Atari 65 XE Najm troubleshooting

[+slx]

An impulse buy has added an Arabic 65XE to my collection. While it seemed to work out of the box, the picture tended to disappear and reappear (without reboot) when the power plug was touched. After a couple of minutes it did not work at all and when I turn it off and on again I get a dark screen, a tiny little bit of the "boot fart" (just one "note", so to say) and the power LED blinks steadily (on with every sound). The frequency is about 72 Hz.

 

Tried both a modern and old PSU, with the old PSU it does nothing at all. Both work on another 65 XE, so the PSUs should be OK.

 

My first instinct would be to check and resolder the power connector connection to the mainboard.

 

Any other hints/suggestions are welcome! Thanks!

[TGB1718]

Could just be cracked solder joints on the power socket on the mainboard, solder re-flow and try again.

[+slx]

On 2/24/2021 at 3:19 PM, TGB1718 said:

Could just be cracked solder joints on the power socket on the mainboard, solder re-flow and try again.

I tried that but it makes no difference. Interestingly I don't get 5V between the power socket solder points on the underside of the mainboard.

[TGB1718]

Maybe some close up photos of top/bottom and plug, maybe might be able to see something.

[+slx]

21 hours ago, TGB1718 said:

Maybe some close up photos of top/bottom and plug, maybe might be able to see something.

Here's a top photo, I'll take one of the bottom the next time. When the power switch is off, I get 5V across the PSU connector terminals, when I switch it on, that drops to about 0.12V. I let it sit with power on for about 30 minutes but nothing feels hot or even warm to the touch thereafter.

[TGB1718]

2 hours ago, slx said:

When the power switch is off, I get 5V across the PSU connector terminals, when I switch it on, that drops to about 0.12V.

That does seem to indicate you have a short circuit somewhere, but with nothing hot or any smoke  a bit difficult to find, to be sure

I would try getting your meter to read the current when you switch on, you would need to use some fly leads to bridge between

the PSU plug and the pins on the XL and put the meter in series with one of the leads, your meter would need to be able

to read 2AMPS or greater though, else you might damage it.

 

 

PSU+__________METER___________XL+ve Pin

 

PSU-___________________________XL-ve Pin

[+Nezgar]

7 hours ago, slx said:

When the power switch is off, I get 5V across the PSU connector terminals, when I switch it on, that drops to about 0.12V.

Sounds like symptoms of a machine that has been subjected to a commodore 64 power supply... Could be multiple IC's blown out from the overvolt+AC potentially subjected.

 

I see your OS ROM is already socketed - pop it out and see if the voltage scenario improves. Then next desoldering & socketing the RAM. Then proceed to the other main IC's. Measure voltage after each is removed. If you measure +5V with the power ON, then you know whatever you removed last was the last short-circuited IC. (There could have been others, so test replacing things one at a time)

[_The Doctor__]

capacitor near power switch looks funky, power switch looks rusty, transistor j6 is bent over, bottom left crystal looks twisted as does the resistor near it(make sure crystal in not damaged), there is a discolored resistor over in the color pot area down and to the left nestled by caps around it. Make sure the mother board is not shorting out to lower shield (there is supposed to be an insulating sheet between the motherboard and shield), make sure no holes are poked through it if is present.

this may be over voltage or ac damage, start with ram testing after checking those other issues first.

Edited March 2, 2021 by _The Doctor__

[+slx]

Thanks for all the tips. There was no insulation between the MoBo and lower shield but I tested the bare board as well. is there anything that would routinely blow when the lower shield touches the motherboard?

 

All in all this sounds like a lot of work and as I bought the machine as working, I'll only do a deeper dig if the seller agrees on a substantial discount, otherwise I'll return it and have a collection without Najm. 

 

The interesting thing is that it worked intermittently for a couple of minutes hooked up to an Atari PSU before dying. 

[_The Doctor__]

not specifically as it depends on where pressure might have been applied from underneath as well as what through hole components legs are sticking down.

You've go a good list to start with though... I think it's easily fixed, so long as nothing is bent or touching and the stand off are still fully extended keeping it clear you will be okay with the bottom shield. It's just something I'd come across so it's on my check list.

 

You all ready have a mix of RAM on the board and they are of different manufacture as well as coloration.... I'd definitely start with ram...

[+slx]

On 3/2/2021 at 7:14 PM, _The Doctor__ said:

You all ready have a mix of RAM on the board and they are of different manufacture as well as coloration.... I'd definitely start with ram...

I thought about taking a shortcut and saving myself a lot of desoldering by installing a Lotharek 64K SRAM which I bought on a whim some time ago. If that doesn't work I still have a socketed CPU to show and can proceed to swap the remaining chips.