Some may recall I worked on this board back in early summer.
https://pinside.com/pinball/forum/topic/how-to-determine-what-game-eproms-i-have
It was working fine. Once I verified the EPROMs I put it on the shelf as a spare. (We’ll call it board A)
Recently I was working on another board where I replaced an IC socket (board B).
https://pinside.com/pinball/forum/topic/how-do-you-remove-sip-sockets
Both boards are Bally -17s – both are jumpered for 2716 EPROMs. When I pulled out board A to compare some voltages and now it has a stuck on LED. I followed the steps in PinWiki. I replaced Q1, Q2, and Q5 (I replaced Q5 last summer but replaced it again). I replaced U8 socket last summer so I replaced all the other sockets. I installed new U9 and U11 ICs. I installed Leon’s test ROM (same one I used for board B). I have 5 volts at U9 – pin 40.
I checked the voltage across VR1 = 7.77 VDC. That seemed low but I checked board B and it was the same = 7.77VDC.
Today I checked the voltages in the Valid Power Detector section and compared them with the working board. Green = board A (stuck on LED) Red = board B (working). Everything seems to match closely.
Then I connected to the oscilloscope to check that the 5 volts was being held low. They both seem to be held low for about 10ms. (Notice that one scan is at 4ms per division and the other is 2ms per division). The yellow probe was on the bottom of R 139 and the blue was connected to TP5.
Scan of board B - working
Scan of board A - Stuck on LED
Everything seems to check out but I still have LED stuck on.
Anyone have any suggestions what to check next?
Thanks
Bob