Have been working on a later (1982) system 7 driver board that I acquired to use a spare for my Flash. Got the board working fine except I had all of row 8 of the switch matrix stuck intermittently.
In test mode with the playfield connectors removed (2J2 & 2J3) I was still getting the stuck row (all numbers in row 8 would cycle & lock) so I knew it was limited to the driver board. I replaced the usual associated 4049B ic for that row also the PIA - still the same, row 8 would lock & cycle between switch numbers & with connectors back in would activate associated coils about every 3 or 4 games. I ended up changing the other 4049B & also the two 7406 column ics as well with no luck.
I checked the schematic for the resistor values & noticed that resistors R77-84 were measuring 1k & they should be 4.7k. I could tell they were factory installed resistors.
I checked schematics for games from 1982 in case it was changed towards the end of production but they remained as 4.7k for these. Can only assume it was a mistake with the 1k resistors?
The resistor for row 8 (R84) was consistant with the others at 1k, I didn't think the 1k resistor was the actual problem as they all were 1k but no issue on other rows.
I swapped them for the correct 4.7k resistors & noticed that the newly installed R84 resistor was measuring 3.3k & not 4.7k as it should have, all the other new resistors were measuring correctly - 4.7K, this left the only possibility as the capacitor C46 was dragging down R84's value & the fault for the stuck row.
Replaced C46 which fixed the problem & R84's value was now correct at 4.7k - all working great. Seemed odd the faulty capacitor did not bring down the value of the original 1k R84 as it did with the correct 4.7k resistor.
