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Model 4 disk drive issue

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    Model 4 disk drive issue

    I went to an estate sale yesterday and picked up a cheap but dirty model 4 26-1067, 64k with two floppy drives. It had been stored in the original owner's garage with the official plastic dust covers on it (and the printer and cassette). I didn't need it, but they were basically giving it away. I already have a mint Model 4 and a Model4P. From the photo i was hoping it was a model 3 (was covered in dust cover). but i got it.

    It popped and smoked a little when i fired it up from the back right. two safety caps were cracked/blown, so i clipped them. From there i seemed to be getting stable voltages with no other work...yet.

    It powers up fine, display (grey) looks fine. holding break gets me to the cass? and memory? prompts and it boots to whatever basic that is. It came with a box of disks including a Model 4 TRS-DOS disk, but when i boot with it in the bottom drive it would not get past a diskette? prompt.

    I pulled it apart, cleaned and lubricated both drives fairly well. Also cleaned all the drive connectors. Still only get the diskette? message.

    I also have a set of Ira's TRS-80 disks including 3 or 4 flavors of model 4 dos disks. same issue, diskette? display.

    Any advice on what to try or check next?


    With Power OFF, DEPRESS BREAK KEY (and HOLD DEPRESSED), turn ON power. When you see "CASS" on Display, RELEASE
    BREAK KEY, then press "ENTER KEY" twice, then type in this:


    and press the "ENTER KEY". It should show the available memory as 48082 or some such number of ram available.
    From there insert Diskette, than depress the RESET Button on the Keyboard. Both Drives should start Spinning and the
    Bottom Drive RED LED should come on and you should hear Drive 0 step to track zero (assuming it's not already on
    Track zero). If it doesn't step, remove Diskette, power off, and reposition the Drive 0 head carriage assembly (slowly
    and carefully) by hand to mid position of the travel. Power on and repeat the above. as you listen for the Drive 0 stepping
    to Track 0.



      thanks (again) Larry. you've helped me greatly over the past couple of years.

      this thing is a beast to work on while open, but with it pulled apart i was able to see that drive 0's motor was not spinning. drive 0 LED lights, drive 1 spins with no LED.

      i pulled drive 0 motor and applied voltage directly, and it will spin with a little help. i'm assuming it is dirty/dusty. i'm currently loading it in isopropyl (i've done this with small motors from flip clocks successfully in the past).

      thanks again, i'll report back soon.


        drive motor now spins freely. steppers were clunky. freed them up somewhat. upon reset with head halfway, it does move to the outside track. but still won't load. i tried both drives on the end cable position, neither will read/load dos. they eventually just shut off. there is not further head movement once it moves to the outside.


          i pulled out my other model 4D (machine #1) which has a fully working drive 0. drive 1 tries to work but gets read errors.

          i put my working drive 0 in the new 4D (machine #2 with non-working drives) and it does not work. just spins for a few seconds and stops with no message and no head movement.

          I tried both drives from machine #2 in machine #1. one of them dives a drive error messages which is further than it got in machine #2.

          so the fact that a known working drive does not work in machine #2 mean there's another issue? what else could cause this behavior?


            It could be the Flex cable from the Motherboard to the FDC, or a problem with the FDC, or bad RAM. You could even have a bad ROM.

            If you want to unplug and re-[lug the flex cable, use a wooden Pencil in the half loop and hold the Flex Cable tight to the
            Pencil to Unplug and Re-Insert.



              Is your machine a GA M4 or older revision?


                it is a 26-1067 which believe was originally a cassette only 16k model. arrow keys are split/non-clustered. I believe that would be non-gate array. i notice the memory label on the front between drives is not perfectly aligned so it could have been upgraded with 64k and the badge replaced and drives installed. variety of RAM chip types installed as well.

                I did pull the board out to clean socketed chips. i'll test the ram chips today.


                  WILL NOT READ
                  Verify the Index Detector circuit is working, refer to the
                  "Index Detector" section of this Troubleshooting guide.

                  INDEX DETECTOR
                  To check the Index Detector type in and run the following
                  Basic program. Connect a Logic Probe to pin 9 of IC U3 on
                  the Disk Drive board.

                  10 OUT 244,2: PRINT INP(240): GOTO 10

                  This program will keep Disk Drive 1 running and continuously
                  display a number on the Monitor screen. The number should
                  be 6 and the Logic probe should read Logic Open with
                  no diskette in the Disk Drive. The number should change to
                  4 and the Logic probe should read Low when a 3 x 5 card is
                  inserted in the Index Detector. If the Logic Probe readings
                  are correct but the number that appears on the Monitor
                  screen is not correct check IC U3. If the Logic probe
                  readings are not correct, insert a diskette into the Disk
                  Drive and close the door.

                  Check for pulses on pin 9 of IC U3. If pulses are missing,
                  check the Index Detector assembly, Connector PlO and
                  Resistor R36.

                  Check the alignment of the Disk Drive, refer to the "Disk
                  Drive Alignment." To check the operation of the read circuits,
                  type in and run the following Basic program. Insert a
                  diskette that has a 125kHz (2F) pattern on track 00 into the
                  Disk Drive and close the door. NOTE: The Dysan 22412A
                  Analog Alignment Diskette has a 125kHz pattern on track 00.
                  The program will turn On Drive 1, set the head to track
                  00, continuously read the track and display the number that
                  is read, which should be 255. To turn On Drive O, change
                  OUT 244,2 in Lines 10 and 30 to OUT 244,1. Press the Break
                  key to stop the program.

                  10 OUT 244,2: OUT 240,3
                  20 F0R T=1 T0 600:NEXT T
                  30 OUT 244,2: OUT 240,228: PRINT INP(243): GOTO 30

                  If the number displayed on the Monitor is not 255, check the
                  waveform on pin 19 of Floppy Disk Controller IC (U75) on the
                  System board, see Figure 7. If the waveform is correct check
                  the Logic readings on pins 1, 2, 5 thru 12 and 19 of IC U75.

                  If the waveform is missing on pin 19 of IC U75, check the
                  waveforms on pin 2 and 4 of Multiplexer IC (U95), see Figure 8.

                  If the waveform is present at IC U95 check IC U98. If the
                  waveform is missing on pin 4 and is present at pin 2 of IC
                  U95 check for pulses on pins 9 and 12 of IC U95. If the
                  pulses are present at pins 9 and 12 of IC U95, check IC U95.
                  If waveform is missing on pin 2 of IC U95, check waveforms
                  on pins 16 and 21 of Interface IC (U3) on Disk Drive board,
                  see Figure 9. If waveform is present on pin 21 of IC U3
                  check the Logic readings on pins 2 thru 5, 12, 13, 15, and 16
                  of IC U3.

                  If Pin 7 of IC U2 does not check a logic High, refer to the
                  "Write Protect Will Not Work" Section of this troubleshooting

                  If pin 1 of IC U3 checks Logic Low, check voltages and com-
                  ponents associated with Read Amp IC (U1). Check the
                  Read/Write Head for continuity at pins 2 and 4 of Connector
                  P6 and check Connector P6 for good connections. Check
                  the waveform on pin 21 of IC U3 while opening and closing
                  the Drive Door. The waveform should change from random
                  pulses when the door is open to 125kHz pulses (see Figure
                  9) when the Drive Door is closed. NOTE: The random pulses
                  are present on pin 21 of IC U3 even when the Drive is not
                  running. If the waveform does not change from random
                  pulses when the Drive Door is open to 125kHz pulses when
                  the Drive Door is closed check the components associated
                  with IC U2. Check for pulses on pin 9 of IC U3. Check
                  Connectors P1O and P21.



                    thanks Larry.

                    regarding this: This program will keep Disk Drive 1 running and continuously display a number on the Monitor screen. does this literally mean drive 1 (top/first)? I only have drive 0/last connected with this huge mess on my workbench.

                    preliminarily, if that basic program is looking at index detector on drive 0, it is not functioning as it should. i get a 0, then i saw a 3. pin 9 on IC U3 has no activity with a logic probe.

                    edit: moved the drive to the drive 1 cable position and the index detector does seems to work. 6 when empty, 4 when card inserted. on to the next step...

                    so my first drive showing a 6 when empty, a 4 when disk in. but pin 9 on u3 has no activity. R36 looks to be correct.

                    Last edited by Dokken; May 31, 2021, 08:55 AM.


                      Well, I should have spelled it out for selecting:
                      Drive 0 10 OUT 244,1: PRINT INP(240): GOTO 10
                      Drive 1 10 OUT 244,2: PRINT INP(240): GOTO 10

                      I figured you would have known to change the drive number in out command.

                      I'll have to look at schematic for your other information.



                        Other things you can look at ate the Control Inputs to the Z80 CPU.
                        Is Pin 16 - INT being held LOW?
                        Is Pin 17 - NMI being held LOW?
                        is Pin 24 - WAIT being held LOW?
                        Is Pin 25 - BUSRQ being held LOW?
                        Is Pin 26 - RESET being held LOW?