That is a new one ! Have heard of running slow or not at all but not that fast. Good luck !

I think you can buy 10 for a buck too, to get some spares on hand.

keenmaster486 wrote on 2024-03-16, 05:46:

So I take my 486 (Dell 466/M) out of storage and I'm greeted with this: the clock is running so fast it counts a minute about every 5-10 seconds. Assuming the RTC crystal is bad.

Tested how? In BIOS or in DOS (or other OS)?
BIOS will poll RTC chip to update what it displays so if the clock is running fast there it's the 32768Hz crystal gone bad (or something wrong with the circuit so it picks up a lot of noise).
Every OS will keep its own clock using system timer 0, after reading the RTC for the initial state. So if the clock runs too fast in OS either something re-programmed PIT channel 0 to unusual value and left it there, rather than cleaning up properly (Landmark can sometimes do that for example) or the system clock is somehow bad - but that would usually affect more than just the PIT, this is usually a single clock source that drives everything, from CPU to ISA bus.

Testing in the BIOS.

I tried replacing the crystal. No dice. Even tried removing it entirely - same behavior, so it's acting as though it isn't there at all. If I touch one leg of the crystal, or any of the small surface mount components surrounding it, I can stop the clock entirely.

Sounds like a bad ground. The metal case of the Xtal needs be grounded or noise effects it......and your finger was causing a grounding effect maybe.
Look for bad via or cold solder on the ground plane in that area..

Reflowed all the solder joints on all the components nearby, didn’t fix it.

Not sure where to go from here.

keenmaster486 wrote on 2024-03-16, 22:45:

Not sure where to go from here.

Mobo pics. If this circuit uses some gates (like 4069) to create the oscillator then perhaps this part has gone bad, or is not powered properly. Even on newer, integrated chipsets there's some extra parts like capacitors and a resistor or two perhaps to properly match the crystal to the amplifier. Corrosion or dirt on these parts can cause such problems as well.

Without a scope it's hard to say. Excessive current draw from the battery is commonly correlated with chipset rtc/cmos failures. You can check that if you have a meter that can do micro amps.

Deunan wrote on 2024-03-16, 23:09:

keenmaster486 wrote on 2024-03-16, 22:45:

Not sure where to go from here.

Mobo pics. If this circuit uses some gates (like 4069) to create the oscillator then perhaps this part has gone bad, or is not powered properly. Even on newer, integrated chipsets there's some extra parts like capacitors and a resistor or two perhaps to properly match the crystal to the amplifier. Corrosion or dirt on these parts can cause such problems as well.

The crystal connects to the chipset. There is no freq division/multiplication logic at work on the RTC clock. The resistors and caps you commonly see are to provide the correct load for parallel resonance crystals.

Edit: Keep in mind there is division within the RTC. But I've never seen this done externally with discrete components.

I like the noise theory, might be noise on the supply line.
Cant find a good picture of Dell 466/M motherboard 🙁