Thumping filter in a memorymoog

[WeAreAs1 at aol.com]

In a message dated 6/8/00 5:24:00 AM, you wrote:

<<Hi Everyone,

I have a Memorymoog that 1 voice is thumping on when I play a note on it.

I removed the line cap (between OSC. output and filter input) and it still

does it. I've tried a second cem3360 VCA chip but that didn't change anything

either I adjust the VCA trim but it just seems to go to a deeper (more bass)

thump. 


Any ideas on this would be great.

I have lots of the right tools so throw any ideas at me.>>

"Thumping" is usually caused by some kind of unwanted DC offset.  If one of 
the 'right tools' you have is an oscilloscope, then you should look at the 
signal starting at the voice output, going backwards through the voice from 
there, looking for unusual DC offsets (compare the DC to the same points in 
the good voices).

In my experience, CEM3360's tend to be a bit less reliable than other CEM's, 
so I might check that again.  It's also possible that you have a bad opamp 
somewhere in the audio path.  Often, when opamps go bad, they will still pass 
signals, but will have a big DC offset at their output.  Also, bad "audio 
muting" transistors and bad CMOS gates (4066, 4053, 4051, etc) can bring on 
similar symptoms.  I don't know if the MM has any of those in that section - 
it might have some that are part of its auto-tuning circuitry.

Oh yeah - and put those coupling caps back in there!

If you don't have a 'scope... buy one!  Seriously, though - Oscilloscopes are 
not absolutely essential for building DIY stuff (as long as you're like me, 
and *never* make any mistakes when you build), but they definitely are 
essential for troubleshooting and repairs.

Good luck,
Michael Bacich