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Williams System 7 Switch Column Issue Help

Fantazia2

Fantazia2

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Ive got a Williams System 7 (Barracora) and activating any switch on Columns 5 and 6 (From the switch matrix in the manual) activates every switch on the same row that I need a bit of advice about.

Ive removed the switch connectors and ran a jumper cable from column 5 and then 6 to one of the rows and it does the same so dont think its a playfield issue, didnt try with columns 7 or 8 as they are not used on this game

I was looking at the schematic trying to get my head round how it works and what the issue could be and think since its the same on Column 5 and 6 and they both share a Hex inverter that it could be the hex inverter at fault, columns 1 to 4 work without issue and only trigger the jumped row.

I havent tested with a logic probe yet but just wanted to use the hive mind of knowledgeable repairers to see if I have this theory correct.

From looking at the schematic am I right in thinking that each columns output from the board has a pull up resistor so should be HIGH when the column is not activated and then when activated by the MPU it should go LOW, and the reverse coming out of the PIA since there is a HEX inverter inbetween, and that something is causing the column to always be LOW so it triggers all the switches in the row even and not just when the correct column is strobed.

So with my logic probe if its the Hex inverter at fault I should see that the output for the that columnsis always LOW but the inputs should be switching between HIGH and LOW when I'm in switch test, and if its the PIA at fault then the inputs should always be HIGH, is that the correct theory?
 
So the columns are actively strobed and the rows are read. You're right with the pull ups but: The CPU knows which column is active at any given time as it sets the register on the PIA accordingly. For the effect you're describing all rows must be pulled low at the same time which can't happen via the column side as that has no connection bar the jumpered column. Measure at the PIA inputs to see if all of the rows are high (think it is inverted) when switches on column 5/6 are closed. Can you switch in another PIA to test if there are internal shorts there?
 
Maybe I didnt explain the issue correctly, its not all rows that are getting activated its all switches in the same row as the one jumpered for all columns not just the jumpered column.

So if I jumper from Column 5 to Row 1, then in switch test all the switches on Row 1 get activated, not just the switch at Column 5 Row 1

I dont have any spare chips to swap out, but have a full driver board I know works to swap in just wanted to try and pinpoint where the issue was with this board before removing it.

My thought was the 2 columns with issues are always pulled low even when that/those columns are not being strobed by the CPU which is why the all the switches on the same row for the other columns are getting activated with a direct jumper as that column is always low even when the CPU is not trying to set it low, so either the outputs from the PIA for Columns 5 and 6 are stuck HIGH or the outputs from the Inverter chip are stuck LOW.
 
Those 7406 ics blow quite often. I’d go for that first. logic probe the 7406 outputs to check they are stuck.
then the pia to ensure it is strobing its col 5 and 6 outputs. If not and they are high all time then could be pia or the 7406 ic. Only way to tell is cut the 7406 col 5&6 input legs so can test the pia output on its own.
 
I’ve got caught out - suspected pia cos it’s outputs were not strobing only to find that the 7406 was the culprit ie it had failed in a way that had taken the input pins perm high - that’s why I now cut em if the input side looks dodgy.
 
drhex said:
OK, are columns 5 and 6 strobing or always on then?
Click to expand...
Havent logic probed it yet, wanted to know that what I was looking for was correct which AlanJ appears to have confirmed.

When any switch on either column 5 or 6 is activated, all the switches on the same row activate.
 
AlanJ said:
I’ve got caught out - suspected pia cos it’s outputs were not strobing only to find that the 7406 was the culprit ie it had failed in a way that had taken the input pins perm high - that’s why I now cut em if the input side looks dodgy.
Click to expand...

Thanks Alan, will probe inputs and outputs on the Inverter. Sounds like the inverter will need replacing either way if I have to snip legs to prove its not the PIA.

Im suspecting the inverter seeing as column 5 and 6 are on the same inverter.
 
Now need to find out which pins are the inputs and outputs for columns 5 and 6 as the schematics I have dont list all the pins only on column 1 and 7, I suppose I can back trace from the jumpers on W13 and W19 and PIA outputs with a continuity test?
 
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