hhos

It is highly unlikely that all of them are bad, so don't throw them away please. If you don't want them send them to me. I don't agree. The CPU, bus management, and the Video processor frequently run hotter, sometimes requiring heat sinks. In any case there is probably only one bad chip in the lot unless it's been abused. Throwing away RAM is a practice reserved for SIMMs, DIMMs, etc.

How many were actually bad?

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

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.

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.

I think that is all pretty much normal, but it has been a while since I've opened one of those up.

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. 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

Were any updated, and complete, schematics ever sent on this? I bought a 400-800 Field Service Manual that looks a little bit more complete than what is shown here. I think it cost me about $30 with shipping from eBay. I did not get the impression that they were rare. Right now it says there are 4 left. hh

BURN HIM!!! ?

I think almost all the home computers of the '80s were pretty quiet RF-wise. The Model 1 TRS-80 was pretty bad, but it was late 70s. I think the expansion box was pretty bad, too. I can't seem to locate any of my QIs. I am sure I had either 3, or 5, of them, but since I can't find them, I can't check this out.

I might find more time for TI99 later, especially if I can discover the safe place I put my stash of TTL logic chips. Right now I have to fix some things on my truck, and repair my air compressor.

Does the QI version even have shields? It seems to me that it does not. If my memory serves, the 9918A has some sort of spring metal heat sink attached to it. I have a few of those consoles. I will have to take a look later. If someone else beats me to it, that's even better. HH

He could have been silenced by a possible ignorance of the difference between the 2 processors. As was pointed out almost immediately, different processors understand only their own language, and it would also be putting an 8 bit CPU into a 16 bit bus. I'm a CoCoNut. Really, I'm more into the MC6809 than the CoCo, though. I do admit I like the CoCo3 much more than its predecessors. Its color pallet is much better and it's twice as fast at 1.7897725 MHz. I found the TI99 an easy target myself. It looked so SLLOOWW the first time I saw one running a BASIC program! It could have been so much better if they had just decided to make it a 16 bit computer to start with, given it 4-16K RAM (the VRAM does not count!), and dispensed with GROM altogether, or just used GROM as a debugging tool. Also, the 4 extra wait states for each memory access to the PEB, not to mention 8 bit ROM and VRAM, is one huge burden on system performance. I am at a loss to find a GOOD explanation for why they put in so many wait states. I always picture the Doonesbury Dilbert cartoon characters when I think of the TI decision making process concerning the TI99/4 design. Even the GROM only needs the last two wait states, if I understood it correctly, and I don't know why it needs wait states added, anyway. It has a ready output that it can use to force as many waits as it needs, doesn't it? Kind of just like normal RAM memory setups in other computers? I don't really know what "A8" or "A2" refer to. What are these? Atari somethings? Apple? I have a C64, and a fair amount of software/hardware to go with it, but I don't know much about it. It is a pretty high performance computer from what I've seen. (Please ignore the oh so slow access to the floppy disk) What little I do know is that Commodore put in a fair bit of "helper" chips to increase its performance, a tactic that RS's CoCo would have greatly benefited from. I assume the Atari computer/consoles also spread their investments out to more support chips, but I know even less about them. As far as Atari goes I have only an Atari 7800, joysticks, and a bunch of game cartridges for it. I found all of this beside a trash dumpster in a small backpack. (I think it was originally in the overfilled dumpster but rolled off) Everything works and it also seems to be a pretty high performance machine. I have a much higher opinion of the 6502 CPU family than most of my fellow CoCoNuts do. And now that I am seeing several ways to improve my TI99/4A's performance, I think I may soon be similarly impressed by the TMS9900. And maybe even a little more so? Perhaps. HH

I agree. It would be better and easier to build a complete new system around the 6309. Trying to put it into the TI99/4A would be almost as bad as trying to shoe horn a TMS9900 into an 8 bit console.? The 6809 is fast,... very fast. If a 1 MHz 6809 running the original Defender arcade game isn't a good enough example, you should see a 1.79 MHz 6809 in a CoCo3 playing a Star Wars, episode 4 video. The hard drive containing the data it was playing was the only speed upgrade if I remember right. The 6309 is even faster than the 6809 when operated in native mode. I agree that the BASIC for the 6809/6309, in all the machines I'm familiar with, is a great deal faster, even when the CPU clock is significantly slower on the 6809 based machine. In my opinion, the difference in speed is primarily due more to the lack of CPU directly accessible RAM than anything else, and as a result of that shortcoming, the BASIC is written in GPL, which is SLOW. (Running on tracks that don't have any railroad ties?) Secondarily, I would blame excessive wait states (stop lights?), and thirdly, the register file being in RAM, right next to the read before write behavior it displays when writing byte values. TI didn't even take advantage of the 16 bit nature of the processor, or the one thing the 9900 is really GOOD at, rapidly handling 16 levels of interrupt conditions. Because the 6809 is pretty good at handling interrupts, too, I think it would probably run rings around the 9900 in most cases. The 6309, in native mode, would be even faster. It's hard to say, though, the larger register file could be a much greater advantage than I'm giving it credit for. Also, the A(dd), S(ubtract), C(ompare), MOV(e), SOC and SZC, all have a great many addressing options (they take op 3/4 of the total number of possible opcodes). That could give significant advantages to some applications by cutting down on the number of instructions needed. (Would that be bigger wheels for the train analogy above?) There really isn't any point in trying to run any BASIC benchmarks between a TI99 and any other microcomputer I can think of. Has anybody ever run any assembly language benchmarks for a 3 MHz TI99/4A vs. 0.895 MHz CoCo1/2? 1.79 MHz CoCo3? 1 MHz C64 (6502 microprocessor), or a 6502 based game console? HH