Keyboard issues

[smitty]

I've started having a weird issue with the keyboard not working on my PCjr. 9 times out of 10 it'll be completely unresponsive but every once in a while it'll work. This happens with it plugged in and I haven't had any luck with it wireless. Just to rule out the keyboard itself, I got the keyboard and joystick breakout board from Texelec to try a regular PS/2 keyboard. Unfortunately I have worse luck with that. Has anyone else had this happen?

[DoctorOctal]

If you’re having trouble with the PS/2 keyboard as well, that may indicate a problem on the motherboard. However, the PCjr keyboard’s symptoms match another common problem that you can check and fix pretty easily.

A particular (later?) revision of the keyboard PCB has a tantalum capacitor, C4, that tends to become electrically leaky. This capacitor is part of a simple RC delay that keeps the keyboard briefly in reset when power is first applied. When the cap becomes leaky it doesn’t hold its charge completely and causes the keyboard to drift in and out of reset (or just stay reset permanently). One moment the keyboard works perfectly, the next it’s completely unresponsive.

Open up your keyboard to check C4. There are six screws on the bottom, one underneath the serial number sticker. Once inside, notice how the capacitor looks. If it has a black, molded case, it’s the type that tends to leak charge. If it’s silver in color, it’s probably OK.



Plug the keyboard into the PCjr with a cable and, while the PCjr is on, check the voltage across C4. It should be steady and nearly 5 V. If it’s less than, say, 4.5 V, the capacitor is leaking to some degree. With the symptoms in your case I’d expect to see a voltage around 2.5 V or less (because 2.5 V is the approximate threshold for the keyboard to switch into the reset state).

If your C4 is leaking, simply removing it will probably cause the keyboard to work. The power-on delay is meant to ensure a known and correct state of the keyboard, however, so it’s best to replace C4. The type of capacitor is not critical as long as the capacitance is about the same (2.2 µF) and it fits the space. If you use a polarized cap, remember to put it in the right orientation.

[Brutman]

Your ability to debug these types of problems continues to amaze me!

(I'm at the point where I know to replace capacitors, which is still a pain. I should get the correct equipment so I can determine if a capacitor is actually bad after I pull it.)

[smitty]

Thanks DoctorOctal for the tip! Unfortunately it measured 5v consistently. I'm guessing it must be with the motherboard.

[DoctorOctal]

Well, so much for the easy fix.

Yes, at this point it appears you have a bad chip or bad connection on the motherboard.

Take a look at the keyboard section of the SAMS Computerfacts schematic of the motherboard. (The PCjr technical reference has most of the keyboard logic on page B-10.) Next to each pin, it shows the logic level you should expect to see when no keys are being pressed and the keyboard is not plugged in. Check for correct levels all the way to the 8255 (ZM22). SAMS has a diagram for locating chips on the board.



When you hold down a key on the keyboard, you should see pulses on ZM22 pin 11 and ZM22 pin 14, though they may be hard to detect with just a multimeter. You’ll probably need a logic probe or oscilloscope.

[DoctorOctal]

Your ability to debug these types of problems continues to amaze me!

Don’t be amazed yet.

Computer Reset had cases full of new-old-stock keyboards, but when I tested them many seemed flaky or completely broken. They had me scratching my head for a while until I finally opened one, looked at the SAMS schematic, and began checking the signals systematically. At that point it only took a few minutes to figure out what was going on.

Later I discovered that Dave Jones of the EEVBlog had tracked down the same problem in his own PCjr keyboard without the benefit of a schematic.

Unfortunately it appears that smitty’s keyboard problem lies elsewhere and will require more troubleshooting.

I should get the correct equipment so I can determine if a capacitor is actually bad after I pull it.

ESR meters and the like are certainly helpful, but in the case of C4 such equipment might deceive you: the bad capacitor looks OK to both my capacitance meter and my ESR tester. It’s only leaking about 25 µA, which might be negligible in some applications but is a show-stopper for the power-on delay in the keyboard.

[smitty]

You’ll probably need a logic probe or oscilloscope.

Thanks again! Would you have any recommendations for a good starter logic probe?

[Brutman]

I use a B&K Precision DP52 from Amazon which is good for up to 50Mhz. They are around $35 now, which is quite a bit less than I paid for it 5 years ago.

[DoctorOctal]

The B&K probe looks good. If price is an issue, the Elenco LP-560 ($20, 20 MHz) might be decent as well. You don’t need much to work on a PCjr, though a faster probe could be handy when working on a faster machine.

I just have an old Micronta probe (Radio Shack catalog number 22-303) that I bought some time in the late ’80s or early ’90s. It’s rated for 10 MHz or 50 ns.

Of course, you only need a volt meter to check steady logic levels. “High” is >= 2.0 V, “low” is <= 0.8 V.