On Friday, November 29, 2013 9:25:55 AM UTC-6, N_Cook wrote:
On 28/11/2013 23:03, wrote:

You can see the service manual he http://manuals.harman.com/hk/Service...ion22%20sm.pdf



The problem is no audio from the right channel. The problem occurred after a lightning-induced power surge so I assume that was the cause. I expected to find bad output transistors but that was not the case. I believe the problem is not related to the protection circuit as I removed Q402 and disconnected 2 leads from Q418 (there is a brief description of the protection circuit on page 4 of the manual).



With an audio signal connected, I can see the the signal at Q404 & Q412 matching the left channel. At Q406 & Q458 the signal is gone.



Measuring some voltages I found Q420 to have -81V (E), -80V (B), +81V (C) compared to -6.7V (E), -6.1V (B), -6.4V (C) for Q419 on the left channel.. Also, the +/-1.2V on the collectors of Q422, Q426, Q424, & Q428 is not there and I cannot figure out where it is supposed to come from. Note that the rail voltages measure about 80V as opposed to the ~58V shown on the print (that is only for the Japan model I think).



Suggestions?





would suspect FETs first as they seem to hate high dV/dt the most

Thanks for the suggestions. I had started looking at this a while ago and put it to the side. It turns out that I had removed one side of a jumper wire to make a resistance measurement or something and it was still disconnected. Replacing the connection brought the voltages back to normal and brought the audio back. It was Q402 that was bad.