View Full Version : 486 RTC mystery

June 6th, 2006, 01:34 PM
I've put together a 486 today from various parts, as a friend of mine gave me an old mainboard and SX25 processor. As far as I know, the mainboard actually hasn't been used for at least the last 5 years.

It had an unknown bootup password set, so I did the old "clear CMOS" jumper routine, which worked. I then set the mainboard, including the date and time.

The problem is that when the computer is turned off, the time does not stay current. So if I turn the computer off at 9:30, when I turn it back on, it is still set to 9:30. I know this could be the CMOS battery, but wouldn't this reset everything else too? I don't get any CMOS Not Set errors on startup, and it does remember everything else I set.

The battery is one of those large non-removable types, but there is the option in the jumper settings to disable it and plug in your own.

Here are the mainboard settings (http://www.peteweb.com/ftp/mirrors/hardware_specs/m/C-D/31672.htm)

Any ideas? Dodgy RTC?

Terry Yager
June 6th, 2006, 01:42 PM
What kind of RTC chip does it use? Is it socketed?


June 6th, 2006, 03:28 PM
Hey there:

I've seen this happen when the battery is nearly, but not totally, dead.

The battery on board is probably a NiCd rechargeable. It might just have enough left in it to retain CMOS data (the CMOS requires very little power to retain data), but not enough to run the RTC.

I'd just disable the on-board battery, and plug in an external one. I used to use a battery pack that holds 4 AA cells. You can buy these in any electronics store.

If you want to test this theory, just find a way to attach some battery arrangement that will put around 4-6 volts DC onto those external battery pins. If this works, then you can search for a battery holder.

Let us know what happens!

Terry Yager
June 6th, 2006, 03:46 PM
I've got a box full of 4-cell AA battery holders, but you'd have to improvise some kinda plug for 'em, they come without any.


June 6th, 2006, 04:02 PM
Sadly the RTC is not socketed, I'll dig out the exact details of it tomorrow.

Is there any way to charge the battery? My Amiga 500 which doesn't have a CMOS (the RTC is aftermarket on the memory expansion) had a similar problem, but leaving it plugged in with the computer switched off for a couple of days charged the battery up enough to keep the time. However, the 486 doesn't seem to want to do this, and I don't really want to keep it powered on unattended...

The voltage is 4-6 volts? I've got some holders I can rig up, so fingers crossed it will work. I'll try it out tomorrow and report back! :D

Thank you :)

June 6th, 2006, 07:57 PM
I rigged my Amiga 2000 with a coin type battery. All you do is remove the battery that is currently in the machine, use a coin type holder, and use a diode so the circuit cannt try to recharge the battery (it will blow up if you try to recharge a coin type battery).

You can do the same with a 486 once you figure out where the battery leads are.

Terry Yager
June 6th, 2006, 08:19 PM
Usually, those boards that make accomodation for external battery packs will have the battery circuit already protected against over-voltage (6 vs 3 volts) and non-rechargeable batteries.


June 6th, 2006, 08:45 PM
Don't they make Lithium-Ion coin-type batteries?

June 6th, 2006, 09:57 PM
Don't they make Lithium-Ion coin-type batteries?
Indeed, most are lithiums, but they are not made to be rechargeable. I think they lack a certain chemical to do so.

June 9th, 2006, 06:19 AM
Here's a picture of the battery that is built in to the mainboard - the external battery can connect to J6. However, J6 has 4 pins... which should I connect +'ve and -'ve to?

The battery on the mainboard is a 3.6V 60Ah.


Terry Yager
June 9th, 2006, 09:34 AM
It's probably pins 1 & 4 (pin #3 is sometimes removed). It shouldn't be difficult to determine the polarity. JP26 probably selects between the on-board battery & an external pack.


June 11th, 2006, 09:40 AM
Sorted it!

I now have 3 1.5v AAA batteries in a holder as the battery, I managed to get a suitable connector to plug it into the mainboard. It's been keeping perfect time ever since :)

Thanks for the help all who replied :D

Terry Yager
June 11th, 2006, 09:45 AM
Don't forget to remove the old battery before it leaks and ruins the board.


June 11th, 2006, 10:20 AM
Will do! :)

June 11th, 2006, 01:53 PM
ok, another problem now...

I put in the new battery at 6:30... at 7:30 the time was still keeping...

but I just checked it at 10:30, computer cannot start!!! unplug the battery and it eventually boots. the time is 8:30 in the setup.

I set the time correct, and put the battery back on... I'll check it tomorrow :)