Technics KN800 MIDI keyboard no sound

[Geoff C]

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.

[stic...@gmail.com]

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 controls open up. My usual approach is to strip
an RCA cord going to an amp, ground the shield, and connect a resistor
to the center pin, then poke around on the board listening for the
audio. Then I follow it until the sound goes away. If that doesn't get
me anywhere in 20 minutes or so I'll start looking in earnest for a
schematic.

[Geoff C]

"stic...@gmail.com" <stic...@gmail.com> wrote in
news:d3b060ed-09e5-4166...@x25g2000yqj.googlegroups.com:

> 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 controls open up. My usual approach is to strip
> an RCA cord going to an amp, ground the shield, and connect a resistor
> to the center pin, then poke around on the board listening for the
> audio. Then I follow it until the sound goes away. If that doesn't get
> me anywhere in 20 minutes or so I'll start looking in earnest for a
> schematic.
>

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.

[William R. Walsh]

Hi!

> 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.

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

[GeoffC]

Geoff C <n...@mail.com> wrote in
news:Xns9DE733AE...@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.