"robb" wrote in message
...
hello,

i am trying to repair a fault with an user interface circuit (UIC) board
and
control board out of an old 1987's computerized programmable Pfaff sewing
machine (made in Western Germany).

a for fun project for me , a challenge

programming consists of storing values in memory locations using keys (
+/-
buttons) on UIC board

PROBLEM : a group of 3 (+/-) *momentary switch* buttons on UIC board used
to change memory values do not make any changes when pressed.

the UIC board has a VFD output display (16 chars) and 12 input buttons and
5
indicator LEDs. All but 3 of the buttons register presses correctly. the 3
broken buttons seem to be individually ok as i get continuity changes when
i
test with multimeter and push buttons.

this UIC board is connected to the control board via 12 conductor ribbon
???

I am not sure what kind of things to do to isolate trouble ........

i have dome visual inspection of solder joints and components with no
obvious issues.

How would the VFD display values , LED indicatiors and inputing buttons be
managed through a 12 conductor ribbon ??? in 1980's style ?? how to trace
ot
monitor value changes.

Any troubleshooting or tracing ideas advice would br greatly appreciated.

thanks,
rob




Well without seeing the circuit I could only guess that they used RAS and
CAS to address the keypad matrix. (Row Address and Column Address).

The ribbon cable is a place to look but the addressing logic could be bad.

Tom