troubleshooting Troubleshooting a flaky 7800

[PatrickMc]

Hello all, 

 

I've been working on my first restoration and associated troubleshooting of a 7800 console.   I'm flummoxed and wanted to reach out to the community to see anyone has had a similar experience.  Sorry in advance for the rambling summary.  ?

 

• NTSC w/ a revision B system board.  Main symptom is a black screen w/ no audio -- sometimes...  The Atari logo consistently shows up on power up.  More below.
• Console all the way apart, cartridge cup removed, pins cleaned, replaced all caps after finding that the 2200uF was not in great shape.  Also replaced the power regulator and power switch while I was at it.  All voltages look good now at the ICs and along the main power rail.

 

In response to the no audio issue, I've walked the troubleshooting flowcharts (many times) and typically end up on flowchart E where it mentions the encryption latch.  I don't have the diagnostic or color cartridge so skipped those steps.  As a result of the encryption latch tip, I've checked the logic ICs and outputs all look correct.  In other words, they produce the correct results based on the input signals they receive. For the vast majority of the time the system swaps between initially working and then mostly failing -- I'd guess the failure rate is easily over 90 percent following this pattern. 

 

For the first week or so I was using a known working Galaga cartridge but then I started to get suspicious.  While it always works in another console, most of the time it does not on this particular system.  Furthermore, I can't reproduce the glitch with other cartridges -- e.g., Centipede and Donkey Kong both appear to always work as do a few others but I don't have a lot of cartridges to test with.  In revisiting the logic chips related to the encryption latch I noticed the signal from the A12 address line from the 6502 looked stuck -- any time I see this particular signal coming from A12 it is a dead giveaway that the system will come up with the black screen and hang (it initially doesn't look like this but then will suddenly "snap" in).  Here's a look at what I see on my scope for A12 -- it will stay like this as long as I leave power to the system the CPU is clearly wedged).  

 

 

In looking at the schematic, A12 comes off the 6502 at pin 22 and routes over to pin 8 of the cartridge slot via U4's (74LS08N) output pin #8.  I re-cleaned the pins on Galaga (even removing the PCB to get a good look at them, and then re-cleaned the slot and reflowed all the pins.  No luck.   My next thought was that the vast majority of the time that Galaga actually runs is when the system sat powered down for a while.  So, if I leave it idle for several minutes it seems to almost always work on the first power up and then fail for the vast majority of the following times when the system is power cycled with little rest in between.  The 6502 does occasionally feel somewhat hot (but not too hot to touch) as I was running through other troubleshooting phases (perhaps not too surprising if it is locked up).   After using some compressed air to "freeze spray" the 6502, Galaga successfully started up 10 times in a row.  Once things had a chance to warm back up again the system returned to its lock-up behavior. 

 

No surprises here: My conclusion is the 6502 is potentially starting to fail but I can't wrap my head around why only Galaga triggers this behavior.  The locked up signal on A12 could easily be the result of something else happening that I haven't observed. I haven't done an exhaustive walk of all address and data lines as I'm not familiar with all the low-level startup details for the system.  I would find it very odd that Galaga just so happens to have a particular set of instructions that cause the system to hang.  What are the odds?  ?

I have removed and socketed the 6502 and the same behavior continues.  My plan is to start a search for a replacement Sally to see if that helps validate the diagnosis...

 

Have I missed something obvious?  Has anyone else seen similar behaviors?

 

Thanks for any insight you can provide! 

 

[fiddlepaddle]

Might try another Galaga cart, just to prove it's not the cartridge, if you can.

[PatrickMc]

Hi @fiddlepaddle -- managed to find another one this evening!  Thanks!  ?