Could use some guidance on a 720º issue

AgentX83

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Hey all-

Long-time listener, first-time caller.

I'm finally getting down to brass tacks with my full 720º restore, and the only real hurdle I'm running into is a pretty major one: scrambled output from the board(s). When I first picked up the machine, last year, it was booting to the 6502 Audio Processor error:

30624511801_34b97a299b_c.jpg


At some point after removing the boards, reseating chips, checking traces and some transistors (all seemed fine), it got WORSE and started booting to THIS instead:

27021929928_a79139c01a_c.jpg


I'm worried that something got zapped when I was re-connecting the boards. I've gone ahead and replaced both crystals on the CPU board (I've read that they are common failure points) and all of the electrolytic capacitors on both boards. Still, no change.

I hear no boot sounds, nothing. And no, the test switch on the audio board does nothing to help.

Any suggestions? I've got the logic probe ready to start checking signals and I'm not against ordering replacement chips for the 6502 and T11.

Thanks in advance,
Kasey
 
720

you most likely have an issue with your mob but its hard to say.

the best way is to test each board with a working one, that way you can find the bad one.

I'm sure you only have one set?

I would double check all the ram chips on the mob and make sure none are upside down and make sure all the legs are in the appropriate socket. you need to really check this cause the last issue your showing looks like its from removing the chips.

that being said you may have a bad socket. look at each socket and check to see if one of the socket springs are stuck. if so you need to replace the socket.

replacing the 6502 chip does not mean that is the issue. I have fixed 6502 errors with finding bad sockets.

also did you replace the two power headers and repin your wire harness? that probably wont fix your problem tho.

chad
 
Thanks for the suggestions. There's a CHANCE that I can get my hands on a System 2 boardset from another local; just trying to avoid that for now. I think I'm gonna pull, check, and reseat every socketed chip tonight, look for damage or corrosion on legs, etc.

Also, I neglected to mention that I do get near perfect +5V readings across both boardsets...
 
Your mob I'm sure is the issue. If you can get a working mob you could at least see if the video board works. 6502 error is the mother board and the fuzzy graphics looks like mother board from past experience......i hope your video board is good cause they are usually the issue.

I always see mob for sale but you usually don't just see video boards for sale...or just complete sets. They are getting harder to find.

If you can't fix it you could send them to me and I might be able to get you a working set if your video board is good.

Also you can try replacing the 6502 chip if you can get back that screen. Did you buy it with the 6502 error and move the game and get the garbage or did you get the garbage trying to fix 6502 error? I know you said you reseated all the chips.

Thanks
Chad
 
Both Andrew and Chad have offered good suggestions already.

The sound board in front is also necessary for proper boot up procedure as it supplies needed voltages back to the CPU (I believe). But I'm just throwing it out there. It's possible the voltage needed from that board is not 100%. However, I suggest you try the other suggestions already given first. It does sound like the CPU to me as well.

Careful pulling chips, could lead to other issues.

Question: Do you get the "Tone" on power up (the moment power switch is turned on)? I'm assuming no.
 
Last edited:
Thanks, all. Been busy at home, so late to follow-up.

No, I don't get the boot "tone" that I would expect from a System 2 game.

Pulled, cleaned and reinserted all socketed chips, no help. I have actually already tried swapping out 6502 chips, to no avail, but the chip numbering was different and I'm not 100% sure on the functionality of that spare chip, so I'm gonna pick up a few of the same/original chip online.

I finally got my hands on (and printed out) the 720º schematic. I'm not great with an oscope, but I have one so I'm gonna try testing it out first with my logic probe before trying to check with the scope.

When I picked up the game last year, it was booting to the 6502 error. That didn't change to the garbage til after I started removing the boards, reseating chips, etc.

Thanks again,
Kasey
 
I would definitely do the header pins to rule that out. If you're hitting dead ends double check your audio amp. It runs voltage also to the boards I think 12 or 15 volts, maybe 30? I don't remember exactly but There are postings on here about it. The game boards absolutely need them to run. The voltage transistor has gone bad on mine twice in about 6 years. When this happens I would get similar screens, periodic random rebooting or nothing at all on the screen. Good luck with it. It's really a great game.
 
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