jham55 Posted June 17, 2022 Share Posted June 17, 2022 (edited) Hey folks I’ve seen a couple posts about this but wanted to get your thoughts. I have a coco2 and coco3. The 2 works on boot to where I can type but any game in it and it turns orange. On the 3 is just shooting a green screen no text boot. I opened them up and don’t see any caps that are bad. Most of the chips are soldered in. I was going to buy a part to reseat the GIME to start. Is that fair? I’m happy to recap the whole thing but not sure it’s necessary yet. Edited June 17, 2022 by jham55 update Quote Link to comment Share on other sites More sharing options...
MrDave Posted June 18, 2022 Share Posted June 18, 2022 You can make a gime puller with a paperclip or something.. . Clean and inspect everything.. try pushing on the board lightly with a eraser and see if anything changes.. Clean and reseat the vdg chip 6847 on the coco2 Check the voltage level, see if it lower with a cart inserted.. They are tough machines..you will get them going Quote Link to comment Share on other sites More sharing options...
hhos Posted June 18, 2022 Share Posted June 18, 2022 On 6/17/2022 at 11:48 AM, jham55 said: I was going to buy a part to reseat the GIME to start. I usually use a jeweler's flat blade screwdriver to take a chip like that out. Truthfully, I have no idea what the proper tool looks like. The main thing is exercising patience and moving each corner of the chip up in small increments. On 6/17/2022 at 11:48 AM, jham55 said: I’m happy to recap the whole thing but not sure it’s necessary yet. If a cap has gone bad, measuring the AC voltage on it will usually tell you something.about its condition. After a short settling time it should stabilize very close to zero volts on a regulated supply. An unregulated supply might tolerate a significant ripple, though. I don't remember there being much of that in the COCO3 though. You might check the DRAMs. They will be socketed so you could reseat those. I can't remember the symptoms of a mismatch on the decoupling caps to the RAMs, but it was the first place my mind went when I read your post. If it has 128K the caps need to be connected. There will be four(4) 64Kx4 DRAM chips on the mother board for 128K. The 512K DRAM will be on a daughter board. Mine all have 16 256Kx1 chips. For 512K the decoupling caps, which the 64K chips needed on their power supply pins, need to be disconnected from the 9th address line for the 256K chips. I'm unfamiliar with the larger memory setups but I imagine they also use this 9th address bus. I hope this helps. Good luck. HH Quote Link to comment Share on other sites More sharing options...
jham55 Posted June 19, 2022 Author Share Posted June 19, 2022 So after cleaning and reseating on the coco2 here is what happened.. robocop game went green to a bunch of e letters and another game went orange with colored lines .. please see pics Quote Link to comment Share on other sites More sharing options...
MrDave Posted June 19, 2022 Share Posted June 19, 2022 Robocop is coco3 only.. what does it boot to without cart ? Quote Link to comment Share on other sites More sharing options...
jham55 Posted June 19, 2022 Author Share Posted June 19, 2022 The coco3 just boots to a green screen Quote Link to comment Share on other sites More sharing options...
jham55 Posted June 19, 2022 Author Share Posted June 19, 2022 No text Quote Link to comment Share on other sites More sharing options...
jham55 Posted June 19, 2022 Author Share Posted June 19, 2022 So after running the coco3 for a bit two things I noticed was that the voltage regulator shield was super hot and the Mc68B09EP chip was warm but not hot.. all the other chips were cool. Quote Link to comment Share on other sites More sharing options...
hhos Posted June 19, 2022 Share Posted June 19, 2022 6 minutes ago, jham55 said: So after running the coco3 for a bit two things I noticed was that the voltage regulator shield was super hot and the Mc68B09EP chip was warm but not hot.. all the other chips were cool. I think that is all pretty much normal, but it has been a while since I've opened one of those up. Quote Link to comment Share on other sites More sharing options...
jham55 Posted June 19, 2022 Author Share Posted June 19, 2022 Sounds like maybe trying to change out the RAM next? Anyone know of any cost efficient replacements? There are 16 of them. Quote Link to comment Share on other sites More sharing options...
hhos Posted June 19, 2022 Share Posted June 19, 2022 That might be a good way to go if you already possess the DRAMs. How much do you know about this particular COCO3? If a previous owner upgraded the RAM, but never got it to work afterwards, you may be looking at a simple botched upgrade. Did you check the decoupling caps that the original 64K DRAMs would have required? With 512K they should be disconnected. Quote Link to comment Share on other sites More sharing options...
jham55 Posted June 19, 2022 Author Share Posted June 19, 2022 I have no history about this one. I reseated all 16 of the ram chips and no change. How do I know if he updated the ram? They are hy53c256ls. No sure about the decoupling caps. Where are they located? Quote Link to comment Share on other sites More sharing options...
jham55 Posted June 19, 2022 Author Share Posted June 19, 2022 Is this what you mean? Quote Link to comment Share on other sites More sharing options...
jham55 Posted June 19, 2022 Author Share Posted June 19, 2022 Looks like C65 was removed. So at this point I need a good replacement RAM for the 16 chips. Quote Link to comment Share on other sites More sharing options...
hhos Posted June 19, 2022 Share Posted June 19, 2022 (edited) I'm afraid I don't remember their exact location. I'll dig out one of my COCO3s when I get home. Looking at your picture I'd say they have been removed already. I'll confirm their location later, after I check one of mine. Edited June 19, 2022 by hhos Left ko it a word Quote Link to comment Share on other sites More sharing options...
MrDave Posted June 20, 2022 Share Posted June 20, 2022 Yes they are removed and thats all ypu have to do to upgrade yo 512k+ besides pulling out the 4 64k x 4bit drams that were in the lower sockets.. those you can find cheap to test your coco Quote Link to comment Share on other sites More sharing options...
jham55 Posted June 20, 2022 Author Share Posted June 20, 2022 I'm not seeing any cheap replacements.. Is there a place you recommend? Quote Link to comment Share on other sites More sharing options...
jham55 Posted June 20, 2022 Author Share Posted June 20, 2022 Also since the ram was upgraded would it make sense to replace the cpu first or is it just as likely that ram can go bad? Quote Link to comment Share on other sites More sharing options...
MrDave Posted June 20, 2022 Share Posted June 20, 2022 Modern ram updrades are surface mount .. if you have one with many chips its very old. Yes you can replace the cpu with new 6809 or even better 6309 Quote Link to comment Share on other sites More sharing options...
MarkO Posted June 21, 2022 Share Posted June 21, 2022 On 6/18/2022 at 11:55 AM, hhos said: I usually use a jeweler's flat blade screwdriver to take a chip like that out. Truthfully, I have no idea what the proper tool looks like. The main thing is exercising patience and moving each corner of the chip up in small increments. << BIG SNIP >> The Black Handled puller in this link: https://www.amazon.com/PHONSUN-Extractor-Motherboard-Component-Extraction/dp/B071HW5FV5/ref=sr_1_1_sspa MarkO Quote Link to comment Share on other sites More sharing options...
MarkO Posted June 21, 2022 Share Posted June 21, 2022 (edited) On 6/19/2022 at 3:05 PM, jham55 said: Looks like C65 was removed. So at this point I need a good replacement RAM for the 16 chips. C65 and C66 are the Caps that are Removed when the original 4464s are removed and a 256KB, or 512KB, or 1MB, or 2MB RAM is installed... There are a couple sources for modern 512KB and 2MB SRAM boards.... To use the RAM beyond 512KB, the CPU needs to be Removed and a PCB is installed with Extra Address Lines. You might as well replace that MC6809 with the HD6309, it's CMOS instead of NMOS and has some very cool features that is leveraged with various Software.. MarkO Edited June 21, 2022 by MarkO Additional Info Quote Link to comment Share on other sites More sharing options...
jham55 Posted June 21, 2022 Author Share Posted June 21, 2022 I'm fine replacing the RAM thats there but I just need to know which chips to buy. Quote Link to comment Share on other sites More sharing options...
hhos Posted June 21, 2022 Share Posted June 21, 2022 Disclaimer: I'm afraid I remembered incorrectly about that cap, C65. It is not a decoupling cap, and it is on the RAS/ signal from the GIME. C66 is not mentioned in the conversion procedure in my tech manual. I have experimented with the memory to see what would cause the symptoms observed. It turns out that any failure of any chip in the DRAM will cause the green screen, so the memory is the probable cause as jham55 has deduced. I have a problem with believing it, though. In the past 40+ years I've had several dozens of memory failures, in computers that is, but only 3 or 4 were caused by the memory chip itself. I've seen the pins corrode off in cheap sockets, but then I've seen boards left out in the weather, covered with rust, clean up just fine and run with no further problem. I am still inclined to look for a more immediate remedy. However, there is a lot of 25 41256-10 DRAMs on eBay for roughly $20, including shipping. Search 256K DRAM. That's 100nS and you only need 150nS chips. Hhos Quote Link to comment Share on other sites More sharing options...
MrDave Posted June 23, 2022 Share Posted June 23, 2022 I have been seeing alot of ram failing in the 1980's computers these days.. although the caps were pretty good for the coco i have had to replace some and recap my msx2.. Those 2 caps that get clipped are in a rc configuration and change the shape/timing of the signal..my guess would be all those extra chips add capacitance to the citcuit naturally Quote Link to comment Share on other sites More sharing options...
MarkO Posted June 23, 2022 Share Posted June 23, 2022 On 6/21/2022 at 4:39 AM, jham55 said: I'm fine replacing the RAM thats there but I just need to know which chips to buy. If your replacing the RAM in the DIP Sockets, they are 4464 at 120ns, because the CoCo 3 does almost 2MHz.. For the "classic" Radio-Shack 512KB, your looking at 41256. Some of the Tandy 512KB boards have 16, P21256-10 RAM on them. For "modern" RAM, the SRAM Boards from Cloud-9 are available, ( http://www.frontiernet.net/~mmarlette/Cloud-9/Hardware/512K.html ). And "someday", when I get my production issues resolved, I'll have 512KB and 2MB SRAM of the Boyson Tech Boomerang boards. Cloud-9 has a 2MB SRAM board, but I can't locate the Link... MarkO Quote Link to comment Share on other sites More sharing options...
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