Troubleshooting Apple II Plus Keyboard

[oldpcguy]

I am trying to troubleshoot this keyboard as a significant number of the keys do not work. I first suspected the keyboard was dirty so I separated the key assembly from the circuit board and visually inspected / cleaned the contacts with isopropyl alcohol and reassembled. That did not solve the problem. This led me to suspect the keyboard decoder board which I swapped with a known working one. That did not solve the problem either. Now I am back to suspecting the keys / keyboard contact circuit board.

A review of the Apple II Reference manual provides a matrix which shows the x, y connections for the keys and how they map back to the keyboard decoder chip. I would like to use an OHM meter to test the continuity for each of the keys by utilizing these x, y mappings. However I cannot easily do so from the keyboard decoder chip so I thought the best place would be the connector which connects the keyboard key circuit board with the decoder circuit board. However there is no pinout of this connector in the reference manual and my, admittedly, limited testing does reveal a complete mapping for each x, y line. This seems odd to me as the reference manual diagram clearly shows the mappings back to the keyboard decoder chip so it seems reasonable all of them have to go through this connector.

My attempt to map the connector was to play one end of the meter on the first pin of the connector and then test each of the pins on the keyboard decoder chip until I heard a continuity chime (i.e. a closed circuit). Once I had the pin on the keyboard decoder chip I would use the reference manual diagram to record the x or y line to the connector pin I was currently testing. I then repeated this process for all the pins on the connector. When I was finished I had mapped six of the y lines and one of the x lines to the connector. According to the reference manual the x lines have nine connections to the keyboard decoder chip and the y lines have 10.

Does anyone see anything obviously wrong in the above methodology? Any advice on how to troubleshoot this?

 

[retrogear]

There are different versions of keyboard but if yours has the 25 pin connector into a piggyback encoder board these are the pin# P1/J1 for the matrix. This is from the Sams Computerfacts CC1
Which keys are not working ? Do they correspond to one of the rows or columns ?

 

[oldpcguy]

Thank you for this information retrogear. The problematic keys are:

3, 6, 8, and 0 along with #, &, and (
RETURN
w, e, r, t, u, and o
s, ;, +
c, n, /, ^, and ?

For the first row the symbols #, &, and ( are the shifted version of the former (3, 6, and 8). The comma works fine, it's in the listing to separate the keys.

I haven't observed a pattern but I haven't had much time to really dig into it.

 

[oldpcguy]

I printed out the diagram provided by retrogear and then used a highlighter to highlight the non-functional keys. Unfortunately, there appears to be no one common connection point between them all. In fact it appears that for every x,y key combination which does not function there is a key a is a functional key using one or the other x or y line.

Either I did not correctly identify the functional / non-functional keys or there is some other explanation. I will use an OHM meter to test the connections but I suspect there are no broken traces (which I was unable to identify using a visual inspection under a magnifying lamp).

 

[oldpcguy]

I think I have the issue resolved. First a thank you to retrogear as the diagram he provided was instrumental in troubleshooting this issue. With it I was able to use the OHM meter to determine there was no visible keyboard circuit board damage (which confirmed the visual inspection). Based on the meter testing I concluded the problem most likely was with the individual keys (even though a visual inspection didn't show any issues).

While performing the meter testing I was working on the solder side of the keyboard circuit board connector and notice it was extremely dirty with solder flux. Knowing the flux can cause issues when surrounding chip pins I decided to give it a clean with IPA. I grabbed a brush, dampened it with IPA, and lightly scrubbed away the flux. It cleaned up nicely and looked much better. While I had the IPA out I decided to clean each key pad a second time. I reattached the keys with the keyboard circuit board and gave it a try. All the keys appear to be working. I will need to install the keyboard assembly back into the case but it's very encouraging.

In conclusion the problem ended up being a lot of flux on the keyboard connector or a second cleaning of the key pads on the circuit board.

Before:


After:

 

[daver2]

You will probably never know...

But I always clean the flux off when I see things like that. It may have been ok when it was manufactured, but some fluxes can become conductive over time.

Nice job in cleaning it up though...

Dave