If you are right, that is an enormous help. Thank you very much.
 
Well bummer - the holes are only access to the bottom plate - not anything of the test point variety.
 
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