[sdiy] perplexing polysix problem

[Bob Weigel]

I....can't can't get rid of the LFO.  Cruel hoax was.. there was an ohm 
and a half baseline resistance on the pot.  Spent most of an hour 
retro'ing it with a shim piece that solved the problem on a weakening 
center trace.  But it did absolutely no good to my surprise/horror.

So basically as any polysix expert might know, there is an LFO line that 
goes to the front panel and the Mod wheel also goes to the front panel 
and the tune knob and bender interact and mix to a 'mod' line (pin 2 of 
CN06 I think it is) which then returns to the CPU/patch manager board.  
There it goes via some trace through hole moves, to a 10K resistor that 
goes to the mixing node of IC8B, a 4558 dual op amp.

Now the LFO signal I saw after fixing the slightly wacked mod pot, 
doesn't exist on any of the things feeding the MOD line on the front 
board so..I cut the mod line.  And the signal (which is more sinusoidal 
and does follow the LFO speed control still though it shrinks in 
intensity a lot at the higher frequencies) persists on the patch manager 
board at all points from the connector to the 10K resistor aforementioned.

I pull the 10K resistor..signal persists just where it had.  It's enough 
to be really annoying.  Always LFO to the vco so it always sounds like 
typical light Organ vibrato.

Ok so this...is the really freaky part.  I loaded it to ground via a 
pot.  I adjusted it so half the waveform's voltage is soaked.  Guess how 
much resistance? 

10K. Within typical precision.  Hmm. I had thoroughly..and I mean 
thoroughly looked for anything else on top or below that board which 
connects to that line.  It's as isolated as it appears to be.  No flux 
bridges even.

Gee..I haven't seen one to add the the Weigel museum of spook electronic 
phenomena for a little while.  I think this one is edging it's way 
towards the collection.

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