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Thread: Model 4GA , no boot and now no Video ?

  1. #1
    Join Date
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    Default Model 4GA , no boot and now no Video ?

    Hi ,

    I was checking to see which disks would boot on my M4GA

    I was pressing the reset each time and after 10- or more disks none would boot. ( even the ones which previously would boot )

    You could hear the heads repeatedly hitting the end of travel at the track 40 end each time i pressed reset.

    I tried replacing all the chips in sockets around the FDC (inc the FDC ) with ones known to work but no change.

    I replaced a few chips U78 & U97 ( no change )

    Then i replaced u95 , u94 & u93. The heads stopped hitting the end but didn't load the O/S

    and then i found out i had no video. i only touched right around the FDC chip , no solder bridges , the right chips replaced

    and now i have 2 problems.

    I hope someone reads this that can help me find out what happened and where to look for the problem.

    Thanks,
    Doug (Woody)

  2. #2

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    When you say it won't boot and then no video, does it boot (drives crunching like normal)?

    This could be timing, or a blown CRT controller (near the Z80, same size, interchangeable on the other model 4 you have).

    I'd first check the timings with it on, just to be sure you didnt mess up anything around that crystal/oscillator.

    Pin 6 of the Z80 should have 2 or 4Mhz on it, honestly dont know on a 4 when it powers on which speed it runs. I just check all the frequencies that come from the GA chip and compare to the manual.

  3. #3
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    Hi Tibs,
    When i turn it on drive 0 red light comes on the motor spools up but no movement of the heads as normal.

    Ill check the other things and let you know.


    Doug

  4. #4

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    Doug,
    Depress the BREAK Key, and hold it depressed when you power up the Model 4. Then when the display is seen, release the Break key
    and press the Enter Key twice. Then type in
    ?mem

    and see what the response is. I bet the Drive's Head guide rails are sticky and need to be lubricated. I've got a Model 4 that
    does this, and I've been able to move the Head Guide rail assembly a few inches, and the system will then power up and
    boot from floppy. You just need to remove both drives, remove the logic boards enough to swing them out of the way, and clean the
    drive guide rods with Alcohol, then lube them with some Dri-Slide (Motorcycle Cable lubricant). It just takes several drops.
    That will fix them for years.

    It's worth a try.

    If you have shuttled 10+ floppy's in the boot drive, also have a look at the read head(s) to make sure there is no Brown Oxide
    remaining on the Head. If there is Brown Oxide, use a cotton Swab dipped in Alcohol to gently clean the head assembly.


    One other suggestion is to not mass change out parts when troubleshooting. We as Technicians, need to be logical troubleshooters,
    and not mass parts changers (swappers). I've seen Technicians take a running production line down to swap boards as a test, and end up
    having two production lines down. Try explaining that to your boss.

    Larry
    Last edited by ldkraemer; March 15th, 2018 at 08:37 PM. Reason: updated

  5. #5
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    Default

    Hi Larry,
    Held down the break key however without video i'm not sure how to verify what the output to the screen is.

    I am using freshly cleaned drives, lubed and i calibrated them and checked them on the working M4GA.

    I swapped the crt inc video board and it's working on the other working machine.

    I also swapped the power supply and it works on the good M4.

    Checked Vram on good M4 , works fine.

    I think i'll try working backwards from U20/21 and compare against the working machine.


    Doug

  6. #6
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    Default

    New info,
    i have a FreHd Boot rom installed and noticed the computer does not load the data from ther FreHd

    as indicated by the green led not illuminating. Perhaps the rom is bad.

    Edit: FreHd autoboot rom was bad. pulled and it did not verify with original programmed code.

    Replaced original rom and i have video but a garbage screen.

    Doug (Woody)
    Last edited by woody; March 16th, 2018 at 11:53 AM.

  7. #7

    Default

    Ok, thats an intersting piece of information.

    Did you either plug rom in upside down, or short a pin the wrong way or was it just bad code in the ROM?

    Garbage is better, because you have video and you have a clock.

  8. #8
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    The Rom code on the eprom was bad. I loaded the original code into the Programming software

    and tried to verify that the code on the eprom was the same as in the programmers buffer.

    Half way through the compare it failed.

    The power supply should be good as i installed it in my good M4GA and it ran fine.

    I don't think its the ram either as i installed different ram and no change.

    To be sure i will install the ram from my good M4GA to verify.

    Possibly the Cpu , im going to check that cpu inputs /WAIT, /INT, /NMI, /RESET) are not stuck low

    or no activity.

    Edit : Power p11 4.8v
    Wait p24 high - steady state
    Int p16 stuck low
    Nmi P17 high - steady state
    Reset p26 High pressing reset has no effect

    Stuck low i know is a problem
    Steady high ? don't know
    Reset ?


    Doug
    Last edited by woody; March 16th, 2018 at 09:16 PM.

  9. #9

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    Doug,
    WOW! I grabbed the incorrect prints and it took me a while to figure out that I was using the 4P Schematic.
    So I've updated this posting accordingly.

    INT* is on Page 17. Page 17 has INT* sourced from U49 (SN7416) Pin 2. So, you will need to verify Pins 1 & Pin 8 of U48 and go further back
    to Pins 9 & 10 of U48 (74LS00).

    Don't forget to check the 4.7K PULLUP Resistor (R2) on Pin 16 of U1 the Z80 CPU. I've seen cold solder joints on some of
    the various types of PULLUP resistors.

    If the computer does a RESET when you depress the RESET Button it's working. If not it's time to drag out the O'Scope
    and do some signal chasing as to where it's not going low. (If you set your O'scope for a low going trigger signal, you might
    see the low pulse easier.

    Is the defective ROM the U4 (B/C ROM) or U29 (ROM)?


    Larry
    Last edited by ldkraemer; March 17th, 2018 at 12:39 PM. Reason: Updated to correct print (GA)

  10. #10
    Join Date
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    Location
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    Default

    Hi Larry,

    I have the 4GA and cant locate a u25. what manual are you refering to ( Pages 5 and 7 ) ?

    If i can find that manual/article i should be able to get something done.

    The defective rom is at u4 (4k). I had a 2764 in that loc for the FreHd autoboot rom.

    I replaced it with the original rom for troubleshooting.

    Doug

    Edit: All signals low back to u37.

    At U37 P3 there is a clock signal but only 1.5V.

    U37 P3 High (5V) pin 11 Low

    U37 P4&10 High(5V)

    U49 p8 low, pin9(in) high 5V , I think the dot on the output P8 means inverted.
    Last edited by woody; March 18th, 2018 at 12:33 PM.

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