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Technics KN800 MIDI keyboard no sound

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.

Reply to
Geoff C
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I've never seen the inside of a technics board, but I do frequently see synths with a "protect relay" that keeps the amplifier isolated until the digital circuitry has stabilized. They will click, but their contacts get oxidized and don't transmit audio.

You can temporarily rejuvenate them by cycling them a few hundred times (try connecting a function generator to the transistor that drives it for a few seconds/minutes), but for the amount of work involved I just replace them.

I've also seen volume c> I was given a Technics SX-KN800 keyboard and am trying to repair it. I ha=

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Reply to
stickyfox

" snipped-for-privacy@gmail.com" wrote in news: snipped-for-privacy@x25g2000yqj.googlegroups.com:

I suppose the isolating relay would be on the power amp baord if it existed, but I can't see it. I'll have another look for it though. I have been using a scope for signal tracing, but your tracer idea has its merits too, since it will be very quick.

Also, I have read the manual for the next model up which mentions a method of resetting to factory defaults, and also a MIDI out only mode. I'll try these too.

Reply to
Geoff C

Hi!

Such a relay may not exist. If this thing's amplifier is anything like the Technics stereo receivers from the same time frame, it may turn the output on and off electronically. (An interesting aside: this electronic protection method won't save your speakers if the amplifier itself fails!)

If you do find a relay, I'll practically guarantee it has burned contacts. It seems that people didn't think about turning these off with a signal still being amplified, which probably resulted in arcing at the relay contact points. In all of my receivers that have one (an SA-929 and 560) the protection relay can be opened without too much violence and the contacts cleaned. I did break the magnet wire on the SA-560's relay, but that's another story.

William

Reply to
William R. Walsh

Geoff C wrote in news:Xns9DE733AE1C5EEGeoffC@188.40.43.230:

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.

Reply to
GeoffC

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