[sdiy] Concertmate MG-1 mods/schematics

[KA4HJH]

>Hello from the new guy :)

Oh no, not another one...  8^D


>I've got an MG-1 that I found at a pawn shop and resurrected. Man, do
>I wish I had found this group about 6 months ago! :) Anyway, it's now
>in good working order, and I've made several mods to it, installed a
>MIDIJack, etc.

At least it's working. Did you get all the decomposed foam rubber scraped off?


>For my next mod, I've decided that I want to try and slow down the
>LFO rate. If I understand correctly, the LFO in this machine is just
>another VCO.

It's a simple triangle/square manually-controlled oscillator.


>There's a slider type variable resistor for the rate
>control, where lfo rate increases as resistance decreases. SO I was
>thinking that I could just insert an extra resistor in line and that
>would slow things down.
>
>Of course not... that would be too easy, wouldn't it? :) I tried it
>out, soldered a 22k resistor in at a couple of different points, but
>there was no change (as far as I could tell, anyway)

Way too small a resistor. The frequency pot is 1M ohm, audio taper. You'd
need another megaohm to cut the frequency in half.

A simpler approach would be to increase the capacitance of C25 (6.8uF
@16V). All you have to do is put another cap in parallel and switch it
in/out with a SPST switch.


>Is anyone here familiar enough with this little quasi-moog to help me
>out? I'm really not even sure if I have the right idea or not. I've
>been using schematics for the Rogue, which is (supposedly) nearly
>identical in design. I've been unable to find any schematics or
>technical information about the MG-1 online, so if anyone knows where
>I can find anything, please let me know.

Some schematic scans are here:

http://world.conk.com/world/indridcold/synthfrm.htm
-- 

Terry Bowman, KA4HJH
"The Mac Doctor"

"By the end of Chuck Statler's 'Rock Videos' of Devo we agreed that even if
Devo did not take the stage it was still the best concert any of us had
ever attended." --Kim Thayil (Soundgardern), 1995