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MF101 - modification for separate audio signal to env.fol.

Posted: Mon Mar 22, 2010 1:29 pm
by JimF
Hi everyone,

Sorry about the delay in getting this info up - I promised it several weeks ago.

The question was how to modify an MF101 to accept a second audio signal in to the envelope follower, so that the filtering of the main signal can be enveloped by a second audio signal.

I did this to my MF101 about 10 years ago, after sending a couple of e-mails to Big Briar, and receiving answers from Bob Moog himself with explicit directions. I no longer have the computer on which the e-mails were received, and I have no backups from that time. So the following is retyped from printouts of the e-mails. Points to note:
- these instructions apply to a Big Briar MF101, vintage 1999. I have no idea whether the board and circuit of the current version of the MF101 are the same.
- I haven't got around to taking photos of the guts of mine - sorry...
- these directions are provided under the motto "no responsibility taken" - if you wreck your MF101, that's your problem :-)
- BUT I know that the instructions work - my MF101 has been doing this happily since 2000, when I made the modification.

First e-mail from Bob Moog, 2 March 2000:
"If you're adept at desoldering resistors on circuit board with plated-through holes, you can lift the end of R5 and connect your external signal to it. If you have access to shop equipment, you can drill an extra hole on the back panel of the MF-101, mount a bridging jack in that hole, and then wire the jack up to the R5 part of the circuit."

Several months later, when I finally got around to doing this, I asked for more explicit instructions:
Second e-mail from Bob Moog, 5th October 2000:
"1. DO NOT REMOVE THE CIRCUIT BOARD. Remove the bottom panel and face the jacks away from you. Identify U2 (LM324A).
2. Immediately to the left of U2, starting at the top, are a) two diodes, b) a 270K resistor, c) a 100K resistor, and d) four more resistors. That 100K, right below the 270K, is R5.
3. Carefully unsolder the left end of R5, that is, the end away from U2. Apply your signal to that end of the resistor."

So that's it, folks. It's actually quite easy, and it works like a charm. Like everyone else on this forum, I'm eternally indebted to Bob Moog (may he rest in peace) for his generosity and willingness to help - as well as for the great instruments!

So, have fun!

Cheers,

Jim F.

Posted: Mon Mar 22, 2010 2:51 pm
by Just Me
Very cool, I'll have to see if it works on mine!

Posted: Mon Mar 22, 2010 4:28 pm
by latigid on
Great mod!

Posted: Mon Mar 22, 2010 6:25 pm
by EricK
SO it would function as the original if you multiplied your audio signal and sent it tothe mod and the audio in?

Interesting.

Posted: Mon Mar 22, 2010 6:53 pm
by DontBelievetheHype
Thanks for posting!
EricK wrote:SO it would function as the original if you multiplied your audio signal and sent it tothe mod and the audio in?

Interesting.
I think that it functions as normal when nothing is plugged into the new Key In.. I have/had a few filters with this option and this is how they functioned, and iirc JimF confirmed the same operation of his in the original thread.

Hopefully JimF or someone else with some actual circuit knowledge can confirm.

Posted: Mon Mar 22, 2010 8:32 pm
by Just Me
That is the reason for the shorting jack in the second email. With nothing plugged in, the resistor is back in circuit.

Posted: Mon Mar 22, 2010 11:28 pm
by ColorForm2113
Wow that's seems pretty easy actually. I don't have the equipment for drilling a new jack hole. And my soldering skills are moderate at best. But I will take it to my tech and have him check it out.

Posted: Tue Mar 23, 2010 11:23 am
by soundxplorer
Bob called it a "bridging" jack in his email, but it is also referred to as a "half normal" jack. Plugging a cable in lifts the "normal" connection. Unplugging the cable lets the metal bend back and make the internal connection. You'll need to use one of these to have it work that way.

Image

Posted: Tue Mar 23, 2010 4:06 pm
by varice
Thanks for sharing this great mod. I had thought about making this type of change to my MF-101 back in 2006 to interface it with my Nord Modular G2X, but I came up with a different solution:

http://electro-music.com/forum/topic-12266.html

Too bad this mod separates the Level LED from the filter audio input, but that’s a relatively small price to pay for greater functionality.

Re: MF101 - modification for separate audio signal to env.fo

Posted: Thu Apr 01, 2010 12:49 pm
by _DemonDan_
JimF wrote:If you're adept at desoldering resistors on circuit board with plated-through holes, you
can lift the end of R5 and connect your external signal to it. If you have access to shop
equipment, you can drill an extra hole on the back panel of the MF-101, mount a
bridging jack in that hole, and then wire the jack up to the R5 part of the circuit."
I don't know what Revision of circuit board Jim had in 2000, but the Rev. D board in
my recently purchased MF-101 is significantly different.

Can anyone definitively say what the new part ID is for that same resistor on a Rev. D
circuit board? (I'm not sure that it's R5 anymore.) And which end should I lift?

Re: MF101 - modification for separate audio signal to env.fo

Posted: Thu Apr 01, 2010 1:47 pm
by varice
_DemonDan_ wrote:
JimF wrote:If you're adept at desoldering resistors on circuit board with plated-through holes, you
can lift the end of R5 and connect your external signal to it. If you have access to shop
equipment, you can drill an extra hole on the back panel of the MF-101, mount a
bridging jack in that hole, and then wire the jack up to the R5 part of the circuit."
I don't know what Revision of circuit board Jim had in 2000, but the Rev. D board in
my recently purchased MF-101 is significantly different.

Can anyone definitively say what the new part ID is for that same resistor on a Rev. D
circuit board? (I'm not sure that it's R5 anymore.) And which end should I lift?
I would recommend contacting the Moog guys then.

My MF-101 board is marked P/N 11-350C and is layed out as described by Bob. The 100K R5 is connected to pin 3 of U2. The resistor leg not connected to U2 is the one to lift. But, on the D board, U2 might not be the same circuit design...

Re: MF101 - modification for separate audio signal to env.fo

Posted: Thu Nov 25, 2010 4:00 pm
by jeepo
I think this has already been brought up, but this mod would be great for the mf-107 too. Has anyone attempted such a thing?

Re: MF101 - modification for separate audio signal to env.fo

Posted: Thu Nov 25, 2010 6:09 pm
by latigid on
jeepo wrote:I think this has already been brought up, but this mod would be great for the mf-107 too. Has anyone attempted such a thing?
Yes :)

It's very cool, the signal at the audio input still interacts with the oscillator (FM, sync and envelope), while the CV signal controls the VCA. Immediately many more things become possible to do with the FB. You can even use the raw oscillator directly and have an ADSR envelope shaping the VCA; a real 'fooger synth!

Re: MF101 - modification for separate audio signal to env.fo

Posted: Thu Nov 25, 2010 6:28 pm
by jeepo
latigid on wrote:
jeepo wrote:I think this has already been brought up, but this mod would be great for the mf-107 too. Has anyone attempted such a thing?
Yes :)

It's very cool, the signal at the audio input still interacts with the oscillator (FM, sync and envelope), while the CV signal controls the VCA. Immediately many more things become possible to do with the FB. You can even use the raw oscillator directly and have an ADSR envelope shaping the VCA; a real 'fooger synth!
That's even better, exactly what I wanted actually.
Can you share the details of this mod?
Did you do it, or was it a factory mod?

Re: MF101 - modification for separate audio signal to env.fo

Posted: Fri Nov 26, 2010 1:26 am
by latigid on
Hi there,

I performed the mod myself. It requires just a few parts but will involve cutting a trace on the PCB. I got sent some directions, but "modified" :) them slightly to what I thought was a better method.

I'm not sure if I should give out the details. One condition on receiving the 104 Spillover Mod directions was that I kept the information private. I sent "rosser" at Moog my instructions, which I'm sure he'd pass on to you. I haven't asked, but if he says it's okay I'll post the directions on the forum. It isn't too much different from the 101 mod given above.


Cheers
Andy


P.S. Happy Thanksgiving! :)