help diagnose old circuit board, fault

"robb" wrote in news: snipped-for-privacy@corp.supernews.com:

They use the same I/O lines to multiplex the display and the keypad. Not at all uncommon.

at

Thanks Gary,

Could you elaborate on that a bit more ? as i am a mere amateur

i have traced one side of switches back to the Q22-Q30 lines ?

the general layout is ... that two button/switches {increment, decrement} will share a line to one Q## output and repeat for 12 pairs of buttons (total 12x2 or 24 button/switches)

the other side of the switches get bundled/connected together into two groups of 12, that each group shares one line down the ribbon to the main controler board (with 12k resistor just before going down ribbon wire)

so the Q## pins on the SN75518 are Outputs ... So i guess there is some sort of timed pulsing and controler board has to figure out which button was pressed ?

thanks for your help, rob

and

ok i have posted partial schemat that i made by hand for the pertinent part of the button circuit board it does not include the VFD connections which are inline with the button connections nor the indiocator LEDS that mostly just connect to the DM7496 chips.

id therea good way to turn a MM into logic probe ? i am using the Fluke 177 true RMS meter

thanks again for any help, rob

that was posted on the

alt.binaries.schematics.electronic

and

first thanks to everyone for your time and help,

OK, so i followed most of suggestions from everyone

check and follow traces from the switches, to ribbon, to the VFD SN75518 and to the main board ( traces ok ) then probe signals from the buttons (problem) so i connected up my toy O-Scope (you'd never guess and fall on floor) and found no signal from the two VFD pins going to two broken butons and a mangled signal to the other button that does not work

i noticed several pins not connected to anything with good wave form can i just reroute the broken lines to those or do i need to just replace ic ?

thanks again for help from all that tried, rob