Regarding hard to fix:

Fix, yes...replace, no. That's easy and they wil be availible for
some time to come.

There are two partts that are involved with the 'temperature loop' -
a special type of resister called a tempco and the dual matched
transister pair, which is being used as a 'current mirror' (it
converts voltages from the freq pots and VC inputs into current
which effects the core oscillating frequency). Without going into
details, those two work together and are actually in physial
contact to one another, the transister pair sitting on top of the
chip resister. I went with a chip resister because it transfers
heat better - no coating to insulate things. The key to good temp
compensation is making sure these two parts are kept at the
same relative temp -and- that they are sheilded from outside
fluctations in temp. You want them to heat up on their own and
each other up in the process - the epoxy helps with this (a lot).

Quickly how it works: The tempco resister changes electrically to
temperature conditions. The current mirror also changes, and if
it does this effects the current coming out of it and THAT effects
the frequency of the VCO (and that is a bad thing). In that the
tempco is in the circuit that drives the current mirror, as it adjusts
itself to the same temperature fluctuations that effect the current
mirror, in so doing it compensates for this delta and in the
process limits the current variations coming from that circuit into
the core. So naturally the key is to keep the two at the same
temp (or as close as you can to that) and keep outside
conditions from effecting the two - enter the potting compound.

I elected to take it back one step and encase the entire expo
converter (OTA and all) in this module. This is an addition four or
so parts. It was a questions of practicality. Doing it this way,
there are only five I/O's coming form/to the module - it makes it
much cleaner.

As far as them blowing up, while anything is possible, those two
parts a buried well within the circuit. I doubt if much damage will
come to them if something is plugged in wrong on the faceplate
(well, if you plugged a wall socket in there, all bets are off but
who's gonna do that ). But if this happens, it's a case of
removing one module and inserting another.

In that these are all going to be tested before the epoxy is
poured, if I find the circuit is stable enough without it I wont be
doing it - I'd like nothing better than not having a mold made, or
dealing with the mold release compound and mixing the epoxy .
We'll see when that time comes. But given that the 258 wasn't
world renown as being the most stable of VCOs and also given
that potting really improves that - it's doubtful, but we'll see.

- P






--- In Doepfer_a100@yahoogroups.com , "Julian" <julian@2...>
wrote:

> >and mounted
> > to that will be an epoxy sealed module.
>
>
> Now im following all this resonabley closely, as i may go for a

module or

> two, but im concerned about the epoxy stuff.
>
> Stuff breaks now and again. Normally its just a case of finding

out what

> exactly has gone, and, providing there are not design flaws

making the

> component break, just replacing it.
>
> I dont think theres a way to ensure stuff never breaks. But

encasing it

> ensures its at best very dificult to fix.
>
> How much is to be encased The temperature sensitive parts

in small groups,

> or the whole lot in one go
>
> Cheers, Julian