harman kardon 800+ DC offset?

[savv]

Does anyone have instructions on how to adjust the trim pots for DC offset? I am getting a reading of 63 on one channel.

Any help is appreciated.

 

[tcdriver]

Does anyone have instructions on how to adjust the trim pots for DC offset? I am getting a reading of 63 on one channel.

I assume that is 63mV. Looking at the schematic of my Harman Kardon 900+, which is very similar to your 800+, I see that there is no offset adjustment. The pot you see on the amplifier board is for bias adjustment. A high offset either means you have some leaky capacitors or the input differential pair of transistors are not well matched. The fix is to replace any leaky capacitors and/or get a matched set of input transistors.

 

[ecluser]

There is definitely no offset adjustment on the 800+

The caps to check for leakage are the 220 uF /16V , two per channel.

The input differential transistors are Qx41 and Qx42, 2SC640L, 50V NPN high gain (400 to 600) They must be matched. They are side by side on the board.

For bias, adjust VRx41, for 5.5mV across emitter of Qx47 and collector of Qx48. x refers to 3,4,5,6 according to the channel to adjust.