R.Cade

Some games and not others? That is quite bizarre, but some cartridges use different pins for ground, etc. than others. You didn't happen to test it fully with the same games before the composite mod?

I wasn't suggesting it was the mod board- I don't know what it is. I was only saying to look at other options closely before getting a new power supply. My comments probably came off as attacking someone and that is my fault - I am sorry. Watching the voltages while it's happening is a great thing to check. It seems like that board would only put a tiny load on the power supply. I have seen something similiar once when I modded a Coleco, but the circumstances were very different. It was a simple transistor mod and I think I used the 12v line instead of 5v, so it would very quickly overheat. That's not the case here, I doubt. It certainly wouldn't explain the RF out variable. As I mentioned, the only thing that should change when you plug RF in is the grounding and impedance. Someone else mentioned physical stresss and that's also possible. Just for kicks, have you tried another television?

So what's wrong with it? You really think 11.6v is not enough? You are going to have him buy a new power supply for a CV that works fine without the mod, but also works fine with the RF connected?

That mod looks like an SMT version of this (http://www.retrofixes.com/2015/06/colecovision-svideo-or-composite.html). However, it uses a different ground point. I would give it a try.

I don't know. If it works with RF connected, maybe it's an impedance or a grounding problem. Both of these will change with something connected to RF vs. unconnected. Also, what kind of output connector is that?

Seems OK at first glance. You didn't show the ribbon to socket connection though. However it should be keyed and obvious how it goes.

I have my SIO cable installed now. Can you send me the invite for the test firmware?

Thank you for the offer- Mcclane already sent one my way.

Some of these switchers do introduce interference due to either cable length issues, the quality of the internal wiring, or strange hacks to convert the signals from Y/C to composite or vice-versa. Also, some mods are not terminated properly with a ~75ohm load, so they may not work properly on a longer cable. One of my switchers (an RCA branded one I believe) will bridge the audio to mono if you only connect the left audio channel, so if I were to reuse that for a video signal I am sure it would cause a problem.

Why isn't/can't the 5v on the side port be used to power the CF7? Is it not rated for enough amperage? I know it won't pass through the speech synthesizer. Is that the only reason for the secondary power design here?

It looks like a decently-installed mod. If it were me, I would leave it and find another 800XL (or 600XL) to mod.

That would be great- PM me and let me know what I owe you. Also, I checked and there is no crack in that chip. It just looks like a white paint spot really. Here is a better photo.

I noticed the white spot on that chip and thought the same thing at first, that it overheated and blew it. That still could have happened, but it is perfectly smooth (not a missing chunk) and seems to just be a white spot. It could still be heat damage. I didn't even notice the crack until you mentioned it and I zoomed in. I'll check that to see if it's real and not a photo anomaly. Everything seems to be working with the cart, though. What does that chip do?

I grabbed the bag and looked again and I must have misplaced it. I'll be happy to pay you for another one+postage if you have one. Otherwise I'll have to wait a couple months for the slow boat.

Thanks very much to tmp's eagle eye! He spotted in the pictures that I must have knocked a diode off the board. By a huge stroke of luck I found it inside the Atari and reattached it, and the AVGCart lives again. When I was testing it outside the cart shell, I was holding open the flaps on the XL, but at some point I must have let it scrape against the board a little and knocked it off. Even more luck that it didn't pull up the trace or anything. Now I just need to source the correct socket for the SIO plug...