EPROM sealant causing shorts

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

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.

Approximately how many ohms is the "short circuit"?

If there's a conformal coating, it always has a phosphorescent dye mixed in to allow for inspection. Where there's no blue or green glow, the coating needs to be touched up. Find one of the cheap UV LED flashlights, and inspect the EPROM for the dye.

If coated, try dipping the EPROM in some nasty solvent, such as paint thinner, acetone, xylene, or trichlorehtylene and see if anything appears that looks like a coating. If you want to go for broke, try paint stripper (methylene chloride) which will surely remove any coating. Scrape off the mess with a wire brush. Oh yeah, wear protective gloves and do it outside so you don't breath the chemicals.

In the distant past, I had to deal with a product that was rather static sensitive. Management didn't want to deal with a redesign, so I stupidly suggested that we spray it with conductive anti-static spray and seal it with acrylic spray conformal coating. I think it was about 100 Kohms/square. The problem was that the resistivity was not stable, and would change with coating thickness, temperature, humidity, handling, and the position of the moon. I eventually did the redesign. Hopefully, wherever the EPROM's originated, did not have the same bad idea.