[sdiy] MG-1 Mods (patching it out)

[phillip m gallo]

Good observation! The schematic indeed indicates a 0-8V level is to be
expected from envelope buffer at pin 7.

The loading effect noticed on the envelope  drive to the VCF maybe an
indication that you aren't using a series resistor (but feeding the envelope
directly) to the passive summing network at the input to the VCO expo.  A
series value of approx. 5.1K would maintain close to 1V/8ve response.

If you want to drive both VCO's you can input the envelope (with perhaps a %
of modulation attenuating pot) to the input of the control voltage summer
for the VCOs at U8.  A 10k resister in series between the envelope  (or the
output of it's attenuator pot) and pin 2 would do the trick.

As to the -.5V offset if you have an o'scope with an X10 probe, i'd do some
investigating first, comparing noted voltages with "found" voltages.  No
point replacing the envelope buffer chip if the -.5V is found at C23.

regards,
p



-----Original Message-----
From: owner-synth-diy at dropmix.xs4all.nl
[mailto:owner-synth-diy at dropmix.xs4all.nl]On Behalf Of WeAreAs1 at aol.com
Sent: Saturday, July 02, 2005 12:09 PM
To: nil at pressurepenetration.com; synth-diy at dropmix.xs4all.nl
Subject: Re: [sdiy] MG-1 Mods (patching it out)



In a message dated 7/2/05 11:20:45 AM, nil at pressurepenetration.com writes:

<< Ok, so I tried the diode and it kinda worked. The OSC doesn't go sharp

when the EG is at rest, but it pulls down the eg amount to the VCF.

I will have to use an opamp and have a buffered output. Not a big deal

as I was going to have a board on the side for analog noise source

anyways and another EG (ADSR). >>

Hello Michael,

I looked at the schematic, and it right next to U13B, it says that the EG
output is supposed to go from 0 volts to +8 volts.  They don't mention
anything
about it starting at 0.5 volts.  This makes me think that there might be
something wrong with your envelope generator circuit.  I think you should
try to
sort that out before you go any further in your attempt to modify it to work
the
way you want.  It's a fairly simple circuit with only a couple of active
components (U14 - the 4007 CMOS IC, and U13 - the 4558 dual opamp).  For
now,
remove that new diode and try replacing the opamp, then the 4007, and see if
things
start working better.

If it turns out that EG is in fact working properly, and that the diode is
still needed, then you can quickly fix your other new problem (the low VCF
EG
amount) by simply changing one resistor value on the existing circuit.  R114
(39K) determines the VCF EG sensitivity.  You can find R114 near the bottom
center of the schematic page, just below the filter circuit, next to the
Filter EG
amount pot (labeled "Contoured Cutoff").  If you change it to a smaller
value,
you will get higher sensitivity, and the VCF will have a higher peak
response
to the EG.  Start with a value of 33K, and if that's not enough, then go
smaller a bit at a time (27K, 22K, etc.).  Those are the standard resistor
values
that are available in that range, so you may need to put two resistors in
series in order to get the exact response you are after (or choose from some
more
exacting 1% resistors, if you have some).  Changing this resistor will get
you
the response you want.

Michael B.
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