Re: Corrosion abatement

Thanks for the excellent information, James. I am using whatever ESD protection my kitchen counter allows: laying down aluminum foil on my work surface, touching the chassis first, etc. Pittsburgh has been cold and dry of late, so the risk of static damage is elevated. I guess you never know until you power back up.

It would be a bit of a hardship to try and replace a COP421, so I hope my ministrations didn't harm it.

In the meantime, I am almost tempted to dike out D8 altogether and placard the charging circuit as inop. for future owners. I'll follow your recommended checks first in any case.

Thanks again,
--Tom

On Tue, Jan 28, 2014 at 3:47 PM, James MacPhail <gg__at_email.domain.hidden>wrote:

>
> Probing the card cage electronics in the Lisa poses very little risk.
>
> Most of the parts are TTL which can stand brief short-circuits between
> signals or to +5V or ground.
>
> The largest voltages in the card cage area are 12 volts, and thus almost
> completely safe in terms of electrocution hazard, although shorting 12V to
> a 5V portion of the circuitry is likely to damage some parts.
>
> Note that there IS an electrocution hazard inside the power supply and at
> the CRT/Video board, but these are not accessible if only the back panel is
> removed.
>
> There are a few parts that are MOS and not so rugged, and thus vulnerable
> to discharge of static electricity or overload. One needs to be
> particularly careful in handling the EPROMs, static RAM and COP421. There
> is static RAM on the I/O board: U1B and U2B, and on the CPU board: U8A,
> U9A, and U10A. Of course it is a nuisance to have to replace ANY part, so
> care is warranted in general.
>
> D8 on the I/O board is a 6.2V zener diode which is used to limit the
> charging voltage for the NiCd batteries (hopefully no-one still has these
> on their I/O board due to the corrosion damage they cause).
>
> I believe that the Lisa will still operate correctly if D8 is missing or
> malfunctioning. However, if D8 is shorted, then R39 will exceed its power
> rating, overheat and fail.
>
> To check D8, turn on the Lisa and measure the voltage across it:
> - If it is close to 6.2V, then D8 is working properly
> - If it is 12V, then D8 is not working (open circuit)
> - If it is close to 0V and R39 is hot then D8 is shorted (or some other
> short circuit)
> - If it is close to 0V and R39 is not hot, then there may be a problem
> with the +12V supply or R39 may have already failed, or there is a broken
> trace, etc.
>
>
>
>
> some idea whether it would be safe to
>> try powering up the boards again if I/O board diode D8 (which looks
>> like it's in bad shape) is no longer diode-ing. Being fairly
>> uneducated w.r.t. electronics, I am wary of probing D8 with my
>> multimeter in case the act of probing would also affect other parts
>> of the circuit. However, I'd be willing to try it if it carries no
>> risk of harm.
>>
>
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