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Thread: Atari 400 has power led but no signal to tv

  1. #1

    Default Atari 400 has power led but no signal to tv

    Hey everyone. I have an Atari 400 that I modded to have 48k of ram. It worked great on my test bench, played some games on it with the top plastic cover off and then I shut it off and let sit for a few weeks. I decided to finally put the top back on it and connected it to the tv with it all back together, loaded basic to make sure all the ram was still working and noticed the screen was a bit fuzzy and then after a few seconds garbage started to form on the screen and then a black screen. Restarted it a few times and it decided to run long enough to do a ram check and found it didn't have the full 48k. So I took it back apart and found a spare 4164 laying around and then I found a wire had broken and a broken leg on the chip that was piggy backed on the ram board. I replaced the chip with the broken leg and replaced the 3 wires on the board with some more flexible wire. I put the ram board back in the ram slot and connected power and connected it to the tv and no signal. The power led comes on but it sends no signal like it's not connected to the tv. So I grabbed my Atari 2600 and connected it to the tv using the same cable and it shows up on the tv perfectly. I tried switching the channel switch back and forth on the Atari 400 and nothing. Checked the resistance of the RF cable that's connected to the power board and found about 1 ohm of resistance on the inside pin and less than an ohm on the outside of the connector and the cable itself isn't shorted. So I don't know what else to do since I don't have any spare parts to use for swapping parts other than stealing some ram from my IBM XT which I'd prefer not to. Thanks in advance for any help!

  2. Default

    Which 48K mod did you do?

    You might try reseating all the chips in their sockets, to scrape any corrosion off the pins.

  3. #3

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    It's the non-official mod by Claus Buchholz with the hand written schematics. I shall do that and check back.

  4. #4

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    Does the screen go from noise to black when you turn the computer on?
    If so, there is nothing, likely wrong with the rf output. It is something in the computer, like RAM, rom, processor, address decoder, power supply, and such.
    Dwight

  5. #5

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    Quote Originally Posted by Dwight Elvey View Post
    Does the screen go from noise to black when you turn the computer on?
    If so, there is nothing, likely wrong with the rf output. It is something in the computer, like RAM, rom, processor, address decoder, power supply, and such.
    Dwight
    I have no noise and the tv isn't receiving any signal.

    I also checked the chips and reseated them and found no corrosion either. I tested the channel select switch and it's ok. All the capacitors, resistors and the transistors check out ok as far as I can tell with my multimeter on the RF modulator on the power board.

  6. #6

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    So I feel like a dummy, I had the chip in the Z503 socket backwards since I had it out to resolder the piggy back chip on it and didn't noticed it was backwards when I soldered it. So now I have signal but I'm still missing 16k of ram. Also it's having problems running BASIC cart, most of the time a black screen, sometimes a garbage screen and then eventually I'll get the proper screen. So progress, but now I gotta figure out the ram issue which I assume is causing the black screen.

  7. Default

    Meaning you have only 32K?

    I can help. I'm Claus Buchholz.

  8. Default

    Can you post some photos?

  9. #9
    Join Date
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    Default

    Quote Originally Posted by hef9800 View Post
    So I feel like a dummy, I had the chip in the Z503 socket backwards since I had it out to resolder the piggy back chip on it and didn't noticed it was backwards when I soldered it. So now I have signal but I'm still missing 16k of ram. Also it's having problems running BASIC cart, most of the time a black screen, sometimes a garbage screen and then eventually I'll get the proper screen. So progress, but now I gotta figure out the ram issue which I assume is causing the black screen.
    Inserting the chip in backwards and turning on the computer may have damaged the chip and/or the one piggy backed on it depending on where VCC and ground wound up.
    PCjr, DTK PC-XT Turbo, 386DX 33, 486 laptop, Pentium 120, Pentium III 500, various old laptops, Commodore Colt, all working. I also have a 286 that I need to see if I can repair.

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