Atari 400 Power Circuit Issue

[mattlacey]

Hey Guys,

I recently got hold of a 400 that's not in working condition (unfortunately it's also missing a bit of the case at the back next to where the RF lead comes out, seems to have taken a hit during shipping).

 

Basically there's nothing on screen (though it's not straight up noise) and no key-clicks, so I set about debugging the issue using the Field Service Manual. I got as far as the step for the black/grey screen chart that says to check for 12V on Pin 2 of the power board -> mother board connector (J201 from memory).

 

I do not have 12V on that pin, (the +5v and -5 pins seem good). The flow chart says to check 3 caps and a diode, the diode seems good and two of the caps have 12V across them which would be what I expect. One cap however, (C203?) is a smaller green type, wedgedbetween the big black sheild/heatsink and RF modulator. I managed to hook some wires around the legs of the cap to get my multimeter onto it, and on there I'm seeing a voltage of around 26V. I know it's a long shot but can anyone confirm if it's supposed to be that high or if that's an indication of the fault?

 

 

[Joey Z]

Hey Guys,

 

I recently got hold of a 400 that's not in working condition (unfortunately it's also missing a bit of the case at the back next to where the RF lead comes out, seems to have taken a hit during shipping).

 

Basically there's nothing on screen (though it's not straight up noise) and no key-clicks, so I set about debugging the issue using the Field Service Manual. I got as far as the step for the black/grey screen chart that says to check for 12V on Pin 2 of the power board -> mother board connector (J201 from memory).

 

I do not have 12V on that pin, (the +5v and -5 pins seem good). The flow chart says to check 3 caps and a diode, the diode seems good and two of the caps have 12V across them which would be what I expect. One cap however, (C203?) is a smaller green type, wedgedbetween the big black sheild/heatsink and RF modulator. I managed to hook some wires around the legs of the cap to get my multimeter onto it, and on there I'm seeing a voltage of around 26V. I know it's a long shot but can anyone confirm if it's supposed to be that high or if that's an indication of the fault?

 

 

C203 is a voltage doubler filter cap, it would make sense to see 26V across it, yes. it sounds like you have a blown regulator (the 7812, A201, it would be mounted on the heat spreader). if you measure across the input pin to ground, you should get that 26V, and if you do the same from the output to ground, you should get 12V. If you get the first, but not the second, you have a bad regulator. They're a common part, if you know how to solder it should be fairly easy to replace yourself, otherwise an electronics repair shop could probably do it for you pretty easily.

 

http://circuits.datasheetdir.com/39/LM7812-pinout.jpg

 

EDIT: On a 400 with a stock RAM board, 12V is needed for the DRAM and the GTIA color adjust. If you have a 400 with the official atari 48K upgrade, they use different DRAMs that only need 5V, so it would actually work, but with no/wrong color.

Edited August 18, 2014 by Joey Z

[mattlacey]

Thanks for the info Joey, will check out that part tonight! More than happy to get stuck in with a soldering iron. I was quite amused to see that the heatpaste for that regulator is still soft after all these years, I had thermal compound on PC chips dry up after a year or two, suspect they got a ltitle more cooked though.

It's a stock 400 so will let you know how I go!

[mattlacey]

Spot on the money! Reading around 24V on the one side, and 0.5V on the output side of things... will replace and let you know how I go!

[mattlacey]

Dropped in a replacement voltage regulator and it worked a treat, 12V showing on pin 2 like it should. That got me to a solid black screen, further investigation revealed that one of the ICs (I forget which, didn't check the code) was inserted the wrong way around. Second computer in this batch of 3 to have that issue!

Now I have a working machine (need to rebuild it to check the keyboard, just wish I could get my PAL TV to tune to the NTSC signal properly, just get a grey scale screen and no sound, which sucks because the composite can deal with NTSC fine. Guess it's just the TV tuner

[Joey Z]

Dropped in a replacement voltage regulator and it worked a treat, 12V showing on pin 2 like it should. That got me to a solid black screen, further investigation revealed that one of the ICs (I forget which, didn't check the code) was inserted the wrong way around. Second computer in this batch of 3 to have that issue!

 

Now I have a working machine (need to rebuild it to check the keyboard, just wish I could get my PAL TV to tune to the NTSC signal properly, just get a grey scale screen and no sound, which sucks because the composite can deal with NTSC fine. Guess it's just the TV tuner

did you check the color pot on the CPU card? also, since the audio doesn't work anyway, there is an adjustable inductor you could try to change the audio carrier of the modulator and possibly get the audio to work. I'd recomend you just composite mod the 400 though.

[+slx]

Now I have a working machine (need to rebuild it to check the keyboard, just wish I could get my PAL TV to tune to the NTSC signal properly, just get a grey scale screen and no sound, which sucks because the composite can deal with NTSC fine. Guess it's just the TV tuner

Unfortunately there seem to be many TVs that can do NTSC composite but no NTSC RF. Shopped for a used TV recently and most had that restriction. Good luck with your restoration project (I am sitting next to an untested 400 as well)!

[mattlacey]

Unfortunately there seem to be many TVs that can do NTSC composite but no NTSC RF. Shopped for a used TV recently and most had that restriction. Good luck with your restoration project (I am sitting next to an untested 400 as well)!

 

Yeah it's annoying! I'll either do a composite mod on it or just sell the machine on. Only bought it to see if I could fix it and seem to be going ok on that front