POLY 800 knobs mods

[Adrian Corston]

>>> i am new to this list and was just womndering if any1 has sone the cuttoff
>>> fequency and resonance knobs additions to a poly-800 b4.  i have heard it
>>> is simple but i dont want to delve without some good advise.
>>
>>Me too.  Got a garage sale Poly-800 sitting useless in the basement
>>right now.  It works fine, but is just so dang boring.  Cutoff
>>frequency, resonance - oh, and routing the oscillator output to
>>frequency cutoff, to have filter FM for extra klangy keyboard
>>intervals.  (Super fun!)  If anyone has any schematics and can suggest
>>some good solder points, I'm willing to Frankenstein my 800.
>
> Here's a url for poly 800 freq.cutoff and resonance knob mods---------------
>haven't done them, but keep us posted...

Ok, I stopped procrastinating and tried it out last night. It works
great, and is definitely worth doing.

Basically, you don't really need a schematic, just good soldering skills.

When you have the case open, try adjusting the VR2 and VR5 trimmers
(I did it with a sequence running) on the main board attached to the
bottom of the case to see what sort of effect you will get. The
trimmers are basically a fine adjust - you still get to set the
basic cutoff and amount of res in the patch, but then you can
adjust it over a usable range on the trimmers. The res goes spastic
all the way up and I don't know how good for the synth it is at
extremes. My personal rule of thumb is, if it is distorting, wind it
back (after you sample it ;-)

I had to unsolder a resistor on the far left hand side which grounded
a metal strip on the bottom of a piece of cardboard which sits under
the PCB. Your mileage may vary here, I expect. There are seven screws
holding the board down you'll need to remove. The board is double-
sided, so be careful removing the trimmers that you don't lift the top
track.

>From there, I ran some ribbon cable to the front panel in the blank
space above the joystick, where I drilled two holes and mounted two
25mm 100K linear pots. Well actually I couldn't find a 100K pot so I
used a 1M one, which didn't work very well - the useful values were
at each end of the turn of the pot and there was little effect in the
middle. I'll be replacing it with a 100K tonight and I'm sure it'll
work much better.

It's not a hard mod at all - unsoldering the trimmers without mangling
them is the only vaguely tricky bit.

Next project: buy another Poly-800 and rack mount it :-)

Hmmm... I should trace some tracks; it may be possible to use an
op-amp and some discrete bits and do a CV cutoff/res inputs mod...

Cheers,

-- 
Adrian Corston
Internet Engineer
Internode Professional Access