Keyboard issues

Hardware questions and modifications

Keyboard issues

Postby Acenewman » Sun Dec 27, 2020 9:17 pm

Recently acquired my first PCjr. Got it home and everything actually worked as it should with the keyboard connected with the factory cable. Of course I couldn't leave well enough alone and wanted to try the keyboard wireless. That is when I discovered the 30 year old exploded batteries. Well couldn't just leave it like that so I disassembled the keyboard, cleaned up the corrosion with some white vinegar, rinsed it, blew the excess water off and dried it with the head gun. Reassembled everything to find that all keys work perfectly except a --J. Did some tracing on the board and all of those keys map back to the 14068 chip. Suspecting that chip may be the source of my issue??? After much googling, didn't find a data sheet of any kind?? Anyone know the particulars of this chip?? Or have any other advice ?? Thanks, Chris
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Re: Keyboard issues

Postby DoctorOctal » Mon Dec 28, 2020 4:55 am

Acenewman wrote:After much googling, didn’t find a data sheet of any kind?? Anyone know the particulars of this chip??


It helps to search for the full part ID: MC14068B

It’s an 8-input NAND gate, and there are plenty of sources for the data sheet out there, including this one. As this is part of the popular 4000 series, other manufacturers have equivalent parts with slightly different designations. TI’s version, for example, is the CD4068B.

All of the keys connect to this chip on one pin or another. It may or may not be the problem.

  1. Get the schematic diagram for the keyboard from the Sams Computerfacts for the PCjr: first page, second page

  2. Using a multimeter, verify continuity from the keys in question to pin 10 of U6 (the MC14068B), pin 10 of RX2 (the blue resistor network right next to it), and pin 17 of U4 (the microcontroller).

  3. With the keyboard PCB lying bare on an insulating surface, plug it into the PCjr with the keyboard cable and turn on the PCjr.

  4. Verify that pin 10 of U6, pin 10 of RX2, and pin 17 of U4 are all reading about 5 V.

  5. Lay the rubber dome mat on the PCB and press one of the problem keys. A logic probe or oscilloscope should show pulses on pin 10 and pin 13 of U6. If you only have a multimeter, you should see pin 10 and pin 13 change to some middling voltage when you press a key.

    If you see no pulses on pin 10, and it just changes to a steady 0 V when you press a key, pay attention to pin 13. If pin 13 is still 0 V, that suggests U6 is the problem. If pin 13 changes to a steady 5 V, the microcontroller may be the problem.
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Re: Keyboard issues

Postby Acenewman » Mon Dec 28, 2020 9:00 am

Thanks for the quick reply. Getting ready to take a quick road trip to Lansing this morning. Will report back my findings later this evening. Chris
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Re: Keyboard issues

Postby Acenewman » Mon Dec 28, 2020 6:47 pm

Ok so finally got around to doing some testing and this is what I found. In order of the procedure you laid out:

1) got the Sam's Computerfacts, thanks for pointing that out. I have gotten them for other machines, but for some reason it slipped my mind to check for one for the PCjr.

2) Checked continuity and it is there to pin 10 on U6 & RX2 and pin 17 on U4 for the bad keys

4) checked pin 10 on U6, pin 10 RX2 and pin 17 on U4 and all are just shy of 5 V - 4.95 or so.

5) Checking voltage on pin 13 with a multi-meter it jumps from 0 to 0.1-0.2 for a key that works. On the bad keys it stays at 0 or jumps at most to 0.03V.

So is this suggesting that U6 is the problem??

Chris
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Re: Keyboard issues

Postby DoctorOctal » Tue Dec 29, 2020 3:33 am

Is pin 10 on U6 going to 0 V when you press one of the non-working keys? If so, then yes, U6 seems to be the problem. It’s not responding to input on pin 10 as it should.

As you see in the schematic, ‘Ctrl’ through ‘J’ make up one row of the keyboard matrix. The fact that all the other keys are working indicates that most of the circuitry is working, and the problem is restricted to just that row.

So what’s attached to that row? Pin 10 of U6, pin 10 of RX2, pin 17 of U4, and the traces and solder joints in between. The objective, then, is to isolate which is going wrong.

Your continuity check indicates that the traces and joints are fine. Having 5 V on the row when powered tells us that the pull-up resistor in RX2 is working (and it’s unlikely to fail anyway). That leaves U6 and U4.

U6 is a NAND gate with all of its inputs normally pulled high and its output low. Pressing a key should take one of the inputs low and cause the input to go high, triggering the startup of the microcontroller and strobing of the column lines (thus causing pulses to appear on the row line, not just a steady state).

If, when you press the ‘A’ key, you see a steady low on pin 10 of U6, that means the microcontroller isn’t walking through the column lines and probably isn’t on at all. Seeing a steady low on the output (pin 13) of U6 shows that it isn’t responding to the input properly and starting the microcontroller. If it were a steady high, that would be the right output (for a steady low input) but raise the mystery of the lack of column probing by the microcontroller. If you saw pulses on pin 10 and pin 13, that would indicate that U6 worked and triggered the operation of the microcontroller (U4), but U4 wasn’t reading pin 17 correctly.

The TI CD4068BE should work as a replacement. Mouser, Digi-Key, Jameco
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Re: Keyboard issues

Postby Acenewman » Wed Jan 06, 2021 9:30 pm

Replacement chip showed up in the mail today. Happy to report that after replacing U6 I have a functional keyboard again. Thanks for the input on troubleshooting. Your advice was spot on. Chris
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