Biasing a KA-7100

[marc brown]

I've got a KA-7100 that needs diagnosis. The DC offset floats and when biasing the left channel, the voltage is always 0mv. When I run a sine wave into it and check the output with an o-scope, the bottom half of the waveform is truncated. The left channel clips far earlier than the right channel. Does this sound like a bad pre-driver or resistor?

Thanks in advance

 

[EchoWars]

Look at the output of the preamp (the wires are right there at the top of the preamp board) to determine if the root cause of the clipped channel is happening in the preamp or amp.

 

[marc brown]

Thanks EW. I went through all the pots and switches with Deoxit then went through with the oscilloscope probe checking the waveform at different points on the board.

I narrowed things down to the Q25 pre-driver on the amp board. To be sure I desoldered Q25 and Q26 so I could switch them and confirm my suspicions. After resoldering them in their new homes, the waveforms in both channels looked good.

I'm still having a hard time biasing the channels. I'm not showing any voltage at the biasing checkpoints.

 

[EchoWars]

Grab the manual, start checking voltages. Confirm that the bias pot is actually functional. Check that the pins you are connecting to are clean. Try connecting to the emitter resistors instead of the test points. Maybe the bias servo transistors (Qe13/14) are flaky? Seems unlikely that both channels might suffer the same fate.

 

[marc brown]

Grab the manual, start checking voltages. Confirm that the bias pot is actually functional. Check that the pins you are connecting to are clean. Try connecting to the emitter resistors instead of the test points. Maybe the bias servo transistors (Qe13/14) are flaky? Seems unlikely that both channels might suffer the same fate.

Thanks for tips for the next step.

When I last tried biasing, I had the speaker outs connected to 8ohm dummy loads. I used my o-scope to analyze the output waveform. When the amp began to distort (as visible in the waveform), turning the biasing pots fully counterclockwise did eliminate a flat spot at the peak of the sine wave.