[sdiy] Discrete OTA

[Tom Wiltshire]

Incidentally, in my view conformal coating is worse than potting. It's easier to put on (a simple spray can) and impossible to remove effectively. At least you can scrape many potting compounds off, at least the rubbery ones. It's fiddly, but possible. Conformal coating makes repair extremely difficult since it burns and sticks to your soldering iron. Horrid.

Perhaps there's been some new formulation since I last had to deal with it though. (Hope springs eternal)

T.


On 4 Apr 2014, at 00:01, Neil Johnson <neil.johnson71 at gmail.com> wrote:

> David G Dixon wrote:
>> The potting is an annoyance, and for $10, it would be nice to be able to
>> fix a bum resistor or something.
> 
> Yes, I don't think it is appropriate for something like a synthesizer
> where repairability is an important buyer requirement.  Just look at
> all the complaints about how small and "unrepairable" surface mount
> components are.
> 
> My view on potting is that it is done for several reasons (sometimes a
> combination):
> 
> 1/ Robustness - it seals the circuit against a harsh environment, and
> provides mechanical support for all components.  Downside is it's a
> real pain to do failure analysis on failed units as you have to pick
> away at the encapsulant to get to relevant circuit nodes for analysis.
> 
> 2/ It can also be useful as a construction technique in making small
> modules, such as George Hearn's modules, or the aforementioned
> discrete op-amp.  No need for expensive custom packaging, you just use
> a standard off-the-shelf potting box, a big dollop of goop, and you're
> done.
> 
> 3/ Thermal uniformity - with the right compound you can spread any
> generated heat throughout the potted module, so good for
> thermally-sensitive circuits, in effect slugging any drift due to
> changes in ambient temperature, circuit dissipation, etc.
> 
> 4/ Keeping it hidden - ARP modules, Yamaha hybrids, Korg 35 filter
> module.  A more recent example being the Schippmann VCF-02 filter.
> 
> In all cases it is difficult to repair the potted module.
> Or as Roman wrote a few posts back:
>> potting is dissed here, so watch out ;)
> 
> Neil (has potted modules, and done failure analysis on potted units,
> and cursed potting compound)
> --
> http://www.njohnson.co.uk