trying to troubleshoot a locked on LED on a bally mpu -35.

with my probe i have a Pulse at U9 pin 5. I see the signal then goes through R136 and proceeds to U19-8.

on my board the output of R136 is High and NOT pulsing. is that correct?

U19-10 (output) is pulsing.

thanks.

now i checked it again and it was pulsing. im not sure why it wasnt before. i had checked tp7 before and it wasnt pulsing as well.

well now that i got that issue resolved lol, along with previous issue, U9 pin 18 (A9) is high/not pulsing as well. is there anything other than a bad U9 that would cause that? U9 is a brand new chip, and i switched it out for another known working 6800 chip, and u9 pin 18 was still high/not pulsing.

U6 and U2 pins 22 (A9) are both high/not pulsing. also U6 pin 18 is high/not pulsing as well. not sure if that one is normal.

to note, im powering on a bench, with U2, U7 and U8 removed. U9 and U11 are new. U6 is recently burnt and verified, ALL reset components replaced, traces inspected with multi meter for continuity.
i have new U14-u20 chips arriving tomorrow.

U9-18 is High/not pulsing

U6 is a 2732
jumpers i have connected are:
13a-4
12-ground
7-8
31-32
33-35
16a-29

looks like it may be a culprit U17. I have a 74LS00 and a 74HCT00 chip arriving from GPE today. I'm hoping one of those is an okay replacement, I was unable to find the exact chip number replacement.
I appreciate the response. ive been going crazy for a month now, staring at two locked on led Bally MPUs. ill save the other one for another post, its in worse shape than this one I think.
ill post results later.
thanks a lot.

I followed the path u19-u14-u15. It all checks out good.
U8 socket does look crappy. It's one of the only ones I haven't replaced yet. I checked continuity of it's traces to the chip legs and it was okay, but I'll replace socket tonight
Just to be clear tho, my issues changed from missing vma signal to now a not pulsing a9 signal. That could be caused by a bad u8 socket?

it was the same. it read .52 for all three

Pins 13 and 11 are both high

Quench ill check a12 in a bit. (im not quite certain if U17-13 was high/not pulsing or just a REALLY high pulse. it sounded slightly modulated compared to the high/no pulse of U17-11)

also just an observation i had: if i reversed the multi meter leads (red on chip legs, black on ground) while testing resistance, U9-19 and U9-20 read 0, but U9-18 was .92

U9 is a new chip i just recently ordered. ive also switched it out with another known working one, with same results. I sadly cant actually test it tho because my other Bally -35 decided to have a stuck led a few weeks ago as well.

this board has never booted for me tho btw. it came in a game i recently got.

it was the resistance setting on my cheap radio shack multi meter. setting has the diode symbol and a V underneath. display reads 0.923. i wish i had a clearer answer.

72438.pdf

aaaaand, low and behold, there was a jumper on back of board between U6-22 (a9) and U7-10 (a9). i cut that and now U9-18 has a pulse.

okay so now..
U9-18 pulses
U9-22 is high
U17-11 is still high
U17-13 is high, (pulse light does not flash on probe, but does sound slightly modulated unlike U17-11's high pitch sound)

still doesnt boot tho. i assume high U9-22 is throwing it off...

also to note, there are no other unmentioned jumpers on this thing. also i did replace U8 socket with no change in results

thanks again.

Quench i will test U17 tonight.

feel I should mention that I received this board with SOCKETED chips for U14, U17, U18, U19, and U20..

incase that brings up any red flags or other ideas for testing a shorted U17.

other than the sockets and the jumper I removed, board is typical to a working one. no other "improvements" made lol

(yesterday I also received new replacement chips for U14-U20, so I have those to help diagonose. of course the U17 are the only ones I'm not sure are correct because chip number varies slightly. I ordered 74HTC00, and 74LS00 as possible replacement U17's)

i bent U17-13 out and CPU-22 had a pulse.

i replaced U17 with a new 74LS00 and GOT A FLICKER.

ive never been so happy to see a light blink before.

got all excited, threw all the other chips onto board, put it in the machine, and got a solid LED.

took out chips realizing i should put them in one by one, and noticed i had bent a leg on U8 chip because im a dingus.

gonna put them back in one by one. will report back with hopefully good news.

I GET A FLICKER. U17 had gone bad, and a incorrect jumper cable screwing up an address line.

with that said, with U2 chip inserted in its socket, i get a solid LED. i will try burning a new one tomorrow, and read up on why that can happen. I will mark this thread as resolved tho, as getting the flicker was the big battle.

i really appreciate the help. this last month i learned way more than i expected about this board. this thread got me to the finish line.
thank you.