720 Garbage and Controller help please

Jedidentist

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Donor 3 years: 2011-2013
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Howdy folks!

So after waiting a month to get my monitor board back (thanks Chad), I finally get everything all plugged in, and Voila! I finally have a picture! This game was bought as "non-working", so I had no idea what was up. However, now I can actually troubleshoot.

Sound is good, RAM and all ROMs test okay. However, I have two issues:

1. Garbage on "moving parts" like the wheels pictured below, and when bees and such are moving on the screen. My skater is fine.

IMG_105150.jpg


Any ideas which specific chip might be wrong or other ideas?


2. My joystick doesn't work at all. I rebuilt it all and have everything with perfect placement and alignment using RAM Control's parts. I have 2 original dual optical encoder boards, and neither work. I've gone through the test mode, and I don't get a single flutter of anything moving in any direction. I do have 2 new dual encoder optical devices/slots, but since I don't have any life at all from either board, I don't really think it's those, so I don't see any sense in soldering in new ones. I've confirmed that I'm running +5.08 V on my PCB, so I'm not running low there. I've disconnected and reseated all of my wires. Could both boards be completely dead? Is there 1 specific wire that might be bad? What else/where should I test?

I'm so close to the finish line after having waited to complete this restore for almost 2 months. I'd love to get this done this weekend. Thanks everyone!
 
1. I had some glitchy graphics on my 720 until I adjusted the voltage. Check and see what voltage you are getting on your chips, and adjust it up a bit if necessary. I think I was close to +5.0, and things got much better at +5.1. The system 2 boardset is huge, which means you might have +5.0V in some places, and a bit below what you need in other places, so a little bit of insurance voltage goes a long way. Whatever you do, don't crank up the voltage needlessly, because it can fry the portion of the board that handles the controller input. (See issue 2.)

If that doesn't help with the graphics issue, I would go ahead and start reseating chips. It will take a while, but it can actually work wonders on graphics glitches on system 2 boards.

In some instances, a bad Slapstic chip is known to cause graphics problems. It's a type of security chip that Atari used.

2. There is a chip on the system 2 boardset that is known to die that handles optical encoder input. It is called a LETA chip. Apparently they are prone to failure if they see too much voltage... so if someone decided to turn up the logic voltage on your 720 at some point, it could have smoked the LETA chip, which handles all of the encoder input. Try swapping your LETA chip with another one (fortunately it is socketed).

It is possible that the infrared LEDs on all of the old encoders are totally dead, which would also produce the behavior you are seeing. On my 720, one of my encoders was dead, and the other wasn't.

I found that a $20 logic probe was useful for testing my 720 encoder board. If you hook up the logic probe to the outputs of your encoder board, you can use it to listen to the pulses from the spinner. The encoder board needs ground and +5 V from the boardset in order to do this test (or you can use a benchtop power supply hooked to the correct pins). The logic probe is a GREAT way to test the encoder boards for functionality without having to second guess the condition of the LETA chip on your system 2 boardset. You should hear a continuous stream of logic beeps (high/low) when you spin the 72 tooth wheel, and 2 beeps per rotation for the 2 notch wheel.

If you end up getting frustrated with it and want me to have a look at getting your controller going, lemme know. I recently went through a similar routine with mine.
 
Using the link on 720zone to an old thread, I checked my board voltage from GND3 to +5V last night and it was running 5.6V. I adjusted that down to the 5.06 I have right now. Could it have fried the LETA chip? Maybe. However, I don't have another one to swap out with. All I have is an extra video PCB that doesn't work. I have no parts and no real way to test anything. I don't have a logic probe and unless they've got one at Radio Shack, I'd have to wait until monday to see if they have it at Do It Yourself electronics.

As far as reseating chips...I was just too tired last night to mess with it. Since everything basically functions, I'm hoping that might be all it is. I'm going to give that a try this afternoon.

Now, if you happen to be out and about this weekend, I'd love to have you drop by, but since you're about 45min-1hr away, I wouldn't want you making any special trips. Thanks for the tips!

Derek
 
Well, I removed all of my chips and reseated them. Now I've got more garbage...great! Well, at least I know that's the source of the problem. The only issue is which one is it? I really don't feel like taking them all out again, but if that's the way it's gotta be...

As for the Leta chip. Well, I didn't know exactly which one it was...of course, it always helps to read the manual, huh? So I find out which socket it is and...

IMG_105250.jpg


Awesome, huh? Well, at least I know where the problem is. I guess I can see why the previous owner was having problems with the joystick. Oh well, off to post in the WTB section...
 
Yeah,+5.6 V is too high for my comfort level. A missing LETA chip, huh? It's definitely consistent with your symptoms. :)

The LETA chip was used on a number of different Atari games. I *think* Roadblasters uses one. It might be easier to track down a trashed Roadblasters board (or something else that uses one) to harvest the chip than it would be to find someone selling that specific chip loose.

Radio Shack used to sell a nifty logic probe, but they don't anymore. They're on Ebay all the time though, i.e. this one:

http://cgi.ebay.com/LOGIC-PROBE-WIT...emQQptZLH_DefaultDomain_0?hash=item5d2731f4b4

It's definitely a nice thing to have amongst your troubleshooting tools. Let me know how things work out after you track down a LETA. This hobby is half scavenger hunt!
 
Well, I ran down to Needham to You Do It and picked up what I thought was a logic probe...ended up being a logic probe kit. Great...now I'll never know if what I'm testing is broken or if it's the probe ;) In any case, I e-mailed Francis at Ataricade.com and he had one leta that he's holding for me. I gave him my symptoms, and he said that it might just be a socket, but without it in front of him, he wouldn't know for sure. He had a long backup of jobs to do, but he says he's free to do it now. So...I think I'm just going to bitw the bullet and send it to him vs. trying to rack my brain figuring out something that I have no clue about. I have fun learning new things, and I'll probably still play around with it and build this logic probe, but when all is said and done, I'll probably waste more time than it's worth to me. I need to get this game in line with my others so that 1. my kids can have room in the basement and 2. I've gotta go pick up my Discs of Tron and need to get this out of the way first!
 
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