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Atari 400 has power led but no signal to tv

hef9800

New Member
Joined
Feb 11, 2019
Messages
4
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!
 

ClausB

Experienced Member
Joined
Jan 22, 2012
Messages
156
Which 48K mod did you do?

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

hef9800

New Member
Joined
Feb 11, 2019
Messages
4
It's the non-official mod by Claus Buchholz with the hand written schematics. I shall do that and check back.
 

Dwight Elvey

Veteran Member
Joined
Jun 21, 2003
Messages
4,735
Location
Santa Cruz
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
 

hef9800

New Member
Joined
Feb 11, 2019
Messages
4
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.
 

hef9800

New Member
Joined
Feb 11, 2019
Messages
4
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.
 

ClausB

Experienced Member
Joined
Jan 22, 2012
Messages
156
Meaning you have only 32K?

I can help. I'm Claus Buchholz.
 

KenEG

Experienced Member
Joined
Oct 20, 2016
Messages
440
Location
Cincinnati, Ohio USA
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.
 
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