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Thread: Tektronix 4907 Floppy Drive

  1. #1

    Default Tektronix 4907 Floppy Drive

    I've made progress on both my 4907 Floppy drive systems- memory errors during boot have been fixed - 2 bad 4027 DRAMs on one and I replaced five 4027 on the other - more difficult on the second one because they were soldered in, so I had to remove the ceramic DRAMs and install sockets.

    Both systems now run the 4907 operators manual example BASIC program for the Tektronix 405x computers to set the time - but both fail to access the floppy disk with Error 16.

    The 4907 service manual downloaded from bitsavers indicates the following items for this error:


    I have connected my HP 1630A logic analyzer probes to the pins in the ribbon cable between the mainboard and the ROM board - and when I trigger on a read of the hex address 4014 RP0 status register - I see the drive reporting not ready.

    Ok, download the Shugart 800/801 Floppy Drive service manual. That manual indicates that the drive is looking for sector pulses from the sensor above the disk and the door closed sensor before reporting drive ready.

    Ah-hah - my 8" NOS disks are double-density double-sided - and the disk hole is offset from the sensor on the 800 floppy.

    More google searches turned up the reason for the disk not ready:

    I don't have the right floppy - I need a single sided single density hard sectored floppy: SSSD, the DSDD floppies I bought won't work.
    These SSSD floppies have 32 sector holes and an extra hole to indicate first sector - see 3M floppy disk manual on bitsavers:

    The 4907 Service Manual shows the mainboard logic has a sector counter - and requires the 32 sector holes.

    I've ordered a box of 8" SSSD Nashua floppy disks - EBAY seller indicated they have lots of holes - and the location of the sensor hole matches the one on the 4907 drive (7 degrees from the center line through the slot for the head)

    I'll hold my breath until I get these floppies next Friday.
    Last edited by nikola-wan; June 29th, 2019 at 05:39 PM.

  2. #2

    Default 4907 repair photos

    Here are some photos of the insides of the Tektronix 4907 Floppy Drive - master drive with the controller:

    Front View after I replaced the bad DRAMs - now only the Clock LED is ON - it stays on until you manually enter the date and time from the computer:

    Top View of the 4907 with 18 socketed 4K bit Mostek 4027 DRAMs. The right photo is the 4907 with soldered ceramic case DRAMs.
    The mainboard continues under the board with the ROMs and has the 6800 CPU. The only other boards are the power supply board behind the drive and the Shugart floppy drive board.

    And the bottom view showing the older Shugart 8" discrete drive controller board, and the second photo is the newer Shugart controller board in the second 4907 with LSI chip.

    In order to debug the DRAM issue on both 4907 - that resulted in the BUSY, FAULT and CLOCK LEDs to be ON, I pushed a header strip into the ribbon cable connector hanging from the mainboard to ROM board. This cable could connect to the 4051 diagnostic tester - which I don't have, plus a 4907 diagnostic ROM which I don't have. But all the signals are documented in the Service Manual on

    I have an HP 1630A Logic Analyzer - and used the wires on three pods to connect to the signals in the ribbon cable by pushing the logic analyzer wires into the exposed header pins.

    Here is a photo of the Logic Analyzer Trace Setup: all eight DATA signals, all sixteen address lines, RWOC, VMA. I didn't use PIAE - and the service manual indicates it is driven from the 4051 tester. The capture spec was looking for a WRITE (VMA=0 and RWOC=0) to address 4000 - the I/O register controlling the front panel LEDs.

    Here is a photo of the logic analyzer pods connected to the ribbon cable. J Clock on Pod 4 was connected to J300-A23 signal 02L-1 service manual page 8-08. B pins are in the foreground of the second photo. First photo shows clock falling edge, I also used clock rising edge with no issues.

    Here is a trace showing the read of a DRAM location didn't match the value written. shortly after this was the write to the register 4000 to turn on the Fault LED.

    Last edited by nikola-wan; June 30th, 2019 at 02:00 PM. Reason: corrected clock pin

  3. #3


    I have a 6800 CPU preprocessor module and other CPUs with a 10269B General Purpose Probe Interface plus mini cassette tapes with Inverse Assembler files coming from Australia. Should arrive Friday.

    The Inverse Assembler files will disassemble a trace listing with the 6800 code, to make it easier to see why the program does not continue.

    Here is the photo of the 10269B probe interface to the HP 1630A/D/G logic analyzer pods

    and here is a photo of the User Definable (wire wrap) interface that plugs into the probe interface:

    I found that one on goodwill website - it should be here tomorrow. It has an 8051 cable attached.

    The seller had manuals for everything - I will be scanning them so Al can post them on

  4. #4


    The small photos were a downloaded as a collection from iCloud. the larger photos were directly transferred. Here are larger versions - edited to brighten them and reduce the size which increases how they are displayed, from the first post:

    4907 front view

    4907 top view

    4907 bottom view - older Shugart floppy

    Newer Shugart floppy in the other 4907

    Logic Analyzer probes attached to ribbon cable between mainboard and ROM board

    Closeup of connections to the ribbon cable

    Where the first DRAM failure was detected. The write to that location was a value of FF at mark 0038
    The trace triggered at 0000 at the write to register 4000 hex.
    Other reads from Fxxx are ROM code fetches. VMA must be zero for valid reads or writes. RWOC is one for reads and zero for writes.
    Last edited by nikola-wan; June 30th, 2019 at 03:11 PM.

  5. #5
    Join Date
    Mar 2006
    Salt Lake City, UT, USA


    I have two of these; one with a single floppy drive and another with dual floppy drives. This information will be very handy when I get around to refurbishing mine! Thank you very much!

  6. #6

    Default Success! I got one of my Tektronix 4907 Floppy Drives to Work!

    Yeah! I received the Athana 8" single-sided, single-density NOS floppy drives yesterday - and got one of my two Tektronix 4907 Floppy drive systems to work!

    Here is a photo comparing the Athana floppy to a 3M double-sided, double-density floppy.
    Notice the sector hole location differences. Also note the Athana has more sector holes - 32 plus additional hole for first sector.

    Next image is the sample program from the 4907 Operators Manual on bitsavers:

    I have uploaded that program to a new 4907 Floppy Drive folder on my Tektronix 4050 program repository:

    One of my two 4907 ran the program successfully! Here is a photo while the drive was formatting the floppy disk and then writing the ASCII and BINARY files:

    The 4907 does not use the LED in the Floppy Drive Eject Button - but has a separate Unit Busy LED and Write Protect Switch with LED for every drive.

    Next is a photo of my 4054A screen showing the program formatting the floppy disk, then creating the two files and reading them back and printing the contents to the screen (just two text messages).
    Also note the formatted capacity matches the capacity listed in the operators and service manuals as double-density, although the floppy disks I found on EBAY were only listed as single-density.

    You can also see the default folder that was created to store these two files. The 4907 has a hierarchical file structure, supporting up to five levels of directories and also supports password protection and secret encryption on any file.

    I also ran the CALL "HERRS" command which returns values: retries in last I/O, accumulated retries, successful I/O recoveries, and unsuccessful I/O operations since power on. I saw no errors for the format and two file writes

  7. #7


    Next, I will debug the issue reported with the second 4907 system - Error 15.

    Check out the screenshot for this error:

    Note the 08 at the end of the Error 15 message.

    Here is what the 4907 Service Manual posted on bitsavers indicates for submessages for this error:

    I think I will try to connect the floppy drive in the working system to the one with Error 15.

    I already tried to add floppy drive terminator resistors mentioned in the service and installation manual - but I don't think that is the problem.

    I also ran the HERRS command after this error and it reports 1 unsuccessful I/O operation.

  8. #8


    I'm reposting the link to my scan of the 4907 File Manager pocket reference. It hasn't been picked up on bitsavers yet.

  9. #9


    Quote Originally Posted by legalize View Post
    I have two of these; one with a single floppy drive and another with dual floppy drives. This information will be very handy when I get around to refurbishing mine! Thank you very much!

    It sounds like you have a master unit - which only has a single drive and the dual floppy slave drive unit.

    You should only have to debug the single drive unit - if you have a working 4050 computer with the File Manager ROM Cartridge.

    I just got one of my systems working - I'm debugging the other one.

    I measured the index pulse width as 106.6ns on the working floppy and about 4x longer on the floppy that doesn't work.

    I'll check the floppy service manual again - there was a paper sticker on the front of that drive that said "Drive 0F". I also think there was no DS jumper - which could result in F or 15 - which doesn't sound right. Shugart manual does indicate they could support up to eight drives.

  10. #10

    Talking Both of my 4907 Floppy Drive Systems - now working!

    I put the 'newer' 801 Shugart floppy drive in my working 4907 - still didn't work.

    I looked at the 800/801 Shugart floppy OEM manual and found a diagram with explanation for how the drive could separate the INDEX from the SECTOR pulses - and the Shugart service manual showed the test points "10" for INDEX and "12" for SECTOR.

    I checked my Fluke meter manual and found I needed to press a different function key to measure milliseconds and Hz. I measured TP10 on the working floppy and found 6.0Hz, which makes sense - as the motor is turning the disk at 360rpm. TP12 showed 192Hz - once again making sense as 6 times 32 is 192.

    I put the 'newer' drive back on the working unit and saw 6Hz on TP10, but flaky readings -sometimes 192Hz on TP12.

    I then closely examined the phototransistor mount on the bottom of the drive. The yellow paint on the screw was 'cracked' - it appears a shock during shipment might have done that.
    Here is a closeup of that screw:

    I loosened that screw slightly and pushed the mount until the paint 'looked right' and TP12 reported a solid 192Hz and tightened that screw.

    I put the floppy in the 'newer' drive, turned on the system - and the first system now worked with the 'newer' drive.

    I re-ran the sample program, re-formatted the disk, re-wrote the two sample files with no errors (and the new timestamps).

    Now time to put the original drive back in the working system - to see if the disk was readable by the original drive.

    Yes - disk read fine and the files printed out with the original drive.

    Last step - put the 'newer' drive back into its system and retest.

    Yippee! that disk reads fine in the second system - also with no errors!

    It's Miller Time!

    Next step - make a program to backup a tape to a floppy.
    Last edited by nikola-wan; July 4th, 2019 at 12:15 PM. Reason: added photo of mount that may have moved in shipping


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