I'm new to the forum.
My name is Andreas. I'm 49 years old and from Stuttgart / Germany.
After reading a while and learning many things about pins and their electronics I decided to subscribe to the forum.
Thanks for your valuable input!
A few weeks ago I startet my first pinball project.
I'm trying to bring a Power Play (MPU 2518-17) back to life. After it's many years of hibernation I'm trying to restore the pin. The MPU board had the expected problems with oxidation so i changed all headers and sockets with new ICs (U8, U9, U10, U11) after cleaning the board carefully.
It boots fine but there seems to be a bigger problem with the switch matrix. After I got several wrong (and differend) results on switch testing, I isolated the MPU from the playfield and connected only J4. Then I measured the Strobes and Inputs on J2 with a scope from PIA up to the header pins.
I got readings of very nice and crisp signals up to the anode of the Diodes. In case of ST0 the diode CR43. On the kathode side of the diodes the signals goes bad. I looks like the signal can't "discharge" fast enough. Or some other signal or potential is crosstalking and keeps the strobes on "High". Sometimes after powering up the signals on the strobes are good. But only for a few minutes the they stay high again.
I'm staring at the schematics for hours and disconect caps and resistors along the trace. But no difference.
Any ideas what can cause the signals to stay high?