Atari TT Repair

[mgarnett]

Hi All,

I am repairing a TT after a power supply failure where the fuse exploded and several diodes failed in the supply. Before installing a new supply I wanted to check to make sure there were no shorts on any of the power pins and on the motherboard.

I checked the power connector on the motherboard and tested the resistances of the 5, 12, -12 and -5 volt pins to ground. There were no dead shorts, however the resistance of the 5v pins to ground was quite low at only 25.5 ohms. The other resistances for the other power pins were relatively high.

I wonder if anyone could tell me if that resistance measurement is normal, or is there a problem with the motherboard.

Thanks

Mark

[exxos]

It doesn't sound right no... Resistance should go up as the output caps in the PSU charge from the meter.

[mgarnett]

For information, have been told by another TT owner that without a PSU installed, the resistance from 5 volt to ground on the motherboard can range from 25 ohms to about 160 ohms depending on what hardware you have installed, so his resistance appears to be normal.

Cheers

Mark

[mgarnett]

Power supply repaired and the machine powers up, but to a black screen.

Damage was obviously more severe.

Troubleshooting time! I’m going to start going through the service manual procedure but additional advice always welcome if there’s other things I need to watch out for.

Thanks

Mark

[exxos]

What do you mean "powers up" exactly, as if it was powered up it would be working ? Or do you mean it has power but nothing happens ? Or its got power and accessing the floppy (as it trying to boot up) and no display ?

[mgarnett]

Yeah sorry, I mean the PSU is on and there are correct voltages coming out of it and I’ve tested many of the ICs and they appear to be receiving the correct voltages.

Just nothing happens on screen, no HDD activity and no floppy activity so something is failing before any of that even starts.

Thanks

Mark

[exxos]

I never had a TT, but I think they are not easy to fix. Normally you start making sure the reset line is working, then check /AS & /DTACK on the CPU to see if its at least trying to do something. Also check CE & OE on the ROM's to see if it looks like they are being accessed.. you need a good scope to do that..

[mgarnett]

Thanks, appreciate the advice.