Fw: [sdiy] Heat Paste/Compound

[Dana.Scott]

I got this reply off list. I hope Terry does not mind my posting it to the
list.
My reply is at the end.  -D

----- Original Message -----
Subject: Re: [sdiy] Heat Paste/Compound


Message text written by "Dana.Scott"
>Only the early modules were 'hard potted'. On the vast majority of
modules,
we 'soft potted' them. The module is in an epoxy cup which was then filled
with soft RTV rubber. A thin layer of hard epoxy then went over the top. It
looks like an unrepairable brick, but they were routinely cracked open and
repaired. Usual failures are shorted tant caps across power supply or
mounting pins that detach from the PC board.
-Dana
ARP inmate 1974 -> 1980<

Hi Dana:

I know from experience you are right.  I ran the service shop for a large
music retailer from 1972 to 1979, and I worked on many ARP products.  One
was an early 2600 that had a power failure that destroyed every opamp in
the synth, and trashed most of the modules also.  I tried to get into them
for repair, but those were hard potted.

Some years later a friend of mine had an odyssey, I no longer remember why
I had to get into the filter module, but I did, and found it had the RTV
with a thin, easily removed hard epoxy layer on the outside.  I was quite
surprised to find this,  I guessed the factory made this change to be able
to repair modules in instruments returned from the field, from wht you said
it looks like I guessed correctly.   It was obvious the RTV modules were
made to look like a hard potted module, you wouldn't know the difference
unless you attempted to open one up. which I did on at least that one
occasion.

One problem that can occur with hard potted modules is, epoxy shrinks
slightly when it cures, the shrinkage might be enough to break circuit
connections within the module in some cases.

I don't know this first hand, but I understand ARP eventually dispensed
with potting their modules altogether.

Terry

Hi,
  Yes, potting was done away with on the later 2600's. The Avatar Guitar Syn
th had two modules named Eprime and Dprime and these were hard potted for
security reasons. Some of the last Avatars had no potting on the Dprime
module, but hard potting remained on the Eprime. Security on the Eprime
module was intense. Testing and calibration of the Eprime modules was done
in a seperate room away from the rest of the test dept. Schematics of the
Eprime module were kept under lock and key and only available to those
people who had a need to know.  In my attic I have 1 or 2 unpotted Eprime
modules. I guess they are rather rare!!
-Dana