Me and this Robotron are fiddna go to fist city!!

Humdinger

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Ok so I started trying to fix another Robo board set last week. Upon initial power-up, I thought it was odd that the rug screen was jittery, almost like a 60-cycle RFI situation. All the components are wired up on my desktop, and I'm using a 13" crt as a test monitor. It stayed in rug and repeated, and it tried hard to boot, but it only drew one-half of a black screen each time. Then things got weird. Eventually the black background became orange. It continued to rug and draw the background from left to right, orange each time. At one point I ran my fingers down the side of one of the 74153's, and by doing so, I could affect the draw rate of the background! I could run it back and forth with my fingers. WTH? So I replaced 3 of the four 153's and—the jittering stopped and the game booted! I thought I had it licked. The next day I walked in and powered up, and it booted. Sweet! That day was the last time it did that. Ever since, the jittery screen returned, the rug freezes, and the background attempts to draw in orange. (The orange NEVER turned to black.)
So over the last 3 days I have replaced all the 166's, the 7489's, the 9316's (with 161's), the remaining 153, a 139, the decoder sockets (I tried another pair of decoders to no avail), moved some rams around due to the random ram errors on the ROM board's segmented display, even tried another ROM board, replaced the 4 caps in the power section, and have new headers and plugs on everything—and NOTHING has changed. There are new AA batteries in the holder, there is no battery acid on this board, and the ribbon cables are fine. I only replaced the 166's in line I because 3 of the 4 were running pretty hot. The 7489's and a few others had blackened legs and had obviously run hot for some time.
I know that 7474's are ridden hard on these boards but I haven't touched them. I don't like randomly swapping chips; I'd rather have an informed direction to go before I do anything else. My voltages are fine. I'm getting tired. I followed the flowchart in the manual but the 139 I replaced at 4G was the only chip that was a touch suspect.
Once in a blue moon, the sound board emits a solid high-frequency tone that won't go away until you turn everything off. It may come back, but you never know when. Wtf?
Any ideas, anyone?
I couldn't post a vid of the jittering and the periodic re-draw attempts, but look at the rug pic and imagine it shaking so rapidly that your eyes ache and you get a headache.
A few other notes:
this is a Revision B board! 😳
I am using the original Williams (rebuilt) power supply.
I resocketed and replaced the 6809 first thing.
As stated earlier, I swapped ROM boards temporarily and nothing changed.
🫩
 

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They're good. I may have narrowed the issue down to a bad 7411. Naturally it's the one chip I don't have in stock, but we'll see soon. Thanks for the suggestion.
 
When you replaceds the 153s were they already socketed? If so, maybe the socket was bad and not the chip?
 
Are you using the 4116 rams, or did you upgrade to the 41256/4164 RAMs?? The reason I ask is because we ran across a similar issue with a Robotron set using the Arcadeshop 41256 RAM/power adaptor upgrade. The power adaptor they sell has one line floating, which won't work when using 41256 RAMs and cause all sorts of issues similar to the one you're having. If using 4164s, their adaptor works just fine. We sent them a message about this issue with their adaptor, but not sure if its been addressed yet.
 
Still using the 4116's. Good info though, since I do have a new set of 41256's. Thanks!
 
Well, after intense research here on the forums, it appears that the random but non-existent RAM errors AND the screen jittering during the repetitive rug were both caused by none other than a faulty 7411 at 3A. It sounded really crappy on all three outputs when probed, and not that the SOUND a chip makes should point you in any particular direction in all cases, but the awful noise seemed to keep time with the "jitter rate" I was seeing on the screen. (Plus, the sound of the one in my working Robotron was nice and clean with typical pulsing.) So after I replaced that chip, no more jittering, and the game booted again. However, red is still missing. I am betting a week's worth of donuts that the nearby 74LS257 is the culprit. My latest order from Jameco is en route. Stay tuned!
 
Also, regarding the random high-pitched squeal from the sound board, I expect the 6821PIA to be bad since they almost always are, but option 2 would be a bad sound ROM 4 Joust chip that was all I had laying around for testing. I ran my fingers down its legs (sounds naughty 🤤) and it squealed like a pig. Desoldering another 40-pin chip sounds like an "after lunch and after nap" project for this old feller.
 
Replaced the PIA and after doing so noticed that the dip switches on the audio board were "ON". Found a write up on here showing various sound boards with jumpers in various locations. It specified that for Robotron, those switches were supposed to be "disabled". Switched 'em off and voila! We have sound. Getting closer…
 
Replaced the PIA and after doing so noticed that the dip switches on the audio board were "ON". Found a write up on here showing various sound boards with jumpers in various locations. It specified that for Robotron, those switches were supposed to be "disabled". Switched 'em off and voila! We have sound. Getting closer…
the sound board dipswitches were mostly intended to be used on the pinball games. one switch would be the toggle between digital sounds and simulated EM chime effects (which sound cool... for awhile) and the other is for enabling/disabling speech (which was prominently required on their pinball games from Gorgar onward)

I have a box of sound boards, one of them was from the Robotron where I work I wonder if I had a captain stupid obvious moment and the dipswitches aren't set right LOL it might be set for chimes or something. which I would venture the Video Sound rom code won't allot for that because it's unnecessary so it'll just be silent.

you had mentioned your sound call pins were all high, so that's a good sign I think. it's funny of all the sound boards I've fixed I don't think I ever logic probed those before.
 
Actually 3 were low and 4 were high. Looking forward to seeing if any sounds are missing when I load all these guts into the cabinet and attempt to play the game. Lots of bondo smearing to be done before that happens. If anything is out of whack, that 4068 with the wrong outputs will be my next target.
 
Actually 3 were low and 4 were high. Looking forward to seeing if any sounds are missing when I load all these guts into the cabinet and attempt to play the game. Lots of bondo smearing to be done before that happens. If anything is out of whack, that 4068 with the wrong outputs will be my next target.
oh, you know what, I did logic probe those once. that's how I found the bad 40xx parts. :ROFLMAO:

it's all coming back now
 
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