ziggy587 wrote on 2022-11-07, 01:09:

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?

The grey things called F1 and F2 are a fuse for the KB and PS2 port, maybe F2 is for the USB power. Generally the +5v from the PSU connector routes around and goes to a fuse then to the KB.
F1 is near corner of the board by the KB port, F2 is at end of first PCI slot.
I suggest you check the volts across the fuses (specifically F1 first) as it could be near burned and acting as a resistor, which would show a voltage drop across when a KB is actually connected.
You should also check the traces from those fuses (again start with F1) to the +5v PSU connector and from the fuse to the KB connector.
In an emergency you could direct wire from the KB +5v pin to the PSU +5v but is not recommended....

Thanks, that helps a lot!

I see +5v going to a small electrolytic cap, then to the F1 fuse. From the fuse it routes to a small SMD chip, 0805 in size I think, but I don't know what this component is. The signal jumps across this component and then to the power pins on the keyboard/mouse ports. Checking from the +5v pin of the main power connector, resistance is very low across the cap and fuse (about 0.2 ohm). But I'm measuring 6.14 K ohm across this unknown chip component.

The other 0805 components in this area, I can see they are clearly marked as capacitors (C7 for example). But this unknown components, I cannot make out what the silkscreen is. Looks like two lines, maybe L1?

You can see the thick trace coming off the fuse, going to the unknown component.

I was checking resistance across these components because it's less risky tracing everything out without power (to avoid shorting anything and causing further damage). I can check the voltage across the fuse though.

edit: OK, I checked the voltage. I'm getting 5.02v across the fuse. But 3.7v at the KB/mouse ports.

Also, I forgot to mention. If I power the computer on with nothing plugged into the KB/mouse ports, they will measure 5.01v. It's only when I start plugging things into them that the voltage drops.

Ahh yes many have a very small inductor for HF filtering. Can you try to resolder one side at time (let it cool before trying otherside) and see if it makes a difference, could be a cold solder joint.
The inline inductor should not be more than a few ohms as it has to allow up to 500mA thru with little v drop for the KB...
If that does not work you can short across it (should not hurt) as it just makes sure some KB noise does not come back into the 5v lines but am sure there are dozens of caps from 5v to ground all over the board.....

Since I was probing the inductor from the top of it, and not from traces, wouldn't that mean that the resistance across the inductor is indeed very high? I can try and reflow the solder joints anyway. What would an appropriate value be for this inductor? If reflowing doesn't help, I can short it to test with. But as a permanent fix, I'd rather replace it.

One of my KB controller schematics lists a 3uH on the data, clock and +5, the other schematic just calls them a RFC...🤣. Let me check some archives.
added: the Holtek KBC datasheet schematic shows no RFC, just a 47pF cap to ground...
Yeah shorting it will not hurt for testing.

Thanks so much for your help!

I had similar issue on one of my mainboards, one newish keyboard worked, but most did not. One even worked "half way", worked fine until I turned any LED on keyboard then it started to turn off/on.

When I tested voltage there was huge drop the moment keyboards were initialized. The one that worked actually was able to work with below 3V voltage and that's why there were no problems...

In my case motherboard had transistor to control power to AT - PS/2 ports and it went bad. When I replaced it all keyboards worked.

If everything else fails try tracing 5V path(s) from connector to see through which components it goes. Culprit is somewhere along the way.

ziggy587 wrote on 2022-11-07, 02:24:

edit: OK, I checked the voltage. I'm getting 5.02v across the fuse. But 3.7v at the KB/mouse ports.

If you have a 5V drop across the fuse then I'm going to say the fuse blown. The only reason you can read any voltage at all at the ports is because a high logic signal somewhere is backfeeding a pull-up resistor. (Like when you plug Sega control pads into an Atari...)

bakemono wrote on 2022-11-07, 09:51:

ziggy587 wrote on 2022-11-07, 02:24:

edit: OK, I checked the voltage. I'm getting 5.02v across the fuse. But 3.7v at the KB/mouse ports.

If you have a 5V drop across the fuse then I'm going to say the fuse blown. The only reason you can read any voltage at all at the ports is because a high logic signal somewhere is backfeeding a pull-up resistor. (Like when you plug Sega control pads into an Atari...)

Even though I'm measuring near 0 ohms across the fuse? And the inductor is measuring 6.14 K ohm? If the fuse was blown, wouldn't I get no resistance reading at all across it? Or as Horun stated, it could be on the verge of blowing. I can try replacing it. Does anyone know what rating I should get for such a fuse?

If the fuse is blown resistance would be infinite, i.e. broken connection.

But I'd guess problem is elsewhere, if fuse was the issue none of the keyboards would work.

How you tried checking voltage on both ends of the fuse (voltage, not resistance, black probe on GND, red probe first on one end, then on the other, just to be sure 😀 ) while it's turned on and keyboard plugged in? If it's 5V solid on both ends then I think the problem is elsewhere.

I had similar behaviour with Commodore 128 PSU, it "worked" while not plugged into the computer, all voltages read fine. But when I tried to turn the computer with it nothing happened. Turns out wire in the fuse was broken, but made contact, so when I measured it unloaded it seemed fine, but under load it would bend and loose connection.
That was "classic" glass fuse, this is different, but who knows, maybe it's possible that something like this cold also happen.

that tiny inductor isn't supposed to measure >6K ohm. it's likely broken. replace it or bridge it with a wire.

Yes, I can measure 5v on both sides of the fuse while a keyboard is plugged in. The voltage definitely only drops after the inductor.

I have removed the inductor and jumped the pads for it. Now I'm reading 5.02v at the power pins for the KB and PS/2 ports... While both a keyboard and mouse are plugged in! So, confirmed, it was the inductor that was the issue.

Moreover, it seems the inductor was affecting some other things that I was unaware of. When I booted to Windows 98 for the first time after replacing the inductor with a jumper, the New Hardware Wizard auto detected a bunch of things. The mouse, of course. But then a communications port (twice) and a printer port. I guess that's the two serial and one parallel port that this motherboard has. So it would seem that the inductor was affecting those ports as well, and I just didn't know it since I have yet to test them. You can see in the last picture that I posted, from the inductor the trace goes to the KB port, but it also goes to C86.

The next time I place an order with DigiKey, I will order a 3µH inductor to replace the one that I removed from the board. Unless someone suggest another value. Also, now I'm kind of nervous about those fuses. Does anyone know an appropriate value for a replacement?

After shorting the pads for the inductor, I've had the computer powered on for at least an hour now. I've done some minor poking around in the OS, and typed a bunch in Word Pad. No issues as far as I can tell. Thanks, everyone!

That is good news. Interesting that all that "new hardware" was found but that could be just because the Keyboard was found which then triggered more items found...
Any SMD Inductor from 1uH to 3uH should work fine as long as it can handle 1 to 2 Amps and the DC resistance is less than .2 ohm (Or else V.drop could be to high under max load) IMHO.
Just for reference (this one is only 2mm long so you will need to measure yours and 130 milliOhms or .130 Ohms :
https://www.mouser.com/ProductDetail/Murata-E … jgC4%2FQbDm8%3D

Those fuses are typical a 1 Amp slow-blow radial lead, doubt you will find any that look exact like yours but these would work, anything similar would be good as these can tolerate the in-rush current to the keyboard that can happen on power up:
https://www.mouser.com/ProductDetail/Littelfu … prZKaU3KQ%3D%3D see datasheet

Thanks! This will be one of those cases where the DigiKey or Mouser shipping will cost more than the parts, so I'll have to wait until I need some other stuff before I order the inductor and fuse. But I will update this thread once I have it completed.

I got the inductor in and replaced the jumper wire on the motherboard. The keyboard still works, and with the mouse plugged in. I'm now reading 4.7v at the KB and PS/2 ports. Before with the jumper wire it was measuring 5.02v. I don't know if this is normal, but everything seems to be working fine so far.

Here's the exact part that I ordered: https://www.digikey.com/en/products/detail/mu … 1R0NGRD/4358093

Thanks again for your help, everyone!

Great !! yes normally you would see a small voltage drop across the inductor with both KB and mouse attached, mostly from the KB.
Being it is many turns of a very fine wire wrapped around a ferrite core (encased in ceramic, epoxy or plastic) there should be some V. drop, 0.3v under full load should be fine....just thinking out loud 😀