I have some EPROMs which show some short circuits between adjacent
pins. I got rid of the shorts by scraping the white sealant with a
needle and now they program and read correctly.
Should I put something to seal the chips again? What can I use?

Was this a white sealant on the chips themselves? or on the PCB?

In either case... there are quite a few "conformal coatings" that you
can use to protect PCBs and chips against moisture infiltration...
acrylics, urethanes, epoxies, silicones, etc. In a lot of situations
they are quite unnecessary.

It's possible that the current leakage which shows up as a short isn't
due to the sealant itself (if those are ceramic-case EEPROMs, then the
sealant is also probably a ceramic) but is due to some sort of surface
contamination... dirt, dust, smoke, etc. which has built up on the
chips over the years.

If the sealant is something that was gooped on the chips and PCB...
then, yeah, it may well have gone conductive over the years, and is
best removed. It's probably not necessary to *seal* the chips, but
may have simply been gooped on them to make sure they didn't work free
from the sockets due to vibration or thermal cycling.

If you're concerned about those EEPROMs, you might want to scrub them
down thoroughly with a toothbrush and some nonconductive solvent
(electronics-grade isopropyl alcohol, or denatured alcohol for
example) and then dry them thoroughly (e.g. shake off all liquid,
air-dry for a few minutes, and put them in a 140-degree oven for a
couple of hours). My guess would be that this is all you'll need to
do. For example, this is the recommended cleaning procedure for the
old CDV-715 ionization-chamber radiation detectors, which are
sensitive to leakage currents down well below 1 nanoampere at a
50-volt potential.

If you want to try protecting the chips or the board, you can spray on
a conformal coating... but you have to make sure it doesn't get into
places where you don't want it (e.g. into sockets, between the socket
pins and the IC pins) or it may make matters much worse!

For some small PCBs that needed moisture protection (high voltage,
leakage-sensitive circuits) I've done the "scrub and oven-dry" steps,
and then given the board two or three very light spray-coast of clear
Krylon acrylic... cheap and effective.

--
Dave Platt AE6EO
Friends of Jade Warrior home page: http://www.radagast.org/jade-warrior
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