Luxman L58a

[Chrisr3521]

Looking at restoring a Luxman L58a and when looking at the schematic I've found an adjustment pot in the psu that isn't mentioned in the adjustment procedure, anyone known what its doing? Please see attached pic

 

[PMik]

Meassure voltage of the fixed -B1 output .., then adjust +B1 to the same value .. done deal

 

[rayma]

That pot sets the exact output DC voltage of the positive power supply.

 

[llwhtt]

Like Rayma said, sets the +B1 and the -B1 should follow, -B tracks +B. The $10,000 question, what is +B supposed to be?

Craig

 

[rayma]

Good question, there's no indication of that in the service manual.

 

[BSST]

Based upon the values in the schematic, I estimate nominal voltage as 23.9V. So, guessing 24V might be a nominal a setting? The 2.2K pot has modest adjustment range re R813 (33k) so the voltage should be within a few percent of 24V.

Estimates:

Q805 base voltage must be about 16.6V, base current assumed neglible.
Pot resistance 1.1k at center rotation, so current through pot and R813 is about 16.6V/34.1k = 0.487mA
So voltage across R612 is 0.487mA*15k = 7.31V
So, regulator output is 16.6V+7.31V=23.91V

 

[mandu]

Looks like that pot should be used to set the positive supply same as the negative supply.
(Fine adjustment).
The opamps if any will have the minimum offset if direct coupled (guessing).
Regards.

 

[llwhtt]

It's a tracking regulator, set the positive voltage and the negative tracks the positive.

 

[mandu]

The service manual says +/-32 volts for the preamp.
Regards.