I am trying to finish up bringing a Black Knight back into fighting shape. I am running into some troubling switch matrix issues and am out of self solutions, so am reaching out to the community for help.

Switch Matrix column drive J2J-5 and J2J-6 are causing me headaches. Both columns were not registering during the switch test, but everything else was working fine. When I measured the resistors, I found that the resistors for pin 5 and 6 were measuring a few ohms instead of the 4.6K they were rated. I replaced them, but testing them out of circuit found that the original resistors were fine. The new resistors were measuring only a few ohms as well.

Next I did a diode test on IC18 and found that a few of the pins were not reading properly. I replaced the 7406 on IC18 and then measured all the resistors on every pin. They now all read 4.7k ohms!

When I put the board back in the game, now every time the column is triggered, every switch on that row is triggered. When I measure the column drives, all the colums read 4.5volts except for pins 5 and 6 which are reading only a few millivolts. I am not sure where these pins are getting ground from and I am not sure where to test next. I removed the connectors and jumpered the pins and confirm the issue is on the board, I am just stuck.

Any help is appreciated.

Just so I am clear, because this is confusing to me. According to the schematis, it is 2J2 pi 3 and 5, which appear to connect to the 0 ohm resistors marked 5 and 6.

Switch matrix short (resized).png

PINTEC

Thanks for your reply. That is definitely happening. I powered on the machine and both those pins short. They do not short when powered off. I removed 2J2 and tested the traces and the same thing happens. I don’t see anything that would short those 2 connectors, and none of the other connectors show a short.

Any idea on how to remedy this?

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Sure thing. Here is the whole driver board and the switch section. The 2 large resistors are the ones I replaced thinking the original ones were bad. They are still 4.6k ohms, but rated at 1watt.

I tested each 2J2 pad to ground. All read 4.5volts except for 3 and 5 (ignoring the key spot)

I then tested the pad of each resistor closest to the header pins and every resistor reads 4.6volts. I thought the resistor lead straight to the header pin, but looking at the schematic you uploaded, that does not appear to be the case. Not sure where to trace the voltage back to see where it is being lost.

I very much appreciate your help with this. As you can see, I am quite lost.

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I did a continuity test from the PIA to both 7406 chips, both chip pins and socket pads. The good news is that my solder job was good.

The bad news comes when I pulled out the logic probe. Pins 14 and 15 of the PEA had no voltage. It was not low, or high; just nothing. I assume that means the PIA is bad. For the record, here are the results of the first 1/2 of the PIA

20 - High
19 - High
18 - Low
17 - Both Low and High
16 - Both Low and High
15 - Nothing
14 - Nothing
13 - Both Low and High
12 - Both Low and High
11 - Both Low and High
10 - Both Low and High
09 - Low
08 - Low
07 - Low
06 - Low
05 - Low
04 - Low
03 - Low
02 - Low
01 - Low

I is going to take me a few days to get things together. I'll post back with my results as soon as I can.

pintech I socketed and replaced the PIA. The board appears to be working great. I'll play for a few days to make sure it holds.

Thank you very much for your help!!!