I finally figured out why one row of my lamp matrix was bad.
(WARNING highly technical engineering rant below)
(see the attachment for the schematic of the circuit)
(and forgive any terminology that may be wrong, i'm a mechanical engineer by trade, but know enough about electronics to be dangerous)
I knew the wiring was good because if you grounded the tab of the transistor (Q80) that whole row would light. There had to be something wrong with the logic circuit. I figured there was something wrong with the U55 logic chip or the PIA. I probed around on the logic chip and I could see the input was the same for pins 9, 11, and 13. So the PIA was working fine.
Then I looked at the output of the logic chip, pin 8 and 10 would jump around on my meter between .5 and 2 volts. But pin 12 would only jump around between .1 volts and .5 volts. So I replaced the U55 7406 logic chip thinking it had gone bad.
Put the board back in and it still didn't work and I had the same levels on the output I had before. I started measuring various resistances in the different lamp row circuits, because they should all be similar. The big 0.4 Ohm power resistors were good. The 2.7k resistors were good. All good until I measured the resistance of R111 R114 and R117. R114 and R117 measured right on at 1.2k. BUT R111 WAS OPEN!!!!
I put in a new resistor and that lamp row started working perfectly. It all makes perfect sense, the 18V on the other side of the open resistor couldn't pull up the signal from the logic chip, and it wasn't powerful enough to latch the Q80 transistor to ground to light the lights!!.
sorry for the long rant but these lights have been bugging me for weeks.
and if i can help anyone else fix their machine, all the better.
Todd
Attachments to this post
-
whirlwind_schematic.jpg (59 KB, 1 downloads) 1 month old
We've got