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Thread: Modified Model 1 shows weird characters

  1. #1

    Default Modified Model 1 shows weird characters

    Hi,

    I recently rescued this Tandy TRS-80 Model 1, including Expansion Interface and Tandy Monitor with a Floppy drive built in.

    The Tandy is bought in 1979 and was doing nothing for the last 10 years. The old owner recently turned it on (without interface) and it worked. Then he connected the Expansion module and since then, it doesn't work anymore. I picked it up before he threw it away.

    I got a ton of documentation with it. Full schematics of basically everything you can imagine and manuals. All good for one banana box of documentation. The old owner told me the Tandy was heavy modified and based on schematics, I'm pretty sure it has:
    - Z80A cpu, probably running at high speed
    - Ramdisk
    - Expanded Memory
    - High Res GFX
    - RTC
    - Line fixer
    - Lower Characters mod
    - Double Density mod

    and probably a few more which I'm uncertain if it there.

    Pictures say more than words:

    This is what I brought home:


    This is what the screen shows (with and without Expansion Interface, with and without pressing the ESC key)


    Here's a picture of the mainboard (clickable):



    And here's a video from this machine 'running'.

    I really like to save this machine from the shredder. Does anyone have an idea what could be wrong before I have to remove all the mods (as far as possible, quite some chips are changed or soldered on top of the other.

  2. #2
    Join Date
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    As always your first check is that all three Power Supply voltages are within specification. (+5, +12, -5)

    Then, you might be lucky and just have a faulty Z80 CPU. If not, I hope the gent kept all the modification documentation.

    Larry

  3. #3

    Default

    I have a schematics of everything, all mods, the entire Tandy PCB and a lot of more mods that could be made. Which makes it annoying, because it's hard to tell which mod is actually done (well, the obvious ones are quite easy to see).

    The problem I'm facing is that it worked a few weeks ago and the only thing happened to it, as far as I can go back, is the Expansion Interface may have been connected wrong by the previous owner (as in, one side upside down). My guess is that's the source of the problem when it happened. So, I'm hoping that somebody can point me out what would happen if this is done and how to get the Tandy out of this state.

    My only guess is that one chip who received some voltage on a place where it didn't expect it, causing the chip to break. Is this a conclusion which is possible? If so, I'll have to find the broken chip which is a whole new challenge.

  4. #4

    Default

    Does it work without the expansion interface connected?

    Man, that MI is modded to the max!

    EDIT: ignore my question as I see you tested without the EI

  5. #5

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    TandyModel!_1.JPG

    This is a photo of this machine working without the EI connected from a few weeks ago. As far as I know, the owner managed to wrongly connect the EI interface and then is was packed in boxes and transported to my house where I've never got this screen.

  6. #6
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    Default

    With the interface connection reversed, I'd start investigating every IC that was tied to that bus. It's been a while since I read up on the Model I but I can't seem to recall if any internal bus buffering existed. My suspicion was it wasn't so you might be running into a number of blown chips.
    [Need something to waste time on? Click here to visit my YouTube channel CelGenStudios]
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    = Excellent space heater

  7. #7
    Join Date
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    Start with just the Keyboard unit. Depress the BREAK KEY and continue holding it while you turn on Power. Do you get the
    MEMORY SIZE Message? Depress the ENTER KEY twice. Type ?MEM and see what the response is.
    Your Previous Photo is too hard to try and read the response.

    If that doesn't work it's going to be time to dig out the O'Scope and start chasing CPU Control signals for one held/stuck LOW.

    Pin 6 Z80 CPU Clock Signal
    Pin 16 INT*
    Pin 17 NMI*
    Pin 24 WAIT*
    Pin 25 BUSRQ*
    Pin 26 RESET*

    Larry

  8. #8

    Default

    Quote Originally Posted by ldkraemer View Post
    Start with just the Keyboard unit. Depress the BREAK KEY and continue holding it while you turn on Power. Do you get the
    MEMORY SIZE Message?
    Nope. Tried this already and can be seen on the video too

    My guess so far is that because of the reverse connection with the EI, there's a fried chip somewhere (and that seems below one of the modifications holding the Z80 CPU). This bootscreen apparently is there because the Tandy can't boot from FDD, but it can't get past this, even with the 'BREAK' key. So, my guess would be that the chip controlling if the EI interface is connected or not is fried.

    Some Dutch Tandy owners told me this is hardly possible since there are only data lines on the connector. That's where I'm having issues with because on one of the schematics, it shows a +5V on one of the pins (the opposite being a GND, so this shouldn't matter).

    But thanks to the Dutch Tandy-scene (which I didn't knew still exists) I got the mailaddress of the guy who created the selector my Tandy is having. Perhaps he can help me out since he still goes to TRS80 Meetings (which are still held in NL apparently, ) and he probably know the rest of the mods done to this machine, which makes debugging so much easier.

    If that fails, I'm going to do this Z80 test and start debugging everything step by step.

  9. #9
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    This bootscreen apparently is there because the Tandy can't boot from FDD, but it can't get past this, even with the 'BREAK' key.
    Don't even bother floppy booting until you can at least get BASIC to work.
    [Need something to waste time on? Click here to visit my YouTube channel CelGenStudios]
    --------------------------------------------------------------------------------------------
    [No time for videos? Click here to visit my Twitter feed @CelGenStudios]

    = Excellent space heater

  10. #10

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    Quote Originally Posted by NeXT View Post
    Don't even bother floppy booting until you can at least get BASIC to work.
    +1

    troubleshooting with all of those mods is going to be a challenge

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