Vintage Synth Explorer Forums

Realistic Mg1 LFO/ENV to VCO2 mod
Found in the yahoo group by an anonymous poster

Mp3 Demo



Please note, I have performed this mod to my mg1 and it does work. however there are some quirks that I haven't figured out yet (those quirks mentioned after these directions.

If this mod interests you, or perhaps you have some input to improve it, please post it here.


"Just thought I would share this mod here while my MG-1 is still open
and the mod is fresh in my head. It sounds great and I encourage you
to try! Its easy to reverse too if you don't like it...

The idea is to route the lfo or env to tone source two's (osc2) detune
amount. It sounds really cool when you turn the osc sync on and have
either the lfo or env modulate the detune - you can get amazing thick
pads with the lfo, or biting lead sounds using the env.
You can also leave the sync off, and by routing just a little
modulation to osc2 detune, you get some nice thickness and dynamic
quality.

This mod was inspired by the moog rogue, which has a 3rd sync option
that makes the sync "contoured". The rougue lets you route the env to
the detune amount. My mod does this and adds the option of lfo
modulation. I don't have access to a moog rogue to compare, but it
must sound similar.

Now don't be scared, get out the schematics, run over my instructions,
experiment a bit with resistors etc, to see if you can improve on
this, warm up your soldering iron and do it! Life is too short to
leave your moog as it is.

INSTRUCTIONS:
What you'll need: wire, a 220ohm resistor, a 10k linear pot, 2
switches. (Simple 2-position switches with 3 legs - are these called
SPDT?)

Envelope Source:
You need a wire going to the common terminal of R138 and U13B. This is
where you tap the envelope ("contour"). From the component side of the
board, I tapped it on the "east" side of R138.

LFO Source:
To get the LFO signal, you can tap from the common junction of R105
and R71. From the underside of the board, I tapped this from the North
pin of R71. R71 is the pot in the modulation section labelled "Tone
Sources".

Detune Destination:
The common junction of R29 and SW3 provides the input for this mod.
This is where you will be routing the signal coming from either the
LFO or Env. I soldered a wire to the West pin of R29 to provide this
access point.

Use some wire to connect Envelope Source to the first leg of a switch,
switch#1. Connect LFO Source the the third leg of the same switch. Now
connect the middle leg of that switch to the middle leg of your 10k
linear pot. That switch will now select the modulation source, LFO or
Env. Connect the first leg of the pot to ground (I'll leave that up to
you to find ground on the board) and the 3rd leg of the pot you can
connect to your 220ohm resistor. The pot now controls the modulation
amount. Connect the other side of that resistor to the first leg of
switch#2. Connect the middle leg of switch#2 to the Detune
Destination. Leave the 3rd leg of Switch#2 unconnected. Now switch #2
turns this mod on or off, by either connecting the modulation source
to detune, or leaving it unconnected just like your MG-1 was before
you started this mod.

By the way, I'm not an engineer or an expert of any kind, so if anyone
sees a mistake or a way to improve this mod, please pass your thoughts
along.

Happy Modding!"

This mod was overall functional.

Pros: With sync on and ENV Mod selected you can get medium sync sweeps. With LFO mod on this can make really thick PWM sounding patches. Can also produce brand new timbres when using the Bell Tone level.

Cons: The Mod Amt knob I installed on the side panel is a 10k linear pot. This pot seems to work ok, but the range of the mod seems limited. There is no effect until about 10o'clock. After 10o'clock the effect is a bit too much for my liking. This limits the ability, without very fine tuning of the knob, to get a beating sound between the oscillators. Past 12o'clock the mod doesnt seem to go high enough for those really drastic sync sweeps.

Does anyone have any input as to what might be causing this lack of amount in the knob? Has anyone tried this out on their MG?

Cheers

Last year I rebuilt a MG-1 and was toying with a similar mod. I posted about it and many members thought a mod might lower the resale value, so I ditched the idea. Let me see if I can find my notes and drawings. IIRC I had to use two op-amp buffers to prevent loading down the other circuits.

I looked at the moog rogue and MG1 schematics side-by-side and going to mod my MG-1 to replicate the circuit from the rogue. All it will take is a 6.8k resistor & a spdt switch. Need to lift one side of a resistor on one leg of the sync switch in order to add the countered sync option. Gonna look at it tonight if I have the parts on hand.