Atari 7800 Not working

[cityside75]

I picked this up at a garage sale with a bunch of games for a few bucks. I'm really anxious to try it, but I just can't get the system to work. Here's what I know:

-The power light comes on when it's powered up

-It does occasionally display a very scrambled picture on the screen when turned on, and even made some random sounds on a couple of power-ups

-It usually displays a black screen when turned on, but it's not straight black but rather a weak-looking (if that makes sense), rolling black screen

-It behaves exactly the same way whether a cartridge is in the slot or not, putting in a cartridge does not change anything (I tried both 7800 and 2600 carts)

-It did not appear to have been opened before I opened it up

-I did not see any obviously burned-out or broken pieces while looking at the board

-There was a small piece of broken shiny plastic rolling around inside the unit, but as far as I can tell it did not break off of anything on the board

I want to check the cartridge connector, but I can't seem to remove the plastic shroud that covers it - any ideas on that?

I know everything else involved works OK, I've tested other systems with the RF connection, I've tested the 2600 games on another 2600 and they worked fine. I've tried messing with the channel switch and trying other channels, nothing helps.

I don't usually post my tech problems because I can usually find a repair link with enough effort, but I figure 7800's must not break often as there is very little info available on the net.

Any ideas where to start?

[Pantechnicon]

Is it possible that your 7800 unit is PAL and not NTSC? The display problem you're describing sounds somewhat like the kind of thing one gets when going from one signal type to the other. Granted this is a longshot, but it's worth asking nonetheless.

Second option - since you've already opened it up - would be to look for an adjustment pot somewhere around the RF box. It's probably orange in color. Put a screwdriver in there and see if you improve anything.

[cityside75]

Is it possible that your 7800 unit is PAL and not NTSC? The display problem you're describing sounds somewhat like the kind of thing one gets when going from one signal type to the other. Granted this is a longshot, but it's worth asking nonetheless.

Second option - since you've already opened it up - would be to look for an adjustment pot somewhere around the RF box. It's probably orange in color. Put a screwdriver in there and see if you improve anything.

Thanks for the feedback. I'm pretty positive it's not a PAL unit, it was purchased with a bunch of NTSC games at a local garage sale, and while the unit itself doesn't actually say NTSC, it does have all of the standard FCC ID's and US Patent numbers on the Serial Number sticker that would indicate it's a US unit.

I did see a couple of adjustmen pots in there, but I've been hesitant to start messing with them since I saw no difference in the picture with or without a game. I'd think that if the rf was out of adjustment, I'd still see some different colors and hear some hints of game sounds when I plugged a cart in. As it stands I see and hear no difference with or without a cartridge. But, if you think it's worth trying maybe I'll give it a shot - I just don't want to mess it up worse in my quest to fix it

[cityside75]

Okay, I got the shroud off the cart connector and cleaned the pins - didn't really make a difference. But I noticed that if you leave the system on you'll see a flickering splash of color every now and then, real faint and gone in a flash. And occasionally it will look as if it's losing power, like flicker halfway off. SO I'm curious about the power adapter now. I don't have a multimeter, but I may pick one up to check the voltage on the adapter.

Any thoughts, let me know...I'll update with my findings.

EDIT -- I forgot to mention I messed with the pots too, and it made no difference whatsoever.

[aclbandit]

Okay, I got the shroud off the cart connector and cleaned the pins - didn't really make a difference. But I noticed that if you leave the system on you'll see a flickering splash of color every now and then, real faint and gone in a flash. And occasionally it will look as if it's losing power, like flicker halfway off. SO I'm curious about the power adapter now. I don't have a multimeter, but I may pick one up to check the voltage on the adapter.

Any thoughts, let me know...I'll update with my findings.

EDIT -- I forgot to mention I messed with the pots too, and it made no difference whatsoever.

Try using it on an older TV -- I had an adventure getting mine working on my newer HDTV, but it works easily with the tube TV.

I doubt it's the TV's problem, based on your described symptoms, but if you are using a new TV, it can't hurt to try an older one, if available.

[cityside75]

Well, I'm just about ready to give up on this thing. I went out and bought a multimeter, and if I'm reading it correctly, it shows the adapter outputting 13 volts directly at the plug. My understanding is that while it's rated at 9 volts, that is under load and the number should be higher when taken directly on the plug. If anything, I assumed that the adapter wasn't outputting enough power, but that doesn't seem to be the case.

I've tried it on my older TV with the same results, and I've tried cleaning everything - games, cart connector, etc with no changes.

Any other measurements I can take with the multimeter?

[gdement]

You could try checking voltage on the ICs (should be 5v). I don't remember which pins for each chip, but generally the +5v (Vcc) and GND (Vss) pins are in opposite corners. Pinouts are easy to find for the small commodity parts like the 74HCxxx chips.
If you're nervous about shorting something then wrap masking tape on the probe, so only the tip is exposed.
They're all connected in parallel, so don't have to check all of them. This rail is also supported by the smaller 220uF capacitors, so you can test there if you prefer. The big 2200uF cap is on the input from wall adapter.

The 7800 uses a 7805 voltage regulator, which converts from input voltage to 5v. It runs hot and is attached to the heatsink above the cart slot.
If you're not getting 5v on the chips then the 7805 might be bad. That's somewhat common because of the heat stress.

Also, you could try replacing the capacitors. It's just a shot in the dark, but I've run into 2 bad caps out of the 3 7800 consoles I've ever had.

[gokugohandave]

turn your multimeter to the 10 dc setting. on the 7805 regulator, with the printing facing you, the middle pin is ground, place the negative probe here. with the positive probe,the left pin is input and should read 9, the right pin is output, this should read 5.