(Topic ID: 244920)

2518-35 booting problem.


By wugly

1 year ago



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  • 111 posts
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  • Latest reply 8 months ago by Quench
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There are 111 posts in this topic. You are on page 1 of 3.
#1 1 year ago

had a corrosion problem on a harlem globetrotters. rebuilt the board using a corrosion repair kit and the game worked fine. decided to write new roms using 2732 with freeplay code, I changed the jumpers. I believe I got everything right. tried to boot the game and the game wouldn't start. game is at friends house and I think the led came on for a moment then went off. took board home and tried to get it to start at home on test bench using a test grade power supply not a pc power supply. If I take off the 12 volt power from tp2 then reattach I get the 6 blinks. seven if I put in the voltage for tp3. what could I have broken during this switch or are the proms causing me reset problems?

#2 1 year ago
Quoted from wugly:

had a corrosion problem on a harlem globetrotters. rebuilt the board using a corrosion repair kit and the game worked fine. decided to write new roms using 2732 with freeplay code, I changed the jumpers. I believe I got everything right. tried to boot the game and the game wouldn't start. game is at friends house and I think the led came on for a moment then went off. took board home and tried to get it to start at home on test bench using a test grade power supply not a pc power supply. If I take off the 12 volt power from tp2 then reattach I get the 6 blinks. seven if I put in the voltage for tp3. what could I have broken during this switch or are the proms causing me reset problems?

The bally MPU's power on reset was designed around how the voltages come up in a linear power supply. A PC power supply may not reset the board properly and leave you with a locked on LED.

If you get all the flashes by manually resetting then the MPU is probably OK unless there is some kind of reset problem beyond that you are using a PC power supply.

You can gently flex on the MPU and chips while you have it on the bench to see if it locks up. Otherwise check the voltages using the test points of the MPU while it is in the game. Taking the board out may have taken out a marginal connector.

#3 1 year ago

checked the test points on the mpu while it was in the game and everything was correct

#4 1 year ago
Quoted from wugly:

checked the test points on the mpu while it was in the game and everything was correct

Perhaps the reset section is damaged. With the 12v off of the MPU the reset (cpu pin 40) should be low, 0v. With the 12v connected it should go high. With a linear power supply you should see p40 of the CPU be low right when you power up the game and then go high a moment later once the 12v raises high enough.

You are in pretty good shape if you get 6 flashes on the bench. Really not that much else to lock up the mpu while in the game vs bench top.

#5 1 year ago

I had the reset circuit working before I changed the rams to 2732. I re configured the jumpers and the game would not go into reset. there is no battery on it. the reset circuit was repaired using a rebuild kit the first time to get it to work. grounding pin 40 on u9 makes the game boot.I have proper voltages on 5 volts and 12.8 on 12 volt line. Have replaced q1 and q5. measured voltage drop through vr1 approximately 8.2 volts. r1 r3 r2 junction 2.6. q1 base .7. q1 collector .03 I was expecting .3. base q5 4.8. tested cr5 and cr7 out circuit and they function c2 and c13 seem to be close to spec out of circuit. What could be wrong and what is the theory of operation.r1 r2 r3 gnding resets board

#6 1 year ago

Can not figure out why the board refuses to boot in the pinball machine. I check the test points on the mpu board and driver board and get 5.03 volts. the mpu gets 13.6 volts tp2. 5.03 on tp1 and 5. game flickers and does not startup when only j4 connector hooked up. take the board out of game and it boots on a test bench setup 6 times and I get the seventh if I rig the 22 volts up too it. ran power in thru the pins instead of test points on bench to check that off the list. reconverted the board back to original roms and it boots only on the bench. found out my friend replaced the plug on the game after we took a working board out and he had reversed the hot and neutral before my first test after rewiring the mpu, power looked good then. We changed the wiring to the correct configuration but the board still only boots on test bench and not in game. nothing else connected but j4 and I unplugged the sound card. thinking about rebuilding the solenoid driver voltage regulator board. put meter on ac voltage to test 5 volt and I got an intitial reading of .4 which cycle down to 0

#7 1 year ago
Quoted from wugly:

Can not figure out why the board refuses to boot in the pinball machine. I check the test points on the mpu board and driver board and get 5.03 volts. the mpu gets 13.6 volts tp2. 5.03 on tp1 and 5. game flickers and does not startup when only j4 connector hooked up. take the board out of game and it boots on a test bench setup 6 times and I get the seventh if I rig the 22 volts up too it. ran power in thru the pins instead of test points on bench to check that off the list. reconverted the board back to original roms and it boots only on the bench. found out my friend replaced the plug on the game after we took a working board out and he had reversed the hot and neutral before my first test after rewiring the mpu, power looked good then. We changed the wiring to the correct configuration but the board still only boots on test bench and not in game. nothing else connected but j4 and I unplugged the sound card. thinking about rebuilding the solenoid driver voltage regulator board. put meter on ac voltage to test 5 volt and I got an intitial reading of .4 which cycle down to 0

Does it boot in the game if you manually put the MPU into reset?

Make sure the zero crossing circuit is working okay and nothing shorting to adjacent pins. I have seen a MPU only boot on the bench but not in a game because the zero cross input track going to the PIA was shorted to something. It only caused a problem when the 43v was attached to J4. The zero cross input to PIA has a long track that goes right next to a bunch of socket pins that can be shorted out.

#8 1 year ago

manually shorting pin 40 on u9 does not cause a reboot it flickers then stops

1 week later
#9 1 year ago

flicker then stop means it's not reading all the rom code or it's failing the checksum. Based on your other symptoms and sometimes it gets farther and sometimes it doesn't there's probably some corrosion that still needs to be cleaned up or your sockets are flaky (I'm betting more on the corrosion, though). The behavior of random stuff happening is pointing towards that.

#10 1 year ago

it always boots ouside the machine it always fails inside the game. will try cleaning the header pins more though

#11 1 year ago
Quoted from wugly:

rebuilt the board using a corrosion repair kit and the game worked fine. decided to write new roms using 2732 with freeplay code...
tried to boot the game and the game wouldn't start.

Quoted from wugly:

it always boots ouside the machine it always fails inside the game.

What EPROM programmer are you using? Did you select a matching manufacturer/chip type or just generic for the type?

Might be that the freeplay EPROMs you programmed didn't get enough charge in some cells leaving them borderline. The EPROMs might work on your test bench power supply because i.e. it has a slightly higher voltage compared to the supply in the actual game.

Try reprogramming the EPROMs without erasing them using the same data they were programmed with so the EPROM cells get a deeper charge.

2 months later
#12 11 months ago

just noticed the last message. I rewired the board back to original strapping and put the old roms back in and it booted on bench not in game. the game is at a friends house so I can only work on it in bursts and lately he has been dealing with bad health for his parents and we have been unable to work on it. last thing I did was recapped his power driver board. same problem. the test points all read good bt was wondering while looking at the schematics. can you get 5 volts on the mpu if you don't have 5 volts going to the board. I thought there was a crossover from unregulated 11.9 volts to +5 through cr7, when I get a chance I am definitly going to double check the wire going to the mpu, i have only been reading at test point. outside the game I have put power in through the pins them selves to test them instead of test points but never checked the wires to the connectors.

2 weeks later
#13 11 months ago

the problem I am having working on this board is that it is at a friends house and he has been unable to have to come over to fix it on a regular basis. the board boots fine when I place it in a test rig with 5 volts and 12 volts. I have replaced the caps on the solenoid regulator board and have checked the power coming down the wires and the 5 volts is clean with no ripple. the 11.9 volt unregulated though I have discovered it running at base line of 15 volt with a ripple that takes it to 16 volts. could this higher voltage cause the game to not boot? the voltage coming out of the rectifier board is also baseline 15 with a ripple that takes it to 16 volts. Should I correct this and if I do how should I do it. when the game is in the pinball I get the initial first flicker and then the led goes dead. outside the game on external power I get flicker then 6 flashes.

#14 10 months ago
Quoted from wugly:

the 11.9 volt unregulated though I have discovered it running at base line of 15

This is a bit misleading on Ballys behalf. That voltage rail coming off the rectifier board will measure around 12 volts when the solenoid driver board is *disconnected*. When the solenoid driver board is connected, the capacitor at C23 filters most of the DC ripple and brings the voltage up anywhere between 14 to 17 volts. Your reading of 15 volts is fine.

Quoted from wugly:

I have discovered it running at base line of 15 volt with a ripple that takes it to 16 volts.

Typical ripple I see is between 0.2 to 0.3 volts. If you're getting 1 volt ripple, it's higher than normal. What kind of capacitor did you install at C23 on the solenoid driver board?

Quoted from wugly:

when the game is in the pinball I get the initial first flicker and then the led goes dead. outside the game on external power I get flicker then 6 flashes.

Have you got a logic probe or oscilloscope? Is there any bus activity after the flicker?

Quoted from wugly:

the problem I am having working on this board is that it is at a friends house

It might be worth your while taking the head home with you so you can diagnose at your will. You don't need the cabinet/playfield to fault find this. You'll just need to make a power plug connector that plugs straight into the J2 connector on the rectifier board. Leave the backglass in a safe secure place where it won't meet any accidents.

BTW, can you ask one of the moderators to move your thread to the early 80's solid state subforum? I don't follow the generic tech forum much.
https://pinside.com/pinball/forum/forum/tech-early-80s-solid-state

#15 10 months ago

just double checking pin 6 and 7 for inputing ac power correct?

#16 10 months ago
Quoted from wugly:

just double checking pin 6 and 7 for inputing ac power correct?

Yes,
Line AC Active goes to J2 pin 6
Line AC Neutral goes to J2 pin 7
Line Ground goes to J2 pin 10

Suffice to say be careful and triple check your work.

Below pin numbers for reference.

RectifierBoard_PinsAC.jpg

1 month later
#17 9 months ago

well my friend has finally dropped off the head to the game and I have been making a power cord for it and checking everything twice about eight times (ha ha ha).

the game will not go into a boot from the heads own power supply. I checked it and I am getting the proper voltages for 5 and 11 volts. disconnected j4 on mcu and hooked power from test power supply into l1 and l2 on the connector pin side, and it boots.

connected j4 back up with the head power supply not plugged into the wall and my external power supply current draw increased by an amp which is limited by a power supply limiter since I don't want to burn anything up, the mcu does not boot with j4 connected.

I unplugged the connector from the solenoid driver regulator board and the mcu will give its flicker and six flashes.

what do you think is the problem and how shall I proceed in determining what is causing the problem. I am going to try to run power through the connector for the solenoid driver/regulator and see if the game will boot. also considering disconnecting transformer power module. wondering if it can supply enough current to run?

ps I checked heads power with a meter set to ac voltage and didn't read any appreciable ripple.

#18 9 months ago
Quoted from Quench:

Have you got a logic probe or oscilloscope? Is there any bus activity after the flicker?

Can you respond to these two previous questions? ^^^

Can you post some clear pictures of the MPU board and solenoid driver board?

Quoted from wugly:

from the heads own power supply. I checked it and I am getting the proper voltages for 5 and 11 volts.

When you say proper voltage for 11 volts, you mean you actually measure 11 volts or something higher?
You should be measuring around 5.2 volts on the 5 volt line.

When you changed the capacitors on the solenoid driver board, did you perform the ground mods for the caps?

#19 9 months ago

both scope and probe i have. actualy if I use the heads power supply i get 14 volts and 12 if I use external. 5 volts from external boots it up but the internal power I get 5.1 or so. no I did not do the ground mod.

#20 9 months ago
Quoted from wugly:

both scope and probe i have

Ok, with the scope, what do you see on the 12V line, ripple etc? Can you post a snapshot? How about the 5V line?

With the logic probe when the board fails to boot, do you see any activity on the address and data lines of either of the ROMs?

For reference we were helping another pinsider with power on issues with his MPU board. We got him to hook up his scope to the 5V line and the reset line on the MPU board to see the relationship between the two on power up. If your scope is able it would be worth your while to try the same. See here:
https://pinside.com/pinball/forum/topic/mpu-100-don-t-boot-on-first-try/page/2#post-5276392

The point was to check that the reset circuitry was releasing *after* 5 volts to the board had become stable otherwise the CPU could start up with unpredictable behavior.

#21 9 months ago

here are your scope views of voltage 2 volts per division and 0 volts is at the base of the screen.

The not booting with an external power supply when j4 was connected between the mpu and solenoid board was accomplished with a high end power supply that has 2 adjustable voltage settings with current limiters and one fixed 5 volt supply in it.

connected 14 volts to tp5 on solenoid board with j4 connected both places. still only flicker. tp3 had 5.14 volts

under all circumstaces in which j4 was connected to mpu and solenoid we had pbo- pb3 precharged no activity observed after precharging. 1 volt on select pin (this includes using power from the head, using power from feeding solenoid board 14 volts to tp5 with transformer module disconnected and finally hooking 14 volts and 5 volts directly to the test points of the mpu)

rom chips u2 and u6, pin 1-5 1 spike at 4 volts at turn on then 0. pins 6-17 had slow pulse activity at 4 volts with exception of 10 and 11 which seemed to be high pulsing low. 20 on u2 4volts and u6 0 volts. 21 4 volts. both 19 and 22 had a slow 1 volt pulse.

interesting occurrence. with both boards connected to each other through j4 and power connected from external power supply to tp2 and tp5 I occasionally saw a first flash that would occur no sooner then 4 seconds after the flicker and up to 30 seconds after the flicker.

I am contemplating maybe something wrong with u11? but u2 and u6 having 1 volt spikes makes me wonder about maybe crossing something when re-strapped rom sockets when it worked. i deconverted it and used the old chips but got no boot with those either so I rewired the roms again to have free game roms

20191103_110922 (resized).jpg20191103_111112 (resized).jpg
#22 9 months ago

removed j4 off of mpu. ran jumper wires for power from solenoid/regulator board and the mpu booted.

Using the setup above I have run single jumpers wires for pb0 pb1,pb2,pb3 and cs at this time and have gotten the game to boot. I need more jumpers to rig every wire on the connection at the same time. I have not yet tried connecting pb4-pb7 yet. plugging j4 into mpu with power jumpered causes the game to only give the first flicker.

I have never checked any activity on the pb4-7 since I fixated on the lower pb's. time to buy some more jumper wires and test the higher ones after work

#23 9 months ago

Hang on, are you saying when you have the 12 pin J4 connected on the solenoid driver board that it causes the MPU board boot failure regardless of using the head power-supply or external power-supply?

#24 9 months ago

I have been haveing a brain fart major time. I got it into my head and confused j3 and j4 on the solenoid board. they are plugged into the right places and j4 on the mpu is plugged in correctly. when I was using external power I unplugged j3 and got the board to boot. when using internal power I plugged in j3 on solenoid regulator and unplugged j4 mpu and use jumpers to tp's and got a boot. I got no boot when j4 on mpu was plugged in so now I got to rethink what I was doing. the thing is I beleive I was getting proper power on the tps on the mpu going to have to double check the connections on mpu j4

#25 9 months ago
Quoted from wugly:

when using internal power I plugged in j3 on solenoid regulator and unplugged j4 mpu and use jumpers to tp's and got a boot. I got no boot when j4 on mpu was plugged in so now I got to rethink what I was doing. the thing is I beleive I was getting proper power on the tps on the mpu going to have to double check the connections on mpu j4

So from what I understand you're saying here, the game boots with the internal head power-supply so long as you disconnect J4 from the MPU but jumper 5V and 12V directly from the solenoid driver board to the test points on the MPU board?

#26 9 months ago

correct

#27 9 months ago

I think I should jumper tp1 to tp3 and do the 2 ground mods on the solenoid board before I continue. what do you think?

#28 9 months ago
Quoted from wugly:

I think I should jumper tp1 to tp3 and do the 2 ground mods on the solenoid board before I continue. what do you think?

Go for it. These mods essentially add redundancy to potential connector issues. If your connectors are 100% in good shape, the mods won't fix your immediate issue.

#29 9 months ago

did the ground mods to solenoid board have not had enough time today to check anything else then the tp on mpu. 5 is 5.06 4 gnd. 2 is 14.0 1. j3 removed from solendoid board does not allow game to boot.

#30 9 months ago
Quoted from wugly:

j3 removed from solendoid board does not allow game to boot.

Power to the MPU comes from SDB J3. Are you still using your external power-supply? If yes can you put it aside because it's only confusing the matter.

Can you setup your oscilloscope so it can take a snapshot of the 5V rail on the MPU board vs the reset line to see what's going on at powerup?

#31 9 months ago

green trace is 5 volts. red trace is reset. looks like 5 volts is established before reset kicks in. when I get a chance I am going to see if any of the j4 pins got cros wired with any of the roms address or data, if they did the precharge on the bus from the solenoid might cause problems. just going to use my multi on continity to one of those pins at a time and stroke the probe down the rom chip listening for a beep

scope (resized).jpg
#32 9 months ago

Looks like your reset is deactivating about 1ms after 5V has reached nominal voltage. Compared to the other thread I linked above their times were 8ms or more.
If you solder say a 4.7uf capacitor between ground and the base leg of transistor Q1 on the MPU board to further delay the reset, what happens?

Quoted from wugly:

when I get a chance I am going to see if any of the j4 pins got cros wired with any of the roms address or data, if they did the precharge on the bus from the solenoid might cause problems. just going to use my multi on continity to one of those pins at a time and stroke the probe down the rom chip listening for a beep

The solenoid/sound signals from MPU J4 go to the small J4 on the solenoid driver board and also the J1 on the sound board. If you suspect they are the problem, disconnect the small J4 from the solenoid driver board and also the long J1 from the sound board will isolate those signals.

#33 9 months ago

it shifted the reset but the game still fails. disconected j4 on solenoid, no joy. did quick continuity check on j4 data pins to roms and got nothing. thinking maybe a wonky u11. should i swap it with u10

20191107_075539 (resized).jpg
#34 9 months ago
Quoted from wugly:

should i swap it with u10

Yes, swap U10 and U11.
BTW what's happening on the VMA signal on the CPU (pin 5 of U9) on a failed boot? Is it pulsing or doing nothing?

#35 9 months ago

very busy today. I checked pin 5 and it is pulsing during boot failure. will hopefully get to swap chips tommorow

#36 9 months ago
Quoted from wugly:

will hopefully get to swap chips tommorow

Can you post some clear high resolution pictures of the MPU board when you get back to it?

#37 9 months ago

any particular areas

#38 9 months ago

All of it, especially closeups of the corroded area and reset section. The more pics the better. Even pics of the backside if you're able.
Just gives us a better idea to see what you're dealing with and incase our eagle eyes can notice anything suspicious.

#39 8 months ago

this picture has an error i corrected. on the left side you can see a trace that was repaired with solder wick. it moved to far to the left and bridged over to 4 104 which is on the j4 connector, I thougt for sure that was the problem. I removed the extra length and it still fails to boot swapping u10 and u11 does not do anything. I tried introducing 5 volts to every resistor in the area for the j4 connector and the board always booted with the connector off and the power connected by jumper wires. next message has more pictures.

oneh (resized).jpg
#40 8 months ago

back side

20191108_070846 (resized).jpg20191108_070856 (resized).jpg20191108_070905 (resized).jpg
#41 8 months ago

20191108_070648 (resized).jpg20191108_070705 (resized).jpg20191108_070719 (resized).jpg20191108_070730(1) (resized).jpg

I have e12 connected to gnd not e9

#42 8 months ago
Quoted from wugly:

the board always booted with the connector off and the power connected by jumper wires.

Cheers, I'll have a deeper look at the pics later.

So are you saying the board always boots from the solenoid driver board in the head if you jumper GND, +5 and +12V from the solenoid driver board to the MPU board?

If yes, my first suspicion is that you might have corrosion under the J4 pin header that might have become conductive between the pins. I'm talking top side of the board at that header and the conductive corrosion might be across the power pin solder pads under the pin header plastic.

What happens when you start isolating power wires on that J4 connector. i.e. remove the two 5V wires from the connector housing (tape them up so they don't touch anything) and jumper 5 volts directly from the solenoid driver board as you have been to TP5 on the MPU board. Then plug the MPU J4 connector back in and power up. Then try the same with the 12 volt wire and then the 43 volt wire. Note, the 43 volt wire is right next to the 5 volt wire on that pin header.

#43 8 months ago

will try that. thought the same though and I set dmm to check conductivity and started checking every pin with every other pin. flipperwinkle said for jumper from e12 to gnd for 2732 but I have seen a picture for the jumper to be e12 to e9. which is right

#44 8 months ago

I am also going to check from tp on solenoid board to j4 connector and look for any resistance.

#45 8 months ago
Quoted from wugly:

flipperwinkle said for jumper from e12 to gnd for 2732 but I have seen a picture for the jumper to be e12 to e9. which is right

Both methods work. E12 to ground is the older recognised method. E12 to E9 is the newer preferred method, the wire link is shorter (neater) and it makes sure the U2 ROM is fully disabled when it's not accessed.

Quoted from wugly:

thought the same though and I set dmm to check conductivity and started checking every pin with every other pin.

For reference, checking one of my MPU boards disconnected on the bench with a fluke multi-meter set to resistance mode:

J4-18 which is ground (black meter lead) to J4-17 which is +5 volts (red meter lead) I read 2500 ohms. If I swap the meter leads around I get 2000 ohms.

J4-16 which is +5 volts (black meter lead) to J4-15 which is +43 volts (red meter lead) I read 6500 ohms. Same reading when I swap the meter leads around.

J4-12 is the +12 volt pin. Pins 11 and 13 are unused. So checking pin 12 to ground (black meter lead on a ground trace, red meter lead on pin 12) I read 1.5 Mega ohms, swap the meter leads around and I get 3.3 Mega ohms.

J4-15 (+43 volts) measured to ground (didn't matter which way I had the meter leads) I measured 4000 ohms.

Your +5 volt resistance reading to ground might be abnormal - see below (zoom in if needed).

HarlemGlobeTrotters_MPU1a.jpg
HarlemGlobeTrotters_MPU2a.jpg

#46 8 months ago

as you can see I have some jumper wires which I can plug into the cable and onto the pins on the board at the same time. unfortunately I do not have enough to connect all the wires at the same time. I first connected 1 5 volt and 1 gnd along with unreg and 43 volts. and the board booted and gave me 7 flashes. I had enough jumpers to connect pins 1-4 at the same time up and got seven flashes. had to remove the 43 volts to hookup pin 10 and I also moved the +5 and gnd pins to the alternate pins and I got 6 flashes. moved pins 1-4 to 5-8 and still got 6 flashes. disconnected the power by using these jumpers and reconnected the data lines I had disconnected and use alligator clips to tp to reestablish power and 1 alligator to connect the resistor on the mpu with its corresponding resistor on the solenoid board. and I got 6 flashes. plugging j4 into the mpu causes the board not to boot. I have since ordered a mpu connector repair kit and additional jumpers in case we still have a problem with this board not booting when I replace the connectors.

swapped cap and resistor
changed e12-gnd to e12-e9.
Added extra solder to the components you specified but the tested connected before I did it, just good to be through.
that possible solder blob connecting 5 volts to gnd I could not find I think it is a trick of the flash since that area is covered with nail polish to protect the corroded traces after they were cleaned.
j4-12 to gnd resistace for me was 8 meg both ways. everything else was reasonably close to yours.

be about a week for the parts to come in.
20191110_110130 (resized).jpg

#47 8 months ago

Very strange.
I took some resistance measurements on my board in the "Valid Power Detector" circuit (reset circuit).

In case you want to follow up on your strange 8M ohm reading at J4 pin 12.
I checked some other MPU boards and they are similar to my first board readings of 1.5M/3.3M ohms.

The values in red below are the red meter lead at that point, black meter lead on a ground trace.
The values in black next to it are the black meter lead at that point, red meter lead on a ground trace.

[Edit] The values in blue are voltage readings on a working system.

MPU Board Readings in ValidPowerCircuit

1 week later
#48 8 months ago

having some very funky results.

cut wires off of j4 connector at mpu. stripped and crimped new pins into connector. double checked color codes to wires in proper position. traced wires with meter to make sure they went were they should and no cross connections.

removed pins from board on j4. cleaned area with fiberglass pen so nice and shiny. used a little bit of flux and soldered new pins to board. tested to see if any were cross connected with a meter and also confirmed they were connected to what needed to be.

first test, got flicker but no flashes.

disconnected from board and started running jumpers from connector to board. started with gnd +5, unreg, 43 volts and it booted.

started plugging all the other wires in and it still booted fully connected with jumper wires. the board at this time was resting on a piece of paper in the bottom of the head unit. I lifted the board so it was now in a vertical stance and it stopped booting.

disconnected one of the 5 volts inputs and the game would boot. swapped with the other 5 volt input and the game booted.

removed all jumpers and and took 5 volt inputs one at a time out of connector and the board does not boot. removed them both and tried to jump the 5 volts from tp3 on solenoid to tp5 on mpu and I get the led locked on. was wondering maybe intermittent connection on caps from 5 volt bus with maybe an intermittent cap there shorting?

#49 8 months ago

Still sounds like issues with the roms or rom sockets.

#50 8 months ago

Have you checked that the inductors at L1 and L2 next to the J4 connector on the MPU board have continuity? They are essentially the difference between you jumpering power to the board or powering through the J4 connector.

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