Atari 800XL black screen

[xXsir_MoneyXx]

I finally managed to replace the chips in the centre of the board (except for the delay chip and the MMU), but it still doesn't boot up

However, I read that it might also be the crystal (I found out that the board is designed to have two crystals, like my board, but some have only one crystal...how could it be possible?), or the Q8-Q9 transistors/C109 capacitor.

The strange part is that I get the initial brown then black screen only when I power the atari for the first time! If i turn it off and on again, I only see a rapid white zig-zag line, and then nothing, only two pops fom the speaker.

Any suggestion of what should I try now?

[xXsir_MoneyXx]

I have just thought about the delay line. Could it be bad? Is there any way to test it / reproduce the delay signal?

It seems tricky to find a working chip on the Net if you live in Europe

[xXsir_MoneyXx]

7 months have passed, and I still haven't fixed my atari...

However, yesterday I thought about the GTIA. I didn't know that the GTIA not only interfaces the ANTIC to the video summation circuitry, but it also controls the function keys and it provides the beep on audio when a key is pressed. I originally tried booting a disk/starting the cassette motor by pressing the option key/typing CLOAD followed by two enter strokes, but since the GTIA controls almost all of the funtions that I tried, it might be the problem. Could a bad GTIA hang the system? I also thought of buying a logic probe analyzer, so I can test the address/data lines from the CPU

[+Nezgar]

Best first step in troubleshooting a non-working system is to swap chips one by one from the 'bad' system into a known 'good' system, and test them. Don't swap good chips into the bad system to test there. Then you might be able to determine the health of each main IC before delving into glue logic...

[xXsir_MoneyXx]

Well, I know that it's the best way to troubleshoot it, but here in Italy the atari 8-bit line of computers didn't have much success during the '80s, and getting a working one from Ebay isn't cheap either (I paid almost 100 euros only for the computer, shipping included). I can buy new chips on Ebay, but they will cost me 10/15 euros each...

 

P.S. I have already replaced all the glue logic chips in the centre of the PCB

Edited July 23, 2018 by xXsir_MoneyXx

[+tf_hh]

If you haven´t the possibility to take a working system for test etc., then I would offer to repair your mainboard. You´ve to pay the shipping costs, of course

[+CharlieChaplin]

If you know for sure the GTIA or CPU is defective, replacements can be found at ebay atm (seller from GB/UK)...

 

https://www.ebay.co.uk/itm/NEW-Atari-XL-XE-8-bit-computer-games-console-CO14889-01-PAL-GTIA-chip-IC/152383452491?hash=item237ac2f54b%3Ag%3AZiAAAOSwo4pYbZAD&_sacat=0&_nkw=Atari+GTIA&_from=R40&rt=nc&_trksid=m570.l1313

 

https://www.ebay.co.uk/itm/NEW-Atari-800-XL-XE-computer-console-CPU-IC-chip-C014806C-29-6502C-Sally/152388495439?epid=28007777933&hash=item237b0fe84f%3Ag%3AJc4AAOSw2xRYcqDQ&_sacat=0&_nkw=Atari+6502&_from=R40&rt=nc&_trksid=m570.l1313

[xXsir_MoneyXx]

Correct me if I am wrong, but I had an idea: there is a special instruction in the 650x family CPUs called NOP. If you send to the processor this instruction, it does basically nothing, and increases the current address decimal output by 1. Placing the CPU onto a breadboard, with a bunch of wires, resistors, LEDs and a slow clock circuit made with a 555 timer, you can effectively feed the data bus with the NOP instruction and see the address output slowly growing up. In this way, you can at least rudimentary test the 6502.

Am I right?

[+tf_hh]

Correct me if I am wrong, but I had an idea: there is a special instruction in the 650x family CPUs called NOP. If you send to the processor this instruction, it does basically nothing, and increases the current address decimal output by 1. Placing the CPU onto a breadboard, with a bunch of wires, resistors, LEDs and a slow clock circuit made with a 555 timer, you can effectively feed the data bus with the NOP instruction and see the address output slowly growing up. In this way, you can at least rudimentary test the 6502.

Am I right?

 

Yes - and no. IMHO this is a nice, but only academic test. Using this way would force you to set $EA (op-code for NOP) constantly at the databuss. In a good case the address buss is increasing with every SYNC = high by 1. But without monitoring every single addressline bit you can´t be sure, that the CPU is working well. I´ve had several defect CPUs (it´s - after DRAM - the 2nd most part with issues) with one or two defect address lines, constant high or low for example.

 

Next: Even if this test runs fine, you can´t be sure that the register (A, X, Y) are fine. Or the status-register (S). Or maybe there´s only a fault when the CPU has to branch or jump, with or without condition, executing any address index method, ...

 

Regarding the effort to build such a test facility, manual "hands-on time" and just a test of a very basic function it´s easier and cheaper to buy a new "NOS" CPU for around 12-15 Euros incl. shipping (see posts above).

[shoestring]

If you know for sure the GTIA or CPU is defective, replacements can be found at ebay atm (seller from GB/UK)...

 

https://www.ebay.co.uk/itm/NEW-Atari-XL-XE-8-bit-computer-games-console-CO14889-01-PAL-GTIA-chip-IC/152383452491?hash=item237ac2f54b%3Ag%3AZiAAAOSwo4pYbZAD&_sacat=0&_nkw=Atari+GTIA&_from=R40&rt=nc&_trksid=m570.l1313

 

https://www.ebay.co.uk/itm/NEW-Atari-800-XL-XE-computer-console-CPU-IC-chip-C014806C-29-6502C-Sally/152388495439?epid=28007777933&hash=item237b0fe84f%3Ag%3AJc4AAOSw2xRYcqDQ&_sacat=0&_nkw=Atari+6502&_from=R40&rt=nc&_trksid=m570.l1313

 

I always buy spare parts from that seller, never had a problem and he ships fast.