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Thread: Help with model iii repair.

  1. #1

    Default Help with model iii repair.

    First post here, thank you in advance for any possible help.
    We got a donation of a trs80 model III for our local retro club, iíve changed the rifa caps and consolidated the power supply of the main board. Powering up the computer without anything pressed it gave me a screen full of dots always same pattern. I did a fast check today before start taking it completly appart and working with the service manual and schematics and i found that the PAL chip in the upper left part of the board near the crystal overheats in seconds. Is there any GAL or similar solution in case is shorted? It has 2 wires on it going both on pin 20.

  2. #2
    Join Date
    Mar 2016
    Location
    Pikesville, Maryland
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    208

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    Quote Originally Posted by Pluto63 View Post
    First post here, thank you in advance for any possible help.
    We got a donation of a trs80 model III for our local retro club, i’ve changed the rifa caps and consolidated the power supply of the main board. Powering up the computer without anything pressed it gave me a screen full of dots always same pattern. I did a fast check today before start taking it completly appart and working with the service manual and schematics and i found that the PAL chip in the upper left part of the board near the crystal overheats in seconds. Is there any GAL or similar solution in case is shorted? It has 2 wires on it going both on pin 20.
    Can you provide photo (closeup) of chip in question - may help others to answer your question

  3. #3

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    Quote Originally Posted by Jay Newirth View Post
    Can you provide photo (closeup) of chip in question - may help others to answer your question
    A photo of the whole board and of the outside of the computer may also help. There are no PAL chips in a Model III. It sounds like this is actually a Model 4 board. Perhaps the system is an upgraded Model III that somehow didn't get the nameplate changed.
    -Alan

  4. #4

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    Here is a photo I took at the moment, of the chip in question.
    AD598CD5-BC87-457A-8964-3AAA8E83B4B3.jpg
    You are right I don’t think is a model 3 board now that i’ve checked photos of boards. Tomorrow I’ll take more.

  5. #5

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    That appears to be a Model 4 PAL board, either 8709296 REV B or 8709296 REV C (confirm by reading the part number printed on the board, near the Z-80). I'm curious what the outside of the system looks like. Does it say "Model III", or is the model name missing and you just guessed that it was a Model III? Does the keyboard have CAPS, CTRL, F1, F2, and F3 keys?

    The PAL chip that goes in socket U3 on Model 4 PAL boards is 8075166 for REV A and earlier boards, and part 8075766 for REV B and later. Those are both programmed MMI 16R6A chips.

    Most REV B and later boards do not have the '5766 part, however. They just have an earlier '5166 hacked to work with the later boards. Namely, they have a '5166 with pins 5 and 7 bent out and connected to pin 20 (VCC). In your picture, I think I can read that the part number is '5166, and I can see that pins 5 and 7 are connected to pin 20, but they aren't bent out. Have they been clipped off? If they're still running into the socket as well as being connected to VCC, then that's definitely a problem.

    If you need to replace the chip, you could ask dealers like Jay or Ian if they have any spares they could sell you.

    You could also make one. Equations for the PAL chips in the Model 4 were included (perhaps inadvertently) in U.S. patent application 06/488,743, "Video Controller", filed on April 26, 1983. Five years later, that became U.S. patent 4,742,342, "Video display generator having alternate display modes", issued May 3, 1988, but the issued patent did not include the PAL equations, and U.S. unpublished patent applications from that era were never scanned into online databases. Fortunately, Tandy filed copies of that original application in Europe, Canada, and Australia, and thus the PAL equations survived. See European patent application publication EP0126307A2, "Video Controller" (1984).

    That document contains equations for PAL chips for each of the five PAL sockets in a Model 4 PAL system. The application appears to have been submitted midway through the transition from the original two-clock board design (REV A and earlier, which required the complicated '5166 PAL in U3) and the later single-clock design (REV B and later, using the simpler '5766 PAL in U3). Some of the evidence that the new design had not been completely finalized yet is that: (1) there's no part number for the equations for the U3 PAL, unlike for the other four PALs; (2) in Figure 6B, there are no component numbers assigned yet for the new ICs, in particular for the SE564 PLL that replaced the second clock (it was eventually labeled U203); and (3) the signal names in the pin assignments on page 33 don't match the names used in the equations on page 34.

    Nevertheless, I think that if you just change FHOLD and 1.25S on page 33 to PSET and 10S, respectively, then you'll get a valid specification for an 8075766. Here's an attempt at writing such a specification in the format Tandy would have submitted to MMI (as documented in the PAL Programmable Array Logic Handbook, Third Edition, Monolithic Memories Inc., 1983, on page 4-7):


    Code:
    PAL16R6A                                         PAL DESIGN SPECIFICATION
    8075766                             UNKNOWN TANDY ENGINEER CA. APRIL 1983
    TIMING PAL FOR U3 IN TRS-80 MODEL 4 PAL BOARDS REV B AND LATER.
    TANDY  FORT WORTH, TEXAS
    
    
    /20MC /20I NC NC NC NC NC NC FAST GND
    GND 20M 2.5S 5S 10S 10M RS232CLK PSET PCLK VCC
    
    
    IF (VCC) PCLK = 20M*/10S*5S*/2.5S*PSET+10M*/10S*/5S*/2.5S*PSET+PSET*PCLK
    
    RS232CLK := 10M*/RS232CLK+/10M*RS232CLK
    
    10M  := /10M
    
    10S  := /10S*/2.5S
    
    5S   := 10S*/5S+/10S*5S
    
    2.5S := 10S*5S
    
    PSET := 10S*/5S*/2.5S*FAST+/10M*2.5S*/FAST+/2.5S*PSET
    
    IF (VCC) 20M = 20I
    
    
    DESCRIPTION
    
    This is an attempt by Alan Petrofsky in 2019 to recreate the file Tandy
    submitted to MMI in 1983 to make part 8075766, the timing chip used in
    socket U3 in the TRS-80 Model 4 single-clock PAL design, revision B and
    later of main logic board 8709296.
    
    This is based on pages 33-34 of European patent application publication
    EP0126307A2 "Video Controller" (1984), with the names in the pin
    assignments on page 33 modified to match the names used in the equations
    on page 34.
    
    As of August 10, 2019, this is completely untested.
    
    The 20MC and 20I inputs are always the same 20 MHz clock.  The job of
    the chip is simply to produce three output frequencies:
    
      10M: the master clock divided by two (10 MHz)
      RS232CLK: the master clock divided by four (5 Mhz)
      PCLK: the master clock divided by five or ten (4 MHz or 2 MHz),
            depending on the value of FAST.
    
    Generating 10M and RS232CLK is trivial.  Generating PCLK is trickier,
    especially because the divide-by-five generates a proper 50% duty-cycle
    clock, unlike the simple divide-by-five in the Model III, which produced
    a 40%/60% duty clock.  Still, this is less complicated than the
    predecessor 8075166 PAL, which also needed to deal with the M1 and MREQ
    inputs and put hiccups in the PCLK output.
    
    Most outputs depend only on the master clock (which is duplicated on
    pins 1 and 2), and repeat the following patterns every 20 cycles of the
    master clock:
    
    20M      0101010101010101010101010101010101010101
    RS232CLK 0000111100001111000011110000111100001111
    10M      0011001100110011001100110011001100110011
    10S      0011001100001100110000110011000011001100
    5S       0000111100000011110000001111000000111100
    2.5S     0000000011000000001100000000110000000011
    
    The other two outputs depend on the FAST input.  When FAST is true they
    follow these patterns:
    
    PSET     0000111111000011111100001111110000111111  (uncorrected 60%)
    PCLK     0000011111000001111100000111110000011111  (proper 50% duty)
    
    and when FAST is false:
    
    PSET     0000000000111111111100000000001111111111
    PCLK     0000000000111111111100000000001111111111
    Last edited by Petrofsky; August 9th, 2019 at 10:46 PM.
    -Alan

  6. #6

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    Thanks Alan, really priceless info. I don’t have a pal programmer and since a gal conversion is not working will contact Ian.
    Today I had more time to check what was given to us, my bad it was a model 4. And the pins on the pal where in the socket, I bent them to try but nothing changed and the ic overheated immediately, clearly shorted.
    Some photos of what we have and what’s on screen with or without break key pressed.
    Obviously any suggestions welcome.

    https://photos.app.goo.gl/dmDRXr3gGN53u8F9A
    https://photos.app.goo.gl/NCDi1SSXDMFmTkun6
    https://photos.app.goo.gl/EM1GSVVyU6J3CfD76
    https://photos.app.goo.gl/q5LpQhMxMSCg5wjT7

    And another faulty one....
    https://photos.app.goo.gl/XcFgXMVR1Rq4V1xB6

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