[sdiy] MG-1 How to get -12v from TIP30 and 741?

[Metrophage]

--- synth1 at airmail.net wrote:
> The -12V is "divided down" (up??!?) by the 2 resistors. The 741 is
> just an follower/buffer, to provide a "current boost" from the 3
resistor
> divider.

Cool, I figured that out as I read your post, indicating that I should
read -12v at pin 3 of the 741.

> -11V is because the TIP30 is 1 base-emitter junction "drop below"
> -12V, with is *about* -11V but probably more like -11.3V.

Weird. I (hopefully!) learn something new every day!

Indeed it was the 741 which was fried! Since the TIP30 and divider
resistors were ok, that left the 741. I pulled U12 off of the control
PCB (replaced with TL071) and used it to replace the fried 741. This
has fixed the main problem which made the synth unusable - the minimal
output volume! I had missed the connection of the -11v points, which
had caused me much confusion.

With -12v, the sharp/flat and detune pots now work as they are supposed
to! The filter still does not work, even with its -12v. I expect that
going through my checklist of filter voltage test points again I will
likely get this working as well. I am also going to try to make better
heatsinks for the voltage regulators, as they are burning hot.

I am getting excited about this synth again!

CJ


		
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