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729 Magnetic Tape Drive Tips

from Ignacio Menendez

- 729 Magnetic Tape Load Problems
- A Procedure on how to diagnose tape breaking problems on IBM 729 Tape Drives

729 Magnetic Tape Load Problems, - Wed, Oct 07, 2015
The most common causes for tape load and dumping problems are the vacuum switches or relays not making contact.

Should always clean the vacuum switches first.

Also, make sure that the lights on the R/W head assembly are turned on, before even attempting to load a tape. One other possibility is poor contact or failure of the ‘stop’ clutches not driving correctly the reel shafts when turned by the load motor worm gear.

Follow the Tape Load Operation Flow Chart, for the sequence in which relays pick, contacts make, and photocells detect and control the mechanical actions, this chart (and other very useful charts on tape motion are found on :

Manual 223-6988 IBM Customer Engineering Instruction – Reference 729 II,V Magnetic Tape Units (30,000 Series) and 729 IV, VI Magnetic Tape Units (90,000 Series)

With Stamped Seal of Mr. John V. Gardner of Smyrna, GA (without the stamp, the flow charts are missing)

The above chart will show you how the LOAD operation progresses, and which devices control the mechanical units, which you can easily verify.

If the above does not help you in finding and fixing the problem, the following may be taking place….

From actual experiences at the Computer History Museum in Mountain View CA.

The first 729 on the German system was dumping the tape because the left STOP ('stationary') capstan was not adjusted correctly...

In a load operation, both left and right stop capstans keep the piece of tape under the head from moving, by the action of the prolays moving the idlers to pinch the tape against the stop capstans.

This is done to prevent the vacuum on the column that loads first from suctioning the tape from the other column before it has enough vacuum to pull it down itself into its column.  (Which is what was happening)

This unbalance comes about depending on how much tape exists on the supply reel, compared to the take up reel, since both turn by the same load motor, and the one with more tape (larger diameter) will put tape faster into that column, than the one with less tape.

I had a déjà vu moment, and remembered that I had fought this problem before, but on the left column dumping, on the 4th 729 on the Connecticut system…..

After much struggling, I finally noticed that the tape was moving and being pulled into the column where it was dumping.... then manually pressed the idler into the stop capstans, to pinch the tape and stop it from moving, and this kept the tape dumping from happening; so I adjusted the stop capstans by loosening their Allen screws and rotating the stop capstans.  

<< This is done by disconnecting the capstan motors, and attempting to load a tape, while manually holding the prolay idlers hard up into the stop capstans.  The load operation will be hung, with tape loaded, but not moving to load point, with the capstans disconnected and retracted.  At this time, the prolays should be having the idlers pushed to the stop capstans holding the tape stationary (pinched to stop)....

Loosen the Allen screws in the center of the stop capstans, and rotate these, until the gap disappears (a flashlight can help in seeing this gap go away, just), then re-tighten the Allen screw on each of the stop capstans. (Do NOT over-pinch, just close the gap to zero clearance) >>

I had documented this earlier along with the passage about the vacuum pump problems on the 4th drive, in the 1401 restoration team site on the net.

A Procedure on how to diagnose tape breaking problems on IBM 729 Tape Drives, - May 26,2019
WARNING... Lethal voltages and moving parts inside the IBM 729 Tape Drives, proceed if trained, and with the outmost CAUTION.
  1. If present, make sure the High Speed Rewind Eddy Current Switch with mercury capsule contact moves freely.
    If the mercury switch is not present, the timed delay of the slowdown is accomplished by a motor timer at rear of the drive.
  2. Follow carefully these steps :
    1. Before loading, remove the R/W head snap cover.

    2. Load a new tape on suspect drive, AND before you make it ready...

    3. Carefully not to get hurt by a/c or moving belts...

    4. Disconnect the load/unload motor, the one on the very front top, that moves the worm gears, that mesh to the 2 front magnetic clutches...
      The connector is on the A/C power covered buss, just below the motors.

    5. From TAU, move the tape fwd until close to the end of tape, stop by pressing RESET on TAU then do a REWIND from the tape front panel.

    6. The tape will go high speed rewind, until the photocell detects the tape pack low enough, at that point, after it slows down, it will bring the head down, and WILL HANG...

    7. ...since it is at a state where it expects the ‘tape in column signal’ from vac. Switches, which will not happen, since the load/unload motor was disconnected above.

    8. At this point, you can take a very detailed view of how the tape is aligned, and if the prolays are pinching the tape against the STOP capstans.... (which they should).

    9. Once you note that, turn the power off to the tape.... (rear switch)

    10. Raise the head manually by moving with your hand the cog belt that moves the head down and up...

    11. Now slowly wind the right tape reel (tape fwd motion) slowly some 10 or 20 feet and closely inspect for tape damage, in the form of stretching or creasing...
      which would mean tape is slowing too fast, causing the stretch, or creasing caused by not enough breaking.

    12. Once you are finished, with power still off, reconnect the tape load/unload motor...

    13. Power up and press rewind to get tape back to load point. Re install the r/w head snap on cover.

    14. You may need to repeat this his process a few times, in case the problem is intermittent.

The tape load/unload and low/high speed rewind operations are shown in the very explicit charts in the IBM-729 Tape Drive Manuals.