[sdiy] Differences between MG-1 filters and broken stuff

[phillip m gallo]

CJ,

Well the MG-1 implements in common parts so if something is popped (other
than the TOG/Dividers) you aren't in a lot trouble other than the cost of
additional effort and perhaps some "popcorn" parts.

Once you've had your morning coffee i would start with the assumption the
new pot's are the problem and the most likely fault would be "interchanged
values".

You indicate the VCA is working, how do you know this?   All VCA propagated
signals traverse the VCF so is it that you "hear" the VCA working, albeit
without input signal (changing noise amplitude?).

When you cause the VCF to resonate does it follow the keyboard when the
tracking switch is "on" (either 1/2 or full)?  If resonation occurs across a
very small portion of the Fz range "tweak" R93 to give yourself a little
more range.  If 8ve's are not accurate tweak R113 until they are.

If you have a 'scope, while resonating, monitor the output U11-#6 (VCA
output) ensure the waveform is symmetrical around ground.  With R102 removed
the MG should "roar", if the max volume is not a lot louder (and the pk/pk
voltage 10x greater) then it's likely this pot was changed and it's value is
not 10k but much less.

R110 is a trim to "center" the operation of the front panel Filter Cutoff
control.  Tweaking this trimmer you can 1) adjust the filter cutoff (to
prove this capability exist) and 2) compensate for new Front Panel control
if it too was changed.

regards,
p


-----Original Message-----
From: owner-synth-diy at dropmix.xs4all.nl
[mailto:owner-synth-diy at dropmix.xs4all.nl]On Behalf Of Metrophage
Sent: Tuesday, September 07, 2004 7:55 PM
To: synth-diy at dropmix.xs4all.nl
Subject: Re: [sdiy] Differences between MG-1 filters and broken stuff


Here's another difference: most Rogue filters work, but since I
replaced the pots in the MG-1, this filter no longer works!

I love it when an apparently simple repair turns into a complete
debacle. Before, the MG-1 ran at a very quiet volume, but with no sync
switch, and extremely crackly sliders. I replaced all of the sliders
with appropriate values and installed the switch yesterday. Today the
board looked clean and dry, so I started to connect everything again
for bench frenzy. No smoke when I turn it on... the LFO indicator
blinks, but no sound. I went ahead and removed R102 which does result
in the synth being a bit audible, but it's even quieter than it was
before when I had initially gotten it "working"!

The new pots do feel great. The pads are all intact, I made certain
that there are no broken traces from my removal of the old sliders.
What works? The VCOs, LFO, envelope, VCA, noise, ringmod, top-octave
chip. I guess that leaves the VCF section. Resonance does whistle at
higher settings, but I do not hear any filter action. The volume is so
minute that I am not sure I'd hear lowpass filtering anyway.

More tomorrow, must pass out!



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