ARP 2600 Ring Modulator #4014

[WeAreAs1 at aol.com]

Hello list,

I am currently trying to reverse-engineer the Ring Modulator submodule in an
ARP 2600 (module #4014 revA).  I have removed the module from the synth and am
now tracing out the circuit.  Fortunately, this module does not have an epoxy
casing around it, so all of the circuitry is visible.  

There are 12 transistors on the board, and eight of them are in pairs that
have been epoxyed together, presumably for thermal contact.  Because these
pairs are epoxyed together, it is impossible to see the part number on some of
them.  Most of the transistors on the board are TZ-581's, including one of
each of the paired transistors.  The TZ-581's have their part number printed
on the side of their "skirt", but the other ones have their part number
printed on their "face", which, because it is glued to the face of the TZ-581,
is not visible.

I am hoping that someone on the list knows the part number of these mystery
transistors, and will share that information with me.  I am guessing that they
are some kind of NPN device, possibly a 2N5172, since that particular device
is used very extensively throughout the ARP 2600, but it would be nice to know
for sure.  As you might imagine, it's hard to trace the circuit without
knowing the footprint of the device, since some transistors (such as the
2N5172) have their emitter/base/collector oriented left-to-right, and others
(such as the TZ-581) have their pinout oriented with BCE going left-to-right.

When I am finished tracing the circuit, I'll draw up a JPG schematic and mail
it to whoever is interested in looking at it or trying it out.  It's a
wonderful sounding ring mod, can be trimmed to work reasonably well with both
AC and DC signals, can also be used as a VCA, and is a nice alternative to
some of the other ring mod designs which are in wide use.

Best regards,
Michael Bacich