The configuration and time-of-day clock are stored in a Dallas/Mostek real-time-clock chip. At least on the 550, this chip is not socketed. It looks like an extra-tall 24-pin DIP. I'm not sure about the 890 motherboard but it's U4 on the 550. Instructions on replacement are here: Re: Atlas 550 Loses Config
It can be replaced relatively easily if you're comfortable with through-hole component level repair. I strongly recommend replacing the old chip with a socket so that a few years down the road it isn't as much of an issue.
Thanks very much for the info on the clock chip. I can do board level. I
will try to get the newer TI chip. Socket sounds like a good idea.
Once replaced, do you know what the lifespan of the chip is in terms of
retaining config and time? Will it fail faster if the 890 is unpowered for
long periods of time?
Besides the 890 I own a bunch of TSU600e, a 550, a bunch of 1224s, a 4305,
about a dozen Tracers, and a DSL (T1) HTU-R/HTU-C pair.
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I think the specification is ten years, but that's assuming that the device is powered on most of the time. Another factor is that these chips aren't in high demand so it's likely that any that you buy have been sitting on the shelf for a while. The clock is shut down from the factory in order to save battery. When you first power it up and set the clock, that "kick-starts" the chip and the battery goes active. Practically I'd expect about four or five years on the low side and ten if you're really lucky.
I recommend installing a socket so that the next time it's substantially easier.
Jayh - I told you this on the phone, but wanted to post it here for others. On the upper middle of the controller modules there is a yellow lithium battery that snaps into the chip under it. I assume this to be the clock and the backup supply for programming memory. These batteries are available from Mouser. I bought 8 to have as spares. The yellow module contains a crystal for the clock and a lithium battery.
Before replacing these batteries on both controller modules of both of my 890s, they could not remember time/date programming after a power cycle -- now they do. I am assuming that the config memory will also be protected after I reprogram the unit.
I have not had a chance to rebuild the config, but I will soon, and we'll know for sure.
Thanks very much for your assistance!