[sdiy] Roland 80017 findings

[Tom Arnold]

Wasted a couple hours on this.

>From the Juno106 service manual you can find a block diagram with pin
numbers of the internals of the 80017.

Hunted down the bad module in my Juno106.  Sure enough, output is clean from
the VCF section, but crackles from the VCA section.  A quick view of the
block diagram shows that one of the VCAs is completely seperate from the
rest of the module, so making an adapter to use an offboard BA662 should be
easy once the proper part values are determined ( 4 resistors and a cap ).

Questions and puzzlements I have...

Looking at the TB303 schematics on Hyperreal, Schem 2 happens to show a
pinout with internals of the BA662 which seems to show a gain control on the
buffer section?  That sound right?  A 662 datasheet would be awesome, but I
dont expect anyone has ever found one.  I'm just puzzled why, if pin 4 was
buffer gain, it'd go to ground unless it was an inverted input.  I'll dig
through more roland schematics and try to find more examples and maybe work
it out.

As far as carving up the 80017A.  My theory is based on the "fact" that its
usually not the IR3109 that goes but the BA662.  So I tried excavating just
a BA662, the one on the right that should actually be a VCA.  Dremel tool
down all four sides since its easy to identify, once you hit pins, stop
cutting and use a small screwdriver to pop the epoxy off.  Clean the
siliconeglue-stuff off the pins, then it was a simple matter to use a sharp
soldering iron bit to lift the pins and once done I popped the old 662 out.
Was able to attach wires to the old pads, so in theory you should be able to
wire a replacement chip onto the module in a truly scary repair job.  Note
that this "fact" could be wrong, it might be noisy thick film resistors in
the module.  Who knows for certain without more digging.

Welp, if we can work out the values of those parts, I have everything ready
where I can doctor up a bypass module that'd use the 80017 for its IR3109
and one 662, but replace the second 662 with a BA6110 or for that matter we
could try an LM13700.  When I located the bad filter in my 106, I installed
a socket just for this kind of experimentation... :-)  Used to be someone
selling a little board to do just this, but I cant find the webpage any
longer so now I try to recreate it.  I could just buy an 80017 from Doug,
but that'd be cheating...

-- 
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 - Tom Arnold       -  Free Synth DIY webspace http://www.sdiy.org 
 - SynthGeek        -
 - BBD Fanatic      -  "...is it a virus, a drug, or a religion?" 
 - echo evho wjxo   -    Juanita Shrugs. "What's the difference?" 
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