[sdiy] Kurzweil K2000 Query continued

[paul brooks]

The protection FET's are a decent guess, but keep in mind that they were an engineering change during the Calvin era of production K2000's.

If it is a Calvin machine, it will not have the output protection FET's located on the analog board.  They will be either on a thin daughter board located along the heat sink near each output jack, or not included at all.

It is still quite worth while to check them, if they exist.  It should be an easy check - one leg of the FET will have the full signal, and the others won't... or they will simply not exist.  make sure the connections are all sound, of course.

Also, the effects / dry mix is accomplished via SSM VCA's.  check the output of those while varying the mix amount.  

Other than that, it should be standard troubleshooting as far as digital synths go - look at your DAC's output, and trace the circuit from there.  looking at every step on the way, you should see a drop - or else it's on the digital side of things.  The DAC's themselves are pretty standard, and have data sheets on line.  Same with the SSM chips, and of course the opamps... the schematics would help, but once you're looking at the right part of the board, just following the traces might be faster anyway.  

Sorry if the advice sounds trite - i haven't read the whole thread... but i've repaired lots of K2000's, and found myself rarely referring to the service manual for schematics.  Though it's a great read...

--- On Mon, 7/13/09, John Loffink <jloffink at austin.rr.com> wrote:

> From: John Loffink <jloffink at austin.rr.com>
> Subject: Re: [sdiy] Kurzweil K2000 Query continued
> To: "DTK" <aurastar at cox.net>
> Cc: "Synth DIY" <synth-diy at dropmix.xs4all.nl>
> Date: Monday, July 13, 2009, 9:48 PM
> Kurzweil K2000 have FETs in the
> output circuitry to mute them when powering off, so you
> don't get audible pops on your speakers.  Since both
> channels have failed, it sounds like the circuitry that
> controls the mute has failed.  The control circuitry is
> a few transistors and resistors, if I recall
> correctly.  It detects a dip in the voltage rails as in
> a power off situation and then turns on the FETs to a low
> resistance path to ground, muting the signals.
> 
> Look for three terminal transistors connected to the output
> signals.  Then track the gate signals for these FETs to
> find the control circuitry. 
> John Loffink
> The Microtonal Synthesis Web Site
> http://www.microtonal-synthesis.com
> The Wavemakers Synthesizer Web Site
> http://www.wavemakers-synth.com
> 
> 
> 
> DTK wrote:
> > A bit more information on this:
> > 
> > First, for those who suggested changing the
> batteries:  There are actually no batteries in this
> machine.  The owner uses the synth as a controller, and
> for stock sounds.  According to him, he removed the
> batteries long ago.
> > 
> > The volume loss has continued, but there is still no
> noticeable change in sound quality.  I'm thinking that
> maybe something is going out in the final output
> stage.  Anyone have any suggestions?  I'll be
> googling for schematics momentarily, but if you know where I
> can get them, or the service manual, feel free to chime in.
> > 
> > Thanks
> > 
> > 
> > 
> >   
> >> I'm taking a look at a K2000 for a friend. 
> It's exhibiting a strange
> >> problem... at least, I think it's a problem. 
> If I hadn't been there when it
> >> happened, I might have thought it was nothing.
> >> 
> >> We were playing, and suddenly the K2000 lost
> volume...  It wasn't a dramatic
> >> loss, the patch was still audible, but there was
> *definitely* a loss in
> >> volume.  The patch wasn't changed or edited,
> mixer settings weren't changed,
> >> etc.  It just got quieter all of a sudden.
> >> 
> >> At first we thought it might be some sort of
> mechanical failure in the
> >> output jack... A solder joint on the way out, or a
> worn out center
> >> connection.  However, that doesn't seem to be
> the case.  The problem
> >> persists on the L, R, and Headphone jacks, and on
> different patches as well.
> >> 
> >> I'm a bit puzzled by this one, frankly. 
> There's no difference in the audio
> >> quality, which leads me to think that the output
> circuitry is ok.  Anyone
> >> have any thoughts?  Suggestions?
> >> 
> >>     
> > 
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