[sdiy] Discrete OTA

[Roman Sowa]

Acrylic coating can be easy removed with solvent. Modified silicone
cannot, but it's rather used to protect the board against nasty
chemicals and temperature so in synth applications I'd expect acrylic. 
Not that's I ever seen any conformal coating in a synth though.
Urethane can be removed by mechanically AFAIK, as for epoxy, there are 
proper solvents for that.

I think it would be good practice to write on the potted cap, among part
number and other stuff "in case of emergency disolve in <put proper
solvent here>". Everybody will still try to remove the potting and
by giving exact solvent type you prevent it from damage caused by trying
various methods.

I sometimes use potting or coating to protect sensitive part of the 
circuitry. Not in terms of protecting the design against reverse 
engineering, but humidity and dust causing voltage drifts.

Roman


W dniu 2014-04-04 01:23, Tom Wiltshire pisze:
> 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
>
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