Mac SE Clock Drift

[Iesca]

I just installed a CR2032 in a Mac SE, and while it's keeping most of its PRAM settings (including the date), the clock drifts significantly when the machine is off. Anything I should know, or is this a sign of a bigger issue?

 

[David Cook]

the clock drifts significantly

Can you be more specific?

I've seen this happen on a couple of Macintosh IIsi computers that I ultrasonically cleaned. The root cause was the crystal was damaged. However, it was cheap and easy to replace.

 

[Iesca]

I will set the time, turn off the machine, turn it on again later, and the clock will be the wrong time, behind by several hours or more.

 

[David Cook]

Assuming the battery is good, then I suspect the crystal. That is Y1 right next to the RTC (real-time clock)

 

[joevt]

I will set the time, turn off the machine, turn it on again later, and the clock will be the wrong time, behind by several hours or more.

Is there a relation between the amount of time the machine is off, and the amount of time that the clock is behind by?

Is the amount of time that the clock is behind by at least less than the amount of time the machine was off?

Assuming the battery is good, then I suspect the crystal. That is Y1 right next to the RTC (real-time clock)

Wouldn't a bad crystal also affect the time while the computer is on?

What voltage does the RTC and crystal have when the computer is off?
What voltage does the RTC and crystal have when the computer is on?

 

[David Cook]

Wouldn't a bad crystal also affect the time while the computer is on?

When the computer is turned on, the clock is kept in a low memory global which is advanced by the interrupt timer. Then, it is written back to the clock chip at shutdown.

 

[Iesca]

I will start with swapping the clock crystal out with one from a Plus (I assume they are the same rate) and see how that goes.

The clock stays in time while on, but after an evening of being off or longer, it's maybe 12 hours behind, somewhere around there, I haven't measured exactly.

 

[cheesestraws]

Yes, sounds like perhaps the oscillator is damaged. The CPU is responsible for keeping time while the machine is on.

 

[Iesca]

I will start with swapping the clock crystal with one from a Plus (I assume they are the same rate) and see how that goes.

The clock stays in time while on, but after an evening of being off or longer, it's maybe 12 hours behind, somewhere around there, I haven't measured exactly.

I assume that a normal 3v CR2032 is adequate in terms of voltage?

 

[Iesca]

Whoops sorry about the doublepost, was having internet connections issues

 

[joshc]

a normal 3v CR2032 is adequate

Yes

 

[Iesca]

Sadly, swapping the crystal does not seem to have helped. The clock basically stopped progressing while it was off it would seem. I suppose next I'll try swapping the RTC ship, unless someone has another suggestion. I'll also check traces around the area.

 

[David Cook]

Yes -- check the traces. Do you have an oscilloscope? If not, the handheld ones are reasonably inexpensive:

https://amzn.to/4oLPdJb