First post, by ziggy587
- Rank
- Newbie
Hey everyone! First time poster here. I've been browsing the forums here recently, and this looks like a great place to ask about a problem I'm encountering.
I've recently purchased a Biostar MB-8500TUD-A motherboard from eBay. This is one of those turn-of-the-era boards that have some AT and some ATX features. So it has an AT style (5-pin DIN) keyboard port, and a PS/2 mouse port. From my testing, so far everything seems to work OK with the exception of the keyboard and mouse ports. The voltage is dropping off on them.
When I first received the board in the mail, I wanted to test it straight away. So I plugged in a PS/2 keyboard (a IBM KB-9930) with a PS/2 to AT adapter. The keyboard wasn't working, so I swapped it out for another random PS/2 keyboard from my stash. Some more modern Compaq, a real cheap one. The Compaq keyboard works fine. Since then, I've purchased a new old stock Acer keyboard with the AT style connector. When I plug it into this motherboard, it doesn't work. I currently don't have anything else to test this Acer keyboard with other than this motherboard. But I have ordered an adapter for it so I can use it in a PS/2 port, when it comes in I'll test it on another computer. But since the Acer keyboard wasn't working, and since that IBM keyboard worked the last time I used it, I suspected something was wrong. So I cracked open the new Acer keyboard. It looks very clean, no obvious signs of something wrong. So while the Acer keyboard was plugged into the motherboard, I powered on the board and used my digital multi meter to probe the connector on the inside of the keyboard. I was only reading something like 3.5v on the pins.
I suspected that maybe the power supply could be the issue. I was using a modern EVGA (this one) at first. The EVGA is basically brand new, I've only ever used it for minutes at a time to test various things. So I powered on the EVGA PSU with one of those little power supply testers, and probed various pins. The 5v pins were reading slightly over 5v, as expected. But I decided to swap out the PSU anyway, just to rule it out as a possible issue. So I swapped in an older Enermax. It's maybe 15-20 years old, but I bought it new and it has low hours on it. The same issue is happening with the Enermax. I also probed the 5v pins on the Enermax and confirmed it's putting out good voltage.
So today, I decided I want to test out more of this board to see if anything else might be wrong. I connected a HDD, CD drive, and floppy drive. I installed Windows 98 SE without issue and was able to boot to the desktop, with the only problem being that error message you get when Windows doesn't detect a mouse. So I powered off and plugged in a no-frills PS/2 mouse, a Logitech 2-button ball mouse. With the mouse plugged in, the computer booted to the desktop but neither the mouse nor the keyboard would work. I unplugged the mouse, and then the keyboard magically worked. So I tried the reverse, I powered off the computer and plugged in the mouse without the keyboard. It booted to the desktop, but the mouse was not working.
So I decided to use my multimeter to probe the pins of the keyboard and PS/2 port. With nothing plugged into either port, with the computer powered on both the keyboard and PS/2 port measure 5.04v on their 5v pins. But, as soon as you start plugging things in the voltage drops. With the Compaq keyboard plugged in (the only keyboard I have that works with this board) the Ps/2 port measure 3.75v, but it actually measures lower if I turn the num/cap/scroll locks on. I guess the LED for those locks draws a little more current so the voltage drops. With just the num lock on, the Ps/2 port measures 3.7v. With just the caps lock on it measures 2.19v. With the num and scroll lock on it's 1.993v, and with all three locks on it measures only 1.905v. So, no wonder the mouse isn't working, there's not enough voltage!
Looking at the motherboard, the keyboard and Ps/2 ports are very close to the AT main power connector. So I would have to assume that the 5v pins on those ports are connected straight to the 5v pins of the main connector. I don't see any major components in between. I guess next I'll have to back trace the 5v pins from the connectors and see if there's any components that could be causing the issue, but there can't be much if any. At least, I'm assuming the power pins are connected to the 5v rail. Or are they actually connected to an IC?
This motherboard has a footprint for the ATX 20-pin connector, but they did not populate it. I did a continuity check between the footprint and the AT power connector pins, and it seems that all the traces are there and they just decided to save the money on the connector. I don't see any missing components around the power connectors. So I'm thinking I can solder in the 20-pin connector and use that instead of the AT connector. I don't have any AT power supplies, only ATX. So for right now I'm using one of those ATX to AT power adapters. I'm wondering if there's possibly an issue with the adapter. I mean, it seems to be wired up correctly, but I wonder if there's a current issue with it? I have a 20-pin ATX connector on order, so once I get it in I'll install it on this motherboard. That'll allow me to use a power supply without any adapters. So I can rule that in or out as the issue.
But the weird thing is that everything else seems to be working fine. Again, I used a FDD and CD drive to install Win98 to a HDD. And once it boots to the desktop, aside from having no mouse, everything seems to work fine when I'm poking around on the computer. So the +5v going to those drives seems to be working OK. And when I probe an unused Molex connector, it's measuring slightly over 5v as normal. It's just the keyboard and PS/2 ports that are having the voltage issue.
Has anyone ever encountered this type of problem? Any suggestions on what I should do? I'm actually still in the return window for this seller on eBay, but I'd really rather not have to return it. I would much prefer to fix it if I can. But aside from ruling out the PSU adapter, or checking to see where the 5v pins go, I'm out of ideas. If it turns out to be an IC, then I guess there's nothing I can do to repair it. I mean, I suppose I could make an adapter that disconnects the 5v from the ports, and instead uses power from something else, but that would be kind of ridiculous. Hmm, maybe I could do that to at least test it.
Any help is appreciated!