[sdiy] Korg 700S downgraded itself to a 700

[Mikko Kuorelahti]

Hello, and nice to see this list back in business.

I sent this message a couple of months ago, but as  I didn't find it
in the archive there must be some problem with the list. I'll try
again, in plain text this time.

A couple of months ago the "effect" section on our bands Korg 700S
stopped functioning. I'm a self-learned electronics hobbyist and I
would be very grateful for any help on this matter. As our band lives
all over Finland and usually I see the synth just on gigs, I rarely
have a chance of dismantling the beauty in peace. So my obvious choice
is to gather as much information as possible before the repair
session, so that I don't have to do any unnecessary work when I find
the time to fix our beloved "Signal", which has become an important
element in many of our songs (yes, it has a name).

 The "Noise 2" setting still works, so my first guess was that the
second oscillator (TS-200011 chip on the "SB-027" board) had popped.
But after looking at the schematics, I started wondering about the
"Noise 1" setting (VCO1 modulated with pink noise) not working. To me
it seems that the ringmodulator on the board doesn't involve the IC at
all, which got me thinking about the connections between the PSU and
the "SB-027" board. Another possibility would be the capacitors on the
ringmodulator, but in that case the "Duet" mode should work. Are my
assumptions correct or am I missing something here?

Of course it would be wonderful to find out that the chip still works,
and that the problem is either in the ringmodulator area of the board
or the PSU lines. But I'm getting prepared for the worst. If the
problem turns out to be the VCO chip, I will solder up a replacement
linear VCO from scratch (which has to withstand +20Vcc). If this turns
out to be the case, I will probably need some help with it. And
hopefully get some from you guys/gals.

http://www.cykong.com/Synths/Korg%20700S/Images/Mini-Korg-700S-Additional-Schematics-Coloured.gif

I'll report back after I get the chance to measure things. I'm opening
the thing as I type.


Mikko