Any tips for diagnosing a black screen on an Atari 130xe?

[shoestring]

Hi. The elenco lp-550 logic probe is affordable, I believe around $30 and I've had no issues with mine.

 

Syscheck II comes in cartridge form and well worth checking out.

 

Mine won't be ready for quite some time but I'll be providing a binary for free once it's finished, I have it booting in my XE but have some testing to do once it's all done.

 

Also check out Adam's video on Digital and using Logic Probes.

 

[KLund1]

DrV. Thanks for the suggestion. I got a logic probe. ($9) from frys, It seems to work. Watched a few vids about them, so i got an idea about how to use one. But I do not know what pins or leads should be high or low or pulse. Sam's manual says in the video section "if the waveform is missing..." A probe will not help with that, and I do not have scope.

[shoestring]

You need a reference such as a datasheet of the chip you're troubleshooting or ttl logic data book. You should be able to download a pdf version of the data book for free as there are plenty links to these floating around the www. Datasheets and ttl data books will tell you which leads should be high, low or pulsing but it's a little tricky with custom chips cause data sheets aren't usually available to the public for these.

 

What a logic probe won't do is tell you "when" in time an output should be high or low if it's pulsing, therefore you can't verify if the waveform is correct without a scope. So you really need a scope for that but there are some tricks to getting around it in some cases. For example, you can remove a CPU prior to probing the output of an AND gate to stop the output from toggling. It will give you some level of control over the inputs by using a logic pulser in conjunction with a logic probe on the output. A logic pulser just allows you to inject your own high or low signals into an IC to troubleshoot a potential bad gate.

 

I find logic probe is handy for quick troubleshooting and finding floating or stuck outputs if I'm too lazy to bring out the scope and it's usually sufficient for my needs.

Edited July 3, 2018 by shoestring

[+DrVenkman]

I’m at work so I don’t have the book in front of me, but I’m pretty sure the SAM’S book has a logic chart in it. You can power the machine and then take readings with the logic probe at the various pins on the IC’s to make sure the results match what the chart indicates. Combine those results with simple continuity and voltage tests with your multimeter, and you can diagnose pretty much any conceivable problem with an A8.

[KLund1]

Thanks, that info helps.

With that, I found that U20 dose not match the logic chart. I swapped it out with a known working ic and get the same results. Pins 2 to 12 are all low. 1, 13-16 are correct. With my rudimentary schematics skills, U20 is connected to GTIA with a few resistors in between. I tested all the resistors in the area with a meter, and they have continuity. The GTIA, and Antic have tested good.

Thoughts?

[+DrVenkman]

Thanks, that info helps.

With that, I found that U20 dose not match the logic chart. I swapped it out with a known working ic and get the same results. Pins 2 to 12 are all low. 1, 13-16 are correct. With my rudimentary schematics skills, U20 is connected to GTIA with a few resistors in between. I tested all the resistors in the area with a meter, and they have continuity. The GTIA, and Antic have tested good.

Thoughts?

 

U20 is a standard 4050 hex buffer that's pretty much common to nearly every Atari 8-bit video circuit. If you're sure the one you swapped in is working, and both ANTIC and GTIA are good. It's possible the socket is bad. What I'd do next is - with the power off - confirm continuity from each pin to the next component on the schematic. It's possible one or more of the isolating caps in the lines from GTIA to the 4050 have shorted to ground, but I can't imagine all of them shorting at the same time. Very odd.

 

And just to be sure, you've already confirmed the CPU, OS ROM and FREDDIE are good?

[shoestring]

I've had one test bad in an 800xl I recently picked up. Looking quickly through the thread you menioned it was working at some point but I don't know if you've established the PSU as the culprit. I'd be looking at RAM first..

 

What do your +5v voltages measure like under load ?

Is there any AC ripple leaking into the DC circuit? It's possible to see this on the screen and causes a rolling effect.

 

But since you have nothing on screen it's time to establish if CPU is actually doing anything with your logic probe. Check the reset pin first and address. + data lines ( these should toggle ),check the output enables and also r/w enables.

 

The Sams 130xe computer facts has a very good troubleshooting guide if you haven't looked through that already.

 

http://www.atarimania.com/documents/130XE%20-%20Sams%20ComputerFacts.pdf

Edited July 4, 2018 by shoestring

[KLund1]

Thanks everyone. I very much appreciate the hand-holding. I'm learning a lot as we go.

CPU is good, but gets warm under load. I have no way to check Freddie. I only have the one 130xe. But it does not get warm at all under load, all the other IC's warm up some but not Freddie. The LP dose seem to show activity on it though, Most pins are pulsing low, if i understand what the tomes mean from the probe.

I can unsolder the OS and swap with a 800XL with the same number IC.

My PSU is in good working order.

I swapped out the under load hot RAM with socketed new ic's. No warm or hot rams now.

What do your +5v voltages measure like under load ? How best to test? I touched several places on the board and get 4.9 to 5.13v

Is there any AC ripple leaking into the DC circuit? How to test.

Check the reset pin first and address. + data lines ( these should toggle ),check the output enables and also r/w enables. How to test?