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Thread: Model 4 Font

  1. #11

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    No luck using the other u23 chips. All have the same 1981 markings

  2. #12
    Join Date
    Sep 2011
    Location
    Melbourne, Australia
    Posts
    997

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    That's a non-gate array board so make sure when people are advising you chip numbers and schems you are dealing with the right board.

    U23 is the character generator so try a different one. U21 is your video SRam and can be replaced with any 6116 they are all the same.

    Also on your non-gate mobo the 68045 (U47) is your CRT controller, its responsible for positioning the characters on the display and does funky things when it's not right.

    Check each of these chips and especially their sockets and if you still have no luck then its time to hit the schems and a scope.

    Cheers,

    Ian.
    *There is never a charge for Tech Support even if you don't purchase from me - We are Enthusiasts Helping Other Enthusiast and that is just the way it is

  3. #13

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    Apologies for the duplicate messages. Posts are being moderated. Thanks for all the helpful information. I've now replaced U23, U21, and U47, with two other motherboards and no luck

  4. #14

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    Ah, so you do not have a gate-array motherboard (and the 14-pin DIP in U17 is unrelated to the problem).

    From your photos, it appears all your major components are working fine, but columns 0 and 7 of each character cell are getting wire-ANDed together. Those are the inputs on pins 2 and 14 of U11 (the 74LS166 shift register). With the power off, try checking those two pins with an ohmmeter. If there's no resistance, that may be your problem.

    Looking at the traces in the technical manual, I'm guessing the problem is right underneath U11, where the trace going into pin 14 comes close to a trace running up the center, which is connected on the other side to pin 2. It's the yellow spot in the pic below. The horizontal DIP-24 in this pic is U23, the vertical DIP-16 with the yellow spot is U11.

    If the traces are being bridged, then separating them may fix the problem. Alternatively, the problem may be internal to U11 and it needs to be replaced.

    vcfed pic.jpg
    -Alan

  5. #15

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    To test my theory that columns 0 and 7 are being ANDed, I'll predict that even through column 7 doesn't show up in m and w characters, it shows up fine in the happy face character, in which the 3 pixels in column 7 also appear in column 0. Try booting to Model III BASIC and entering "POKE 15487,196" to put a happy face in the upper right corner.
    -Alan

  6. #16

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    @Alan The happy face character is truncated (see below) but I've noticed that some of the other graphics characters are okay so maybe it's column 7 and another column. I'll look under U11 when I take it apart again soon.

    happyface.jpg

  7. #17

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    Hey Larry, when I tried to respond to your private message I got "ldkraemer has exceeded their stored private messages quota and cannot accept further messages until they clear some space."

    Quote Originally Posted by ldkraemer
    Where can the Model 4 Non Gate array Manual be located that has a good set of schematics attached?

    The manuals I have found and downloaded just have cut up sections with no full schematic.
    There were two Model 4 Technical Reference Manuals. The second one, the "TRS-80 Model 4/4P Technical Reference Manual", Cat. No. 26-2119, is the one you're thinking of. It covers 4 and 4P, gate-array and PAL (non-gate-array), but with just the cut up schematics and no circuit traces. On the plus side, it includes lots of data sheets for components, and includes some details about the five custom gate-array chips.

    The earlier manual, the "TRS-80 Model 4 Technical Reference Manual", Cat. No. 26-2110, covers only the original (two-crystal) PAL board, but it includes full schematics and traces.

    You can find both at archive.org. Here's a link straight to the PAL board component-side circuit trace.
    Last edited by Petrofsky; May 15th, 2019 at 06:50 PM.
    -Alan

  8. #18
    Join Date
    Mar 2013
    Location
    Chaffee, MO
    Posts
    1,102

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    Alan,
    Thanks, I was busy trying to send you another PM when I discovered the full inbox.

    Larry

  9. #19

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    Quote Originally Posted by Petrofsky View Post
    The earlier manual, the "TRS-80 Model 4 Technical Reference Manual", Cat. No. 26-2110, covers only the original (two-crystal) PAL board, but it includes full schematics and traces.
    I should mention that taos23 doesn't have the original two-crystal PAL board (labeled "8709296 REV A"). He has the later single-clock PAL board (REV B or C). Most of the changes are in the upper left quadrant. I think the tracing in the area we're looking at (around U11 in the upper right) is about the same. From what I can see in your photo, it looks the same.
    -Alan

  10. #20
    Join Date
    Mar 2013
    Location
    Chaffee, MO
    Posts
    1,102

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    taos23,
    I've written up a PDF document that is on my Dropbox account with the following link: (Anyone with link can access for some duration.....)
    https://www.dropbox.com/sh/8zcx0rhgz...zbzs5FeWa?dl=0

    Download everything at that link. 4 items.

    Print the PDF after viewing, and have your yellow marker ready to mark the places I have in RED on the PDF. Those are the KEY PLACES.

    I've tested the basic program on my Model 4 so I know 100% it works.

    It shouldn't take long to locate the problem socket, trace, IC, or bad solder joint.

    I'm restoring a model 3 as well and it seems to have a power supply issue.
    If it's a Tandy Power Supply (I'd have to go look at the exact model number) be sure to turn the Power Supply over and VERIFY
    that the PINS that are soldered to the Supply have a good solder flow and no circles around the pins or weak looking solder joints.
    That was a problem with the Tandy Power Supplies.

    I'm always reluctant to pull the motherboard because of those thin flex cables--I ruined one!
    Yes, those are a PAIN, until you know the secret. Get yourself an old wooden pencil that is about 5" long.
    Lay the wooden pencil in the bend of the flex cable and hold the flex cable tight to the wooden pencil
    as you remove the flex cable. Insert the cable the same way, by wrapping the cable around the wooden
    pencil and insert the cable into the connector. It's easy to do now!.


    TROUBLESHOOTING M4:
    If you just use an OHM meter to OHM the Trace (and IC socket) for the Char Gen IC to the Shift Register IC
    I'm 99% sure you will locate your problem. It could be a cold solder joint, bad IC Socket for Char Gen IC,
    cracked or broken trace, or bad Shift Register IC or bad IC socket for Shift Register.

    If you have an O'scope or Logic Probe you should be able to trace the signal by running the Basic Program.

    It shouldn't take more than 20 minutes to isolate the problem.


    Thanks for the update.


    Larry
    Last edited by ldkraemer; May 17th, 2019 at 09:55 AM. Reason: typo

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