I'm not going to provide a ton of pics on this one because it just isn't needed. But I had been fighting with a 7800 that I've been servicing for a total of just over 12 hours in trying to figure out why it would only play 7800 games? If you put in a 2600 game, it would only give a black screen and not much else. If I put in my Plus+ cart, I could barely make out the plus cart logo on the screen as it would flicker all over.
This told me that the 7800 was in fact switching over to 2600 mode properly or at least would appear to be in order for the plus cart to start loading up. But as you can see the picture is not correct and what you can't see in this static photo is that the image would only blink in and out quickly for me to see this. It took some doing to capture this frame on my PVM to show it.
I first replaced some of the 3906 transistors that sit above the MARIA IC chip as I had one in the past with a similar issue and one of those had been the culprit. In the end, I ended up replacing all of the transistors on this board along with trying alternate ICs from my working test spares to isolate the issue. Nothing worked and I was about to toss in the towel and tell the owner that it would forever have to be a 7800 only console.
Then in checking some traces (suspecting a bad trace somewhere), I happened to touch my meter probes to two of the pins on the cartridge port and was surprised when the metered 'beeped' at me?! None of those cartridge pin fingers should be touching unless a cartridge inserted is causing a connection to do so. In looking further, I could see a dull looking color in the bottom of the far left side of the 7800 cartridge slot. I thought it was paint because it was so flat and kinda dirty looking. I cleaned and cleaned with 99% IPA and contact cleaner and whatever it was just would not clean out of the slot. Eventually I use a sharp larger end of my soldering boring tool to poke and scrape and it came loose. After some mangling with my tweezers I managed to pull this out of the cartridge slot...
It looks like a blob of solder, but in fact it is foil tape?! It is very similar to the tape used on the 2600 switches from Atari as part of the ESD protective measures. It is all twisted up here because of my scraping on it with my tool and using the tweezers to pull it out. But yes, this little bit of foil tape had gotten into the slot at some point in time (Prior to it being send to me), and was the issue. Removing it, removed the short on the cartridge pins and 2600 games now come up and work properly again.
The reason 7800 games would work, is because when you inserted a 7800 cartridge into the slot, it would push the pins back enough to NOT make contact with the foil tape. But with out a 7800 cartridge inserted, the pins were all grounding out. This was confusing the 7800 and so it wasn't really switching fully to 2600 mode as it should have been. Basically, it was running 2600 code in 7800 mode so the clock signals were all out of whack as a result. This is why the video output was all goofed up.
So the lesson here, is that you always need to check power first when encountering issues with a console, but be sure to look into the cartridge slots for any foreign debris as well.
BTW... when the console first arrived to me, this is what I found inside. It came to me in power on but otherwise non working condition. I wonder why?
Edited by CrossBow