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

[Michael -nil-x]

I did use a summing resistor already. 15k if I recall is what worked 
best for the eg sweep of vco2 - I have no plans on adding the eg to vco1.
The offset is not -.5v it is +.5v but on measuring, the EG only goes to 
+5.2v (read with my voltmeter, with sustain on)

I suspect a bad 4558 opamp as the output on pin 3 on U13A is the same 
voltage. +5.2v to +.5v at rest which is exactly the same as pin 7 and at 
pin 3 there is the same +.5v offset.
Is the 4558 compatible with a tl072? I see the pinout is the same from 
the datasheets.

- Michael nil-x
- Pressure : Penetration
Music: http://www.pressurepenetration.com/
Photography: http://nil.pressurepenetration.com/

WeAreAs1 at aol.com wrote:
> In a message dated 7/2/05 1:34:27 PM, philgallo at attglobal.net writes:
> 
> << 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. >>
> 
> Michael,
> 
> Phil is absolutely right about this.  You didn't ever mention if you had 
> installed any sort of series summing resistor between your EG output and the CV 
> input at your VCO expo amp.  I just assumed that you did add a summing resistor, 
> because, well, that's what you're supposed to do.  Such a resistor is 
> definitely needed.  If you don't have this resistor, it can result in some strange 
> and unwanted interaction between your EG, your VCO, and even your VCF -- exactly 
> the kind of interaction you have been observing.
> 
> If you don't already have the resistor, you should remove the new diode, 
> install the summing resistor as per Phil's suggestion (either at pin 2 of U4 or at 
> pin 2 of U8, depending on whether you want to modulate just one or both of 
> the VCO's), and ignore all the other previous advice I gave!
> 
> Michael B.
>