• Strange Model 4 behavior.

    From Robert Bond@21:1/5 to All on Fri Apr 21 22:25:12 2017
    Hey, guys & ladies,

    I really hope someone can help me with a very strange problem I'm having with my TRS-80 Model 4. I bought it off of ebay. It's the gate array model, has 2 single-sided drives, 128K RAM & a Tandy Hi-Res graphics board. I also bought a FreHD which is a
    hard drive emulator. Here's what's going on with it. It won't reliably boot into DOS. Sometimes it will boot, but a lot of the time it returns a Disk Error. Also, when it does boot, it doesn't always install (is that the right term?), the FreHD. Here's
    what I've done to try to figure out what's causing it to operate the way it does.

    1. I swapped my Drive 0 with a known-good different one of the same model, taken from a different gate array Mod. 4 (64K only) that the CRT went out on. That didn't seem to make any difference, but it's only been tonight since I did the swap.

    2. I swapped the Floppy Disk Controller IC (U75) with the one from the previously mentioned Mod. 4 with the dead CRT. That also didn't seem to make any difference, but again it's only been tonight since I did the swap.

    3. I swapped out the entire motherboard with the motherboard from the previously mentioned Mod.4 with the dead CRT. (Actually I just put that motherboard behind the motherboard in the 128K Mod. 4 and then disconnected all the connectors from the 128K mb
    and connected them to the 64K mb. That also didn't seem to make any difference, but again it's only been tonight since I did the swap, so I hooked the 128K mb. back up.

    Two other things I should mention about the 128K Mod. 4 are:

    1. The image on the CRT doesn't seem to be rock-solid steady like the image on an LCD is, that is, sometimes it seems to shrink in a little on the sides, and it's flickery.

    2. Tonight when I was desperately trying to get the computer to boot after spending most of today trying to figure why it won't reliably boot - I even bought some supposedly good boot disks off of ebay, but I have no way of knowing whether they're good
    or not, I put a Model 3 TRS-DOS diskette in the drive and tried to boot with it. It booted to the point where it asks you to enter the date & the time, but after I entered the time, it wouldn't proceed any further, and just kept rebooting and
    continuously never proceeding past where you enter the time.

    I was finally able to get LS-DOS to boot, but it didn't install the FreHD, bummer :(, so I don't know what is wrong with the machine, and I don't know anyone else to ask. I do have the Sams Model 4 Gate Array Service manual, but I don't really know that
    it's going to be of much help to me, as the only test equipment I have is a digital multi-meter and a logic probe, and I don't have experience troubleshooting computer hardware, so any help at all would be greatly appreciated. Right now, the computer is
    powered on with LS-DOS 6.3.1 booted, but the top is off, the CRT is lying on it's side beside the base, the shield is off, and I'm hoping that the computer isn't going to catch fire and burn my apt. down while I'm asleep. Actually, I'm not really sure if
    that would be such a bad thing. I apologize for the length of this post, and I would be truly grateful if someone could help me figure this machine out.

    Thanks so much,

    Robert Bond

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  • From lafos33@gmail.com@21:1/5 to All on Sat Apr 22 11:10:16 2017
    Did you check the power supply voltages and contacts? The flickering display suggests that the contacts might need to be cleaned.

    If you boot into Model III ROM BASIC mode by holding the break key when hitting reset, do you get a stable display? If so, can you enter a simple program to see if it works? Something like

    for j=1 to 1023: print j;: next

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