[sdiy] OB-8 Question

[Magnus Danielson]

From: "J.D. McEachin" <jdm at synthcom.com>
Subject: [sdiy] OB-8 Question
Date: Thu, 19 Apr 2001 14:07:52 -0700 (PDT)

> Hi All,

Hi JDM!

> I know it's been discussed on list before, but can someone tell me if the
> OB-8 uses the 3340's sync pin, or does it generate sync w/ an external
> circuit?  I've got a friend w/ a broken OB8 in the shop, and they're
> quoting him US$60 for a 3340.  I'd rather just give him one of my
> spare 3340's,  but I've only got a couple that are 100% functional, and a
> few more of the defective 3340s w/ a broken sync circuit.  Of course I'd
> prefer to use the latter if I can get away w/ it.

If you look at the OB-8 service manual page 24 (look at my OB-8
website http://home.swipnet.se/cfmd/synths/companies/oberheim/) you
can see how the sawtooth output of UX01 is being sent through a 4016
CMOS switch UX04 (with the SYNC control signal as gate) and then being
passed through a pair of capacitors and resistors and a 2N3905
transistor to the soft sync input as well as the triangle output.

This is a form of modified sync input. By just taking a quick snap at
the internal structure of the 3340 it looks like this is a modified
form of hard sync while not using the hardsync input at pin 6.

If one looks at the internal structure one sees that the hardsync
input affects the comparator level for a comparator which switches two
switches. The OB-8 setup will instead modify another comparator by
overriding the normal triangle output (it is being clamped to about -5
V when the sync arrives) so that the softsync comparator will compare
diffrently. I guess this is a way to get a diffrent sync type out of
the CEM 3340.

Either way. One should check out that the sawtooth make it through the
analog switch and also kicks the input as well as the transistor.

Cheers,
Magnus