VOGONS

ASUS ISA-386U3Q does not keep BIOS settings for longer time (several+ hours)

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First post, by maciejd

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Hello All

Perhaps someone can help, because I've given up.

I have an ASUS ISA-386U3Q motherboard that was saved from a NiCd battery that was about to start leaking.

I cleaned it, soldered in a new NiMH battery, charged it (48 hours of computer running).

The battery is 3.8V. The solder joints seems to be fine, following the "+" path, it maintains 3.8V. Then, on the TC4069UBP chip, it drops to about 3.3V (not sure if it should). But it's still there.

The beast isn't maintaining BIOS settings, however. It still works fine for short periods, but if left for 10-11 hours it loses memory - checksum error.

The measurements were taken after the computer was off for about 11 hours, so it's not like the battery is draining.

Does anyone have any ideas on how to troubleshoot further?

Reply 1 of 6, by weedeewee

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I don't see why the battery voltage would be going to a hex inverter.

Battery voltage should be going to the 82C206 either pin75 or pin 32, the datasheet isn't quite clear on that.

Could be that you're dealing with a dead or failing diode in the battery backup circuit. D1 or D2. those solderjoints do look a bit dull.

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Reply 2 of 6, by majestyk

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The TC4069UBP is the (rt-) clock oscillator, if it´s power supply was insufficient, the clock would run too slow.
You have to trace the battery voltage connection to the chipset / CMOS, if settings fade. It´s probably the 82C206 in this case (pin 15 and / or 32).

The attachment 82c206pwr.jpg is no longer available

Reply 3 of 6, by maciejd

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Weedeewee, Majestyk, thank you for super fast response.

On 82C206 I have 2.86V on 32 and the same on pin 75. A bit too low I suspect.
I also checked the D1 and D2. Soldering is a bit dusty. D1 give a voltage drop of about 0.623V, so in the mid of tolerance for silicon diode. D2 seems to be dead / open in both directions.

I need to replace the D2. Then we will see what next.

Thank you one more time!

Reply 4 of 6, by majestyk

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D2 is a low power Schottky with very small voltage drop. If it´s broken, replace it by a new Schottky.

Reply 5 of 6, by MikeSG

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Worn capacitors, most commonly the 10UF's, can cause voltage drop too.

Reply 6 of 6, by maciejd

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majestyk wrote on Today, 12:49:

D2 is a low power Schottky with very small voltage drop. If it´s broken, replace it by a new Schottky.

Ordered and will replace during the week. Originally it's 1N60 and mot probably I'll get 1N60P, pure 1N60 is quite hard to find.

MikeSG wrote on Today, 13:10:

Worn capacitors, most commonly the 10UF's, can cause voltage drop too.

Not sure if correctly, but I measured C18 (path going to pin 75 of 82C206). Minus of the multimeter on the battery, plus on input to C18 - showing 3.8V, plus on the side of 82C206 gives 2.8V. Multimeter on the capacitor itself also shows approx 1V, so the math checks out (potential difference between the capacitor terminals). I'm not the electronics master, but I guess capacitor is to compensate short power outages, here "tries to act" as voltage converter. Or the Alzheimer did its job on my physics lessons taken 30 years ago 😉

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