Re: Corrosion abatement

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From: James MacPhail <gg__at_email.domain.hidden>
Date: Tue, 28 Jan 2014 12:47:43 -0800

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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Received on 2014-01-28 16:27:27