Geoff C wrote in
:

I was given a Technics SX-KN800 keyboard and am trying to repair it. I
have opened it up and the six or so PCB's are easy to get at. The
symptom is no audio. The PA thumps through the speakers on turn off
and the PA has power supply OK so it is not a concern at the moment.
The line out has no audio either. Of course it is possible that a
crash has occured but I cannot find a reset so far. No audio is
getting to the PA. I could not find any signal at the master volume
either, though it may not have real signal at it.

I believe it was made early to mid 90's. There are a lot of Technics
proprietery chips in it. I have an op manual for a KN1000 which is
similar, but no schematics appear anywhere I have seen.

What I would like to know is a general question about how the audio
signal is generated from the PCM pulse stream. It may be a simple LPF
from a digital chip which regenerates the audio, or there may be a
specialised audio chip to look for? If I can't go much further I will
have to junk it.


I have fixed it so I thought I'd follow up the fix. There was no -15 volt
rail due to a 4.7 ohm fusible resistor on the main board being open. This
was adjacent to the connector which received the power supplies from the
power amp/power supply board.