Hi!

Pretty weird.
Technically, there was no such thing as a 75 MHz 486 chip, you'll agree.
If not just bogus, that would have to be a DX4 with 3x multiplier with 25 MHz FSB, but it seems unlikely that a DX2-66 would behave in any similar fashion.
Maybe that's what is actually jumpered in, but that would be strange.

Do you have a manual for the board, is the model known?
Is it this bugger here?
http://j12345.users1.50megs.com/menu/pb450/pb450man.asp.htm

Maybe post a good photo of it.

Also, not a solution, but sometimes you can set the bios to jump the memory test. "quick power on self test" or something like that. Just to see if that changes anything.

Good luck!

yep it is a PB450 and thanks for that link - I have a decent link http://www.uktsupport.co.uk/pb/mb/450.htm but your link above is a little more comprehensive on the cache chips, which is my next upgrade (since I have none at all installed). I'll look into changing that setting and see if it does anything at all. Thanks again!!

::edit:: it seems like I don't have a quick setting in my bios at all - it's a super basic bios with not many options at all.

::edit2:: this time i saw it said 40mhz first boot

Maybe there are some bad contacts buzzing you. You could try to clean all jumpers and contacts you can find with isopropyl alcohol and see if that's the issue.
I had something like that with a vlb system lately, the video sometimes works, sometimes not. Then the keyboard wouldn't work. Than there was no ide or floppy.
Cleaned all contacts and took me two hours, but the system now runs flawlessly.
The contacts did look clean at first, until I used the wipes and isopropyl... The wipes went black instantly.

waterbeesje wrote on 2022-04-12, 08:42:

Maybe there are some bad contacts buzzing you. You could try to clean all jumpers and contacts you can find with isopropyl alcoh […]
Show full quote

Maybe there are some bad contacts buzzing you. You could try to clean all jumpers and contacts you can find with isopropyl alcohol and see if that's the issue.
I had something like that with a vlb system lately, the video sometimes works, sometimes not. Then the keyboard wouldn't work. Than there was no ide or floppy.
Cleaned all contacts and took me two hours, but the system now runs flawlessly.
The contacts did look clean at first, until I used the wipes and isopropyl... The wipes went black instantly.

Thanks for that suggestion! I have a spray can of some contact cleaner. I'll try to spray under the jumpers and the power connector today and see if that helps at all. It sounds possible, for sure!

I think I have another symptom. The sound card that came with this computer was a Washington 16 (mmsn824) but it was giving me issues when I first got the computer and assumed the sound card was bad. It was showing corrupt characters kinda going from the top left diagonally down to the bottom right. I ended up just buying another cheap card, a Nova 16 Extra (mmsn811) but it ended up being a downgrade and only had mono sound in Duke 3d, etc. but it worked well enough and booted correctly.

A few days ago I decided to look for another card and found an exact replacement for the original one (mmsn824) at a decent price. I received it today and it did the exact same thing with the diagonal characters! Rebooted it and the symbols went away, but the processor wasn't recognized correctly again so i rebooted it yet again and it finally worked.

The CMOS battery was dead but not leaking anywhere and I removed it and replaced it with a AAx2 external battery box and made the proper connector.

The computer also seems to take much longer than I remember to turn the monitor on and start the post from the time I hit the power button and the green light comes on. Could this be a power supply issue? It's the original power supply - that thing can't be long for this earth... it's like 300 years old in power supply years 😀

With my new sound card the computer sometimes takes 5 or 6 resets before it boots correctly now. With the old sound card it would take only one reset almost always.

waterbeesje wrote on 2022-04-12, 08:42:

Maybe there are some bad contacts buzzing you. You could try to clean all jumpers and contacts you can find with isopropyl alcoh […]
Show full quote

Maybe there are some bad contacts buzzing you. You could try to clean all jumpers and contacts you can find with isopropyl alcohol and see if that's the issue.
I had something like that with a vlb system lately, the video sometimes works, sometimes not. Then the keyboard wouldn't work. Than there was no ide or floppy.
Cleaned all contacts and took me two hours, but the system now runs flawlessly.
The contacts did look clean at first, until I used the wipes and isopropyl... The wipes went black instantly.

I took the board out last night and cleaned it really good using spray contact cleaner in all the plugs, sockets, connectors, and jumpers and reassembled it, but it's the same problem still even when booting with no cards installed 🙁

Hmm I have a similar old PBell but board 180433, it started giving issues which lead to the PSU failing but after a year on a better PSU it started having issues again and is near dead now. I do not think the boards were made to last this long and suspect bad capacitors but is not worth my time to fix it since I have other 486's based on better boards... wish I could help but cannot think of anything.

Horun wrote on 2022-04-19, 22:46:

Hmm I have a similar old PBell but board 180433, it started giving issues which lead to the PSU failing but after a year on a better PSU it started having issues again and is near dead now. I do not think the boards were made to last this long and suspect bad capacitors but is not worth my time to fix it since I have other 486's based on better boards... wish I could help but cannot think of anything.

Thanks for the response. I'm thinking capacitors in the power supply or on the motherboard but I'm not sure where to start. I'm proficient enough with a soldering iron to do some simple capacitor swaps, I just hate to risk "fixing" something that might not need fixing.

I figured out the problem - it is the external AA battery box I put in. If I remove it, it recognizes the processor every time and the monitor turns on and starts to POST much quicker - like I remember it from my teen years. I was told that a diode is needed, still trying to figure that one out though. Another day, I guess 🙁