If the chips are socketed or you have good soldering equipment so the
risk of board damage is low, I would replace them.  But if there is any
risk of damaging the PCBs and the chips weren't out stressed beyond spec
I would leave them be.  Check if their outputs may have been stressed by
high currents at the higher voltage.  Also check any bypass caps that
may have been over voltage.

Sherpa Doug

> -----Original Message-----
> From: Jinx [mailto:joecolquitt@CLEAR.NET.NZ]
> Sent: Monday, January 14, 2002 7:59 PM
> To: PICLIST@MITVMA.MIT.EDU
> Subject: [EE]: Replace old chips ?
>
>
> I've got this ship's clock (Wempe Chronometerwerke) to fix. The
> PSU had a hissy (literally) fit and has taken out a few components.
> Now I have to decide on the extent of the repairs. The ship is on
> a world trip and is out of port temporarily until the 30th, when it
> returns and then continues its voyage, so whatever I decide has
> to be doable in a fortnight
>
> The unit was built in 1977 and I feel I should replace all the ICs,
> even those that aren't faulty. Yet. There are 11 180mm x 120mm
> cards in a 19" rack, quite a few dozen ICs. The display's 74s are
> history because of the PSU's fault 14V output, but the rest are
> 4000 series that I think will have survived
>
> Has anyone an opinion on the lifetime of CMOS ICs, with regard
> to atmospheric ingress or just electronic wear and tear ? This clock
> is on 24/7/365, so there's some constant warmth and it shows no
> signs at all of salt corrosion. I know that ICs unpowered for a long
> time can fail when powered up but I'm not sure of the exact cause
>
> The last job I had like this was of a similar vintage and was flakey,
> to say the least. I ended up replacing a very large (450mm x 95mm)
> PCB with 60 ICs with a small PIC circuit. I'm tempted to do the
> same with this clock, but I'd need to do a cost analysis first and
> run that by the customer. Time and the need for them to carry on
> with their journey is a factor
>
> --
> http://www.piclist.com hint: The PICList is archived three different
> ways.  See http://www.piclist.com/#archives for details.
>
>
>

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