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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?

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

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

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

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?

Hey, I finally got out of these awful busy months, and I decided to socket and replace all the chips in the middle of the board, except for the delay chip. They are pretty cheap, although the shipping price is a pain in the wallet (about 25$ from Mouser). I think I would probably check the continuity and substitute the 40 pin sockets, because there is no evident damage on them. Hope I'll figure out the cause of this curse!

Sure, I'll do! I have taken a little break for Christmas holidays, but now I'm on again. I'm going to replace the three oxidated chips (respectively the U19, U20 and U28). Try checking the U2 (the service manual says to do so) decoder chip, and also check the three IC's I've mentioned before.

I found a strange yellow colour on the pins of some chips in the middle section of the atari board. It's the same color of heated iron. May these chips be the cause of black screen? I was also wondering if there's a way to test the OS and BASIC roms...

Well, I found a Star Raiders cartridge online for about 15/18 euros, so I think I'll buy it. From what I have read and watched in this forum, my PSU isn't the ingot one, however I think mine has been refurbished, because I can see some residue that seems super glue. Also, I followed the schematic from Nick Kennedy (using a MAX232CPE) for my SIO2PC interface, it worked just fine for an entire day (and I turned the atari on and off several times!). I can't understand how the interface could damage the computer after a day of intensive usage... Here are some photos of my PSU:

Well, I just resoldered and double checked EVERY socket pin (I used my tester in continuity mode), but it hasn't solved the problem. I turned the volume at the maximum level, and I heared three pops during startup: one when I switch on the atari, one when the brown/red color changes into black and one after 0.5 seconds after the second pop. Unfortunately i don't have any atari spare part, so it seems I have to order a new SALLY and a new ANTIC online (I thought atari put an entire computer inside a 1050 drive, like Commodore did, but I read it didn't, right?)

Hello I have just bought an atari 800XL and a 1050 from an online seller. The machine is almost pristine (only the metal buttons are a bit scratched) and IT was never modified until now. When I first set it up I didn't notice any problem (I ran the self test, and it all worked perfectly), however I hadn't got any game to test, so I built an SIO2PC interface myself. Unfortunately, when I booted the atari with the SIO2PC it went straight to the self test, displaying bad memory. I found out the ram chips were branded by MT (i have read MT 4264s fail very often), so I removed the chips, installed sockets (my atari is a PAL machine) and placed new TI 4164s. Luckily it prompted to basic, so I used it the whole afternoon without issues. Now here's the strange part: when today I powered on my atari again, the screen went brown for a fraction of second, and then it just changed into black!!! I know there are lots of possible bad chips/electronic components, but I made some tests, and here's what I can say: 1) I tested it via RF and via monitor output, and I got the same result 2) I reseated all the socketed chips 3) I tried typing something, but i didn't get any beep 4) I checked the conductivity of the ram sockets 5) I tried booting a game (draconus) from disk, but the disk didn't even spindle 6) I checked the electrolytic capacitors, and they appeared to be OK 7) I checked the output of the PSU (it's a british one, part CO61763-34) and it displayed 4.7/4.8V, and inside the machine i got 5.18/5.22V (I think i got 4.7V because the PSU is designed for 240V AC input, but here in Italy we have 220V). I think the atari isn't responsive at all (no commands from the SIO port, no audio...), the only thing I get is the brown screen when i boot it up. This is frustrating! What can i do? May the CPU be bad? Sorry for my english, it's my first topic. Thanks in advance P.S.: Sadly I don't have any cartridge or spare part to test. The only atari microcomputer-related thing I have is the 800XL, a 1050 and a 1010