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Thread: Model 1 Video Damage

  1. #1
    Join Date
    Mar 2017
    Location
    Pickering Ontario Canada
    Posts
    80

    Default Model 1 Video Damage

    Hello,

    I have 3 model 1's and i damaged the video on one when pluging in the power

    without making sure it was in the correct connector.

    I found a schematic and it says q1 & Q3 are both are 2n3904.

    I believe q1 is an 3904 and q2 a 3906.

    Is there any other parts susepteble to damage this way.

    This board also won't boot. I get a screen of repeating characters "BS" sometimes but mostly garbage.

    I need some help here also.

    Thanks
    Woody (Doug)

  2. #2

    Default

    Woody,
    The initial Schematic in the 1978 Manual had that error. The later version (1982) had the correct parts.
    Q1 = MPS3904
    Q2 = MPS3906

    SAM'S Photofacts of 1985 have the same parts as the RS 1982 version.

    I've read that somewhere and captured it but didn't mark up my PDF's. I'm still trying to locate that reference for the correction.

    Ian, would be the best person to answer that question. I believe he has responded to that exact problem before. You might be
    able to search for that posting.

    Larry

  3. #3

    Default

    Is there any other parts susepteble to damage this way.

    This board also won't boot. I get a screen of repeating characters "BS" sometimes but mostly garbage.
    You might have a lifted trace under the cassette relay from the variable resistor, and also the three CMOS chips could be zapped. Possibly Z41 (8 pin chip). The chips are Z5/6 and I think 56, they are C00/C04s and suitable replacements are available that work perfectly fine.

    Now, if you get the characters and they are stable and visible everything I just talked about is more than likely fine. You could have bad video memory, bad CPU, bad ROMs in some combinations of another.

  4. #4

    Default

    The first thing I'd do is pull the Z3 Shunt (Jumper):

    The address lines are outputs from the Z-80 used to specify memory locations. If any address line is not active (stuck on one state), there will be 'garbage' (random characters) on
    the screen at boot. To check the address lines, remove the DIP Shunt at position Z3 and power-up the system. If the screen fills with a pattern of “@9@9@9@9...”, then all the
    address lines A0 – A9 are good. The remaining address lines A10 – A15 can be checked for activity with a scope.


    If the screen shows “@9@9” in some areas and garbage characters in others, in a repeating pattern, the defective address line can be found easily. For example, if the pattern is
    alternately four characters of @9 and four of garbage, then address line A2 is stuck because 2^2=4. If the pattern is a full line of @9 (64 characters) followed by a full line of
    garbage, then address line A6 is stuck because 2^6=64.

    That is a good first test.

    Larry

  5. #5
    Join Date
    Mar 2017
    Location
    Pickering Ontario Canada
    Posts
    80

    Default

    Larry,
    All Address lines are active. All data lines are active. (z3 removed)

    I am getting a repeating pattern K@b AND KBK at the same locations across the whole screen (Z3) Removed

    I also removed all Dram and no change to pattern (Z3) still removed.

    Woody (Doug)
    Attached Images Attached Images

  6. #6

    Default

    Woody,
    The first two Characters are:
    @ = 0x40 = 0100 0000 = CORRECT
    B = 0x42 = 0100 0010 = Not Correct needs to be 9 = 0x39 = 0011 1001
    0011 1001

    So, almost all of the Video RAM bits are incorrect.

    Larry

  7. #7
    Join Date
    Mar 2012
    Location
    Augusta, Georgia, USA
    Posts
    473

    Default

    I did the same thing to mine and many things were bad... A couple of the logic chips in the video section (they provide the H and V sync) and a couple of RAMs. Also, ultimately I had to replace the keyboard cable also...

  8. #8
    Join Date
    Sep 2016
    Location
    Melbourne, Australia
    Posts
    616

    Default

    Sounds like the perfect time to buy one of my model 1 bus exercisers. At least you wont be working blind trying to locate what is wrong.

  9. #9

    Default

    Woody,
    Why don't you make an Address Line tester first. Take a known good Z80 CPU and slightly bend the D{0..7} pins out
    so they don't go into the Motherboard Z80 Socket. Using some Wire Wrap wire jumper each D{0..7} pin to each other,
    and then terminate that wire on the Z80 GND - Pin 29. Then plug in the Z80, making sure NONE of the D{0..7} pins
    go into the Motherboard Z80 Socket.

    Then with an O'Scope look at each address line A{0..15}. Each should have a good Signal that isn't stuck LOW, HIGH,
    or at .7 VDC. And each should be 1/2 of the previous Pins Frequency as you go from A0 to A15.

    This will allow you to check each and every IC that has INPUT Pins that are tied to the Address Lines. The ONLY thing
    remaining that is going to be tested is the Data Lines and the Data out from the various IC's. That is where David's
    Arduino Mega 2560 Model 1 Bus Exerciser will come in handy. You can set memory, or sections of memory to any value,
    Test Memory or Video, and Ports.

    Arduino Mega 2560:
    The exerciser is controlled by a simple command set via the Arduino USB port. This can be used via the Arduino IDE console or any terminal emulator.
    The wiring is quite simple, just connecting Arduino pins to the Z80 connector. The shield, flatcable and 40-pin socket connector are completely passive,
    only 1-to-1 connections.

    Commands:

    -- Z80 exerciser 0.4 command set --
    Aaaaa - set address bus to value aaaa
    Bpp or B#ss - blink pin p (in hex) or symbol: A0-AF,D0-D7,RD,WR.MQ,IQ,M1,RF,HT,BK
    D[ssss[-eeee]|+] - Dump memory from ssss to eeee (default 256 bytes)
    H - This help text
    Issss-eeee - Generate hex intel data records
    MRaaaa[+] - Read memory address aaaa, optionally repeating
    MWaaaa vv[+] - Write vv to address aaaa, optionally repeating
    PRaa[+] - Read port address [aa]aa, optionally repeating
    PWaa:vv[+] - Write vv to address [aa]aa, optionally repeating
    R[+|-] - Refresh on/off
    Qn - Repeat rate; 1, 2, 4, 8, 16, ..., 32678 ms (n=0-9,A-F)
    Sssss-eeee:vv - fill a memory range with a value
    TD - Exercise TRS-80 display memory
    TK - Reads TRS-80 keyboard matrix
    Ussss-eeee - test RAM range (walking 1s)
    V - view data bus, pins INT, NMI, WAIT, BUSRQ, RESET
    Wpp v or W#ss v - Write pin (in hex) or symbol: A0-AF,D0-D7,RD,WR.MQ,IQ,M1,RF,HT,BK; values 0, 1
    Xp - exercise port p
    ? - This help text

    Some commands have a repeat suffix; allowing the operation to be repeated. The repeat rate is determined by the Q command, and
    exponential: 1, 2, 4, 8, 16, ..., ms. Of course some commands like 'D' are so slow fast repeat times do not make much sense.

    Larry

  10. #10
    Join Date
    Mar 2017
    Location
    Pickering Ontario Canada
    Posts
    80

    Default

    Larry, David , Rittwage,

    Thats alot to think about.

    1. Do i need the keyboard always connected when checking. It gets in the way.

    I replaced the keyboard connector with a 20 pin sip soldered into the MB and KB to use a IDC 40 pin (20 per side) cable.

    Did this on 2 M1's (one tested the other on the broken M1). I had to repair a few traces .


    2 With all the video Ram bits incorrect does that need replacement or is this the result of another problem ?


    3 David, how do i get one of your model 1 bus exercisers ? $ ?

    4 First I'll do the address line test.

    Woody (Doug)

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