CONCERTMATE MG-1 LFO RATE MOD

***PURPOSE***

This document contains instructions for a modification to the 
Concertmate MG-
1 synth. This mod gives the user a choice of 3 speed ranges for the lfo 
rate: 
normal, 1/2 speed, and 1/4 speed (approximately). It requires soldering 
(what 
good mod doesn't? *G*) and drilling a hole to mount a switch.  This 
document 
is written specifically for the MG-1, but this mod should work on any 
synth 
that uses an R/C circuit to control LFO rate... about 98% of the 
analogs out 
there do.

I don't have any way to make schematics or take pictures, but this is a 
pretty easy mod and I don't think you'll need anything like that.

I've been told that I explain things to death... So I'll be giving you 
a lot 
of information that you may not even need. Also, please forgive me for 
writing this like a lab report...it's a format I'm used to using :)

***MATERIALS***

Go to Radio Shack (or your parts supplier of choice) and get the 
following:

spdt center off toggle switch
10uf capacitor 
22uf capacitor
wire (if you don't have any just lying around)

::notes::
-- Both caps should be electrolytic, polarized, and 
   rated for at least 16v!
-- The installation of the switch will be easier if you
   select one that can be mounted in a round (drilled) hole  

***BACKGROUND***

The MG-1 (and many many other analogs) uses a resistor/capacitor 
circuit to 
control the lfo rate. The idea here is to set up our new circuit so 
that when 
the switch is in an "on" position (ie, not in the center) you connect 
another 
capacitor in parallel with the existing one. This increases the 
capacitance 
of the circuit, thereby increasing how long it takes to 
charge/discharge, 
which slows down the LFO. 

2 notes are relevant at this point:

1) The total capacitance of multiple caps in parallel is the sum of 
   their values
2) The relationship of total capacitance to lfo rate is inversely 
   proportional: if you double the capacitance of the circuit, the 
   lfo rate is reduced by 50%

The size of the capacitors can vary depending on how much you want to 
slow 
the lfo down. I chose the 10uf and 22uf caps because they were the 
closest 
readily available values for what I wanted to do. The value of the 
original 
cap in the MG is 6.8uf. When the 10uf is added to the circuit in 
parallel, 
the total capacitance is 16.8uf, which reduces the speed to 41% of 
normal. 
When the 22uf is added, the total is 28.8uf, which gives a speed of 24% 
of 
normal. If you really wanted to be anal about it, I guess you could 
special 
order caps to get 50% and 25%, but I'm not into all that :) 

***PROCEDURE***

- UNPLUG THE SYNTH!! You'll be working around the power supply a
    bit, and trust me... it will shock the bejeezus out of you if you 
    catch it wrong :)

- Open up the MG and remove the top circuit board (where all the
    sliders and knobs are located). If your MG-1 still has the black
    foam/goo on this board REMOVE IT NOW. Scrape off as much of it
    as you possibly can. Be advised, it's a messy task.

- Locate capacitor C25 on the board you just removed. It is located a
    bit to the left of the Auto Contour Trigger switch. This is the
    discharge/integrator cap for the LFO circuit.

- Solder a wire to the negative (-) end of C25. Solder the other end 
    of this wire to the center contact on the spdt switch. Make sure
    you have enough wire to reach your intended mounting location
    for the switch! Insulate the connection with electrical tape or 
    heat shrink tubing.

- Solder the negative(-) end of one of the capacitors to either of 
    the open contacts on the switch. 

- Solder the negative(-) end of the second cap to the remaining open
    contact. Insulate the connections with electrical tape or 
    heat shrink tubing. 

- Solder a wire to the positive(+) side of C25. Solder the other end 
    of this wire to the positive lead on BOTH caps at the switch. 
    Insulate the connections with electrical tape or heat shrink
    tubing. 

That's all the connections that need to be made. Check for shorts and 
bad 
connections and put the top circuit board back into place. Then give 
the 
synth a test run to make sure the mod works as it should.

The final step is to mount the switch. The location is really a matter 
of 
personal preference. I was going to put my switch right next to the 
rate 
control slider, but I wasn't sure if there was enough room there to 
install 
the switch without having it touch the circuit board. I ended up 
mounting it 
on the flat ledge to the left side of the keyboard. There's plenty of 
clearance underneath, and it's not used for anything else, so it made 
an 
ideal location. The switch is out of the way, but easily accessible. 
Just use 
a low speed drill so you don't burn the plastic.

***CONCLUSION***

Re-assemble your MG-1, and bwoooooooOOOOOOOOOOOOoooooooooowwwwwwww....  
:)

Enjoy!


!!!!-- Perform this mod at your own risk! This document is an accurate 
!!!!-- representation of the procedure I used when I preformed this mod 
on 
!!!!-- my MG-1. Mine works like a charm. I will not be held liable if 
yours 
!!!!-- doesn't, if you smoke your board, bla bla etc 
!!!!--           IF YOU DON'T UNDERSTAND IT, DON'T ATTEMPT IT!

Please send comments, questions, or criticisms to aurastar@mailcity.com
