jmb

Good suggestions, I’ll try these out and report back. @andY - I’ve tried the self test but the screen usually messes up right as the tests start.

Hi folks, I recently took my old 800xl (stock, no mods) out of storage and after powering it on and getting to the READY screen the screen becomes completely scrambled about 3-5 seconds later. It’s not a static scrambled screen either, the garbage displayed on the screen changes at random time intervals. I tried all the obvious fixes (took out every removable I.C. and reseated them, double checked all ICs were firmly set in their sockets, replaced RAM, ANTIC and CPU). I also threw out the old power supply and made a new one but the problem persists. Note that about 25% of the time when I turn on the machine I just get a black screen. Power Supply ======== I had the old ingot style power supply that I’m pretty sure must have fried something when I initially powered up my 800xl. It was reading ~ +6.5 to 7 VDC! The first thing I did after reading about those ingot PSs was to throw it away and build a new PS using an old USB cord. The power from that new PS reads +5.50VDC. I verified that the new RAM chips are getting power within tolerance: measured +5.3VDC , tolerance is between +4.5 and 5.5 VDC. RAM === Next, I bought two different sets of RAM chips off eBay but none of them fix the issue: * Motorola 4164 (MCM6665BP20) 64K DRAM * Oki M3764-15RS 64K DRAM note that I am not mixing the RAM chips (E.g. I am not using some of the Oki’s and some of the Motorola’s together). Replacing the ANTIC and CPU ================== After replacing the RAM and seeing that it did not improve things, I read all of the Atari 800xl tech manuals I could find (great stuff btw, really wish I had access to that back in he 80s!). After understanding exactly what the different ICs do, I came up with a hypothesis: if the machine was operating normally, then on the READY prompt screen the video out signals on the ANTIC and GTIA chips would have a consistent pattern of pulses because that screen is static (it only displays READY). The converse should also be true: when the screen on my 800xl becomes randomly scrambled, then the video out signals from the chips would vary with the randomly changing scrambled output I was seeing. So I should be able to work backwards from the composite out port -> display circuitry -> GTIA chip -> ANTIC chip using a logic probe and find the source of the scrambled display. Sure enough, I could see that the signals on pins 2,3 & 5 of the ANTIC chip (pixel data & H/V sync command output to GTIA) were indeed varying with the randomly changing scrambled output I was seeing on the monitor. So I ordered an ANTIC chip (C021697-11) and a CPU just for good measure (C014806C-29). I put those chips in, turned on the machine and..... the problem still exists. Unless I somehow received two bad batches of RAM, the schematics show that it could be a bad MMU chip or the two multiplexer ICs (u26 & u27; 74LS158’s). But before I go buying more chips, I wanted to see if anyone else has encountered this issue or if anyone has advice on next steps to troubleshoot. Thanks, Jeff