I recently got a 486 Compaq Presario, which is the only all-in-one system from this era in my collection, so I’m keen to get it running again. However, I’m facing some issues.

On startup, the BIOS beeps once long and three short, then hangs on the dreaded 102 error, with a blinking cursor below it. A PC analyser card displays codes 90 and 92, though I’m not sure if those are meaningful in this case.

The chassis was extremely rusty, and based on my past experiences with heavily corroded vintage systems, the CPU is usually gone too. That turned out to be true (https://ibb.co/rf2XqXNQ) —when I removed the original 486 DX2-50 CPU, several pins were either broken or fell off completely (https://ibb.co/Lhnp8xd5) I installed a working DX2-66 and swapped the RAM, but the issue persists.

I have a couple of questions:

External BIOS Battery – If I want to use an external battery as I think it will be better choice, do I need three AAA batteries (4.5V) or two? Should it be 3V or 4.5V? and where exactly should it be connected? (https://ibb.co/4Rjxz2qH) I see three pins, but which two are for positive and negative?

Motherboard Cleaning – I already washed the motherboard with hot soapy water and a brush, but I think it still needs further cleaning. Since I don’t have an ultrasonic cleaner, what’s the next best alternative for removing residues from a motherboard? more hot soapy water?

I've already checked this topic (SOLVED Compaq Presario CDS 524 - 102 - System board failure), but it didn’t answer my questions. Also, is there a service manual for this computer that I can download?

thanks 😀