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About shoestring

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    C64, Apple, A8 & Amiga junkie. Video game & electronic repairs, homebrew dev (arcade diagnostics software), guitar & music.

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  1. Those 80s era mT ram chips don't age very well do they. Piggy back doesn't work most of the time. If the soldered IC is shorted and getting hot, you risk frying the RAM on top instantly. It only works when the ICs gate is open which isn't common, not stuck permanently in any position [ hi, low or floating ] which is usually the case If there is any failure mode that causes the bad chip to drive 0 or 5v, the good chip on top drives the opposite value which is the correct one.. . the two chips end up fighting each other where you have one driving 0v and the other driving 5v... not good.
  2. Yeah I can see that being a massive pain. Especially the earlier non socketed boards that used the 1bit DRAM chips. I'm also kinda new, I grew up on Apple IIs, the c64 then the Amiga. The Amiga shares many things in common with the Atari 8 bit, so I like programming on these. In regards to the built in ram diagnostic in the OS Rom, it's still pretty good. It can only verify 48k because 16kb of it is used up as work ram ( storing variables, setting up display ..etc ). Initially my diagnostic runs with the screen blanked out on purpose whilst checking the most critical area of memory, this is why you hear the most annoying sound for 10-15s.
  3. I do plan on doing a 128kb version just for 130XEs to avoid having to switch the two banks around. Shouldn't be too difficult. I just need to stop being lazy. Good to see you have it working now.
  4. Blank screen with screech noise is normal. Just let it finish.
  5. Nothing else at all. A bad OS rom can give you a red screen too. So I suspect there was some problem in the process of programming the EPROM.
  6. The firing sound in Dropzone on the Atari 800 is iconic IMO, makes the game so much more enjoyable. Although it's obviously very Defender-esque if you compare the sound effect from the arcade, the c64 version firing sound pales in comparison.
  7. Thanks. I'd say you still have a partially bad RAM chip(s) within the first 64kb of addressable memory. But you might want to check the MMU whilst your at it as emphasized by others. Some bits will fail verification at specific addresses and not necessarily at every address. But from experience these DRAM chips tend to fail completely, most likely due to some faulty power supply or as someone here once put it... a "Commodore attack" ( the act of trying to use a C64 power supply in an Atari 8 bit).
  8. Mine is still far from perfect but what do ya expect for free . For instance it's accurate for partially bad ram, depending on where the fault is. Today I was testing some ram chips from an aux 80 col card for an Apple II, I knew one or more were bad..the built in Apple IIe diagnostic was giving me different results each time I powered up, its usually great for testing the RAM on the motherboard through. After installing those two 4464s from the 80 col card into my XE, it was able to identify U9 as the culprit and did it consistently. When I swapped the two around in the XE it told me u10 was bad. I wanted the ability to test 4464, 4164 ram chips easily. I also aimed at making something that would display a picture on the screen even with no RAM installed in the machine at all, as long as the vital components in the machine are OK... you should always see something.. even if the screen flickering like mad. Sys-check should be much more reliable because I believe it has it's own work ram on the cart.
  9. I've personally known someone who lived next door exchange their c64c several times before they got a reliable one ( machines exchanged stopped powering up after a week or two ) that survived the summer holidays in 86. All of them came with the white brick with the embossed chicken head logo and potted inside. So given they were new and those problems existed back then... the only suitable use would be as a doorstop.
  10. You could also go a step further and type in this basic program which copies the character set from the character rom to the RAM. https://www.devili.iki.fi/Computers/Commodore/C64/Programmers_Reference/Chapter_3/page_110.html 5 PRINTCHR$(142) :REM SWITCH TO UPPER CASE 10 POKE52,48:POKE 56,48:CLR :REM RESERVE MEMORY FOR CHARACTERS 20 POKE56334,PEEK(56334)AND254 :REM TURN OFF KEYSCAN INTERRUPT TIMER 30 POKE1,PEEK(1)AND251 :REM SWITCH IN CHARACTER 40 FORI=0TO511:POKEI+12288,PEEK(I+53248):NEXT 50 POKE1,PEEK(1)OR4 :REM SWITCH IN I/O 60 POKE56334,PEEK(56334)OR1 :REM RESTART KEYSCAN INTERRUPT TIMER 70 END Once, you type in the above listing and run it, then type the following statement to tell the VIC II to fetch the glyphs from ram. POKE 53272,(PEEK(53272)AND240)+1 List the program or type some stuff on the screen, do you still see tearing on the bottom half of the screen ? In some cases character rom can go bad and glyphs can have additional artefacts or missing pixels, so you can also change the letter T to some other shape to confirm that. In your case, I'm confident that the character rom and ram is OK. 10 FOR I=12448 TO 12455: READ A:POKE I,A:NEXT 20 DATA 60, 66, 165, 129, 165, 153, 66, 60
  11. Looks like a bad counter somewhere in the video circuit, could be the VIC II. Is your machine socketed and does this also affect sprites or only character objects ?
  12. From 2016 but still interesting if you haven't seen it before. http://www.pouet.net/prod.php?which=67366
  13. Was definitely the first computer I ever used, I believe a non enhanced IIe from 1983. I was in third grade. Wanted one but we could never afford one. First program I ever used was the Apple IIe Introduction on disk and favourite game at the time was Choplifter. Favourite all time Apple games were Karateka, Lemonade stand, Where in the world Carmen Sandiego ,Swashbuckler and Aztec. Neighbour had a TI, so we played that. I vaguely recall using an 800xl on display at a K-Mart which was running basic but there was also a c64 right next to it running BC Quest for tires. We ended up with a c64 but I always loved using the Apple in school as well and I was constantly dropping into the built in machine code monitor and messing around with the 6502 instructions.
  14. Hi Graeme. Been following your posts on eab and YT updates since Bombjack. Nice to see you here.
  15. Don't you mean c50? Try a polymer solid electrolytic cap ( more reliable ) if you know c50 is bad. It's not rare for those to fail, just search the forum and you'll come up with some results. But I think you'll get better results if you try a proper 5v power supply.
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