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386 external battery undervoltage?

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First post, by brian.t

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I have a Jamicon KMC-40A motherboard. I recapped the mobo, removed the onboard battery, and replaced it with an external 3.6V Tadiran TL-5242. Motherboard seemed to hold CMOS settings for a couple months now (although the clock may have been running a bit slow). Now the computer can't keep bios settings after a power cycle. I measured the voltage at the RTC Vdd pin and it's about 0.4V with power off, but the external battery is 3.63V. Just wondering if I may have selected the wrong battery. The attached photo shows the battery connection, the diodes on the mobo, and the RTC. Thanks!

Reply 1 of 3, by TheMobRules

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That battery should be fine.

Are you sure pin 1 of the external connector is GND? Usually it's the other way around with pin 4 being ground (however I have seen examples of both).

Also, I see some corrosion on the traces, maybe there is an interruption on the way to the RTC and the chip that stores the CMOS settings. So I suggest tracing the path starting from the battery '+' through the diodes and beyond.

Reply 2 of 3, by brian.t

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TheMobRules wrote on 2023-07-18, 17:28:

That battery should be fine.

Are you sure pin 1 of the external connector is GND? Usually it's the other way around with pin 4 being ground (however I have seen examples of both).

Also, I see some corrosion on the traces, maybe there is an interruption on the way to the RTC and the chip that stores the CMOS settings. So I suggest tracing the path starting from the battery '+' through the diodes and beyond.

Pin 1 was 0 ohms to ground, and pin 4 was in the Meg ohms. The corrosion seemed to be on the silk screen; I've inspected some other parts of the board and scratched it off showing copper underneath. I can do some more testing with the multimeter and report back.

Reply 3 of 3, by brian.t

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Looks like it could be a corrosion issue now after all. Weird that it was working for a few months already. Seems the path from pin4 (battery+) to D3 is broken. The path is fine from D3 to D2 to RTC_Vdd. I'll try to fix and report back.

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