26/R118?RE: [sdiy] MG-1 issues

[Metrophage]

--- phillip m gallo <philgallo at attglobal.net> wrote:
> Recalling your recent post's you had to replace the CD4007.

I think the 4007 fried because of an accident inolving the wire spiral
of my notebook, but I am not completely certain. It's the other
problems which are the real mysteries!

> I understand you used you meter to validate the filter ladder
> voltages. How close where they to expected value especially the
junction of
> R82/R83?

Very, there as well as other VCF test points. R82/R83 I measured as
being 2v.
 
> Now that your envelope is working, if you put it in AR mode (where it
> stays high until key release) do the old "knife in the key thing" (i
> use match packs myself) do you see it at  R117  

Good thing my Keith Emerson sword was nearby! R117 measured as being 0
to 5v, depending where the slider was set to. The voltage ticked
slightly in time with the LFO.

> removing the knife when you press keys do you see voltage change at
Q226/
> R118?   

This junction stayed at a constant 1.5v, whether keys were pressed or
not.

> Q25/26 are the "expo current source " for the filter if when in
resonation you do 
> not get key control, Env control, or trimmer control one or both of
these is 
> probably the fault.  Note they are cheap transistors (now if you find
they are at 
> fault replace 'em with what you've got but make a mental note to
select some 
> better trannies in the future if keytracking is useful to you.

This is what I was planning to do tonight. Crazy thing is that I've got
hundred bags of 3904's and 3906's, but I can't find either of them.
Maybe 2N2712 and 2N4126 will do? What makes transistors good for expo
current? I see matched ones on the same die for dual/quad of a given
PNP or NPN type, I don't know about when one is one and one is the
other. Wouldn't it help also to make R112 a 1k tempco? Filter
keytracking would not always be useful to me, but there will certainly
be occaisions when I will want it to track.

Thanks much, Phil, for helping me out here!
CJ

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