Monday, October 28, 2019

Diagnosing failure in tape load operation in Telex 8020 drive cabinet A


IBM introduced a feature on its tape drives, during the 360/370 era, where the operator only had to push the closed tape cartridge onto the drive and hit the Load button. The drive would open the seal belt around the outside of the tape and thread the tape automatically. My drives have that feature as well, but I am unable to test it because I don't have the right kind of tapes.

The normal seal is a band that clamps shut around the perimeter of the tape to keep out dust, but is not designed to work with the automatic openers. A different band, called either the wrap-around or autoloader cartridge, is instead placed on the perimeter of the reel of tape. With this band on the tape, it can be opened by the drive.

All the tapes I own have the standard seal on them. I have dim memories of having to take new tapes out of the box, back in the mainframe days, take off their normal band and put on the autoloader types before putting the new tapes into the tape library. I am seeking such a band but so far can't find any. Without that, I won't be able to test out that part of the loading function of my drives, instead having to remove the seal and stick the tape reel on sans band.


At the point that the load fails, it has powered up the motor for the vacuum pump and air blower, begun rotating the feed reel slowly counter clockwise, and energized the diverter solenoid. It then waits for the PEOT sensor to indicate that the tape is present. After a watchdog timer goes off without any PEOT signal, the logic records a tape load check condition and stops.

There are a number of possible errors that could cause this:
  • Diverter valve not operating to switch vacuum
  • Insufficient vacuum or air leak
  • Inadequate blower volume or air leak
  • PEOT sensor opening clogged
  • Component error on board responsible for sensing PEOT and advancing load state machine

My first action was to watch the diverter valve to see if it operated. My VOM showed the operating voltage applied to the coil and I felt air coming out of the loading shoe. Additionally, I could feel vacuum at the PEOT sense port. The shaft in the solenoid moved fully and promptly.

This narrows the problems down a bit. The diverter is working and I have some vacuum and air pressure. Time to move the meter to the PEOT switch and see if that is tripped. I can see this on J3 on the motherboard, pin 15.

I then realized that the vacuum front plate wasn't closed properly. It was snapped shut at the top but there is a second snap at the bottom. Without that closed firmly, the vacuum wasn't able to pull the tape where it had to go. With the door shut properly, the drive would almost complete loading.

Notice that the vacuum front plate is not on my list of possible causes. This is always a possibility, thus debugging requires flexibility and adaptability if you want to succeed.

The tape was threaded through and onto the takeup reel, but then it just stopped. This may be caused by the previously noted problem with the Load Point light dimly lighting. If the drive believes it found a load point, that would stop the seek operation.

I also noticed that it won't unload from this point. Pushing unload does nothing, while it should rewind the tape and then unload it completely.

I watched the BOT and EOT lines (beginning of tape or load point; end of tape) and they were properly low while tape was in the column. They are not detecting a reflective tape spot. These should trigger a clear logic 0 or 1 level on the motherboard, driving the indicator with a logic 0 to light the bulb.

I watched the output that drives the control panel light and it stayed at logic high, meaning the bulb should not illuminate. I don't know why it is partially lit but it is not confusing the load logic.

The load process seems to stop at the point where it should enter the 'dump' state where the two reels move to lower tape into the columns now that the vacuum has switched back to run state from threading state. I found a pin where I can watch to see if the state machine advances to 'dump'.

It does not. Vacuum drops after the tape winds on the take-up reel but before it tries to lower the tape into the vacuum columns. The state machine never advanced to this point. I will now check a vacuum switch that should indicate that the tape has wound onto the take-up reel. If that doesn't occur within the timer limit, the drive would give symptoms like I am seeing.

After hooking the VOM up to the switch at J03 pin 14 it is definitive. The switch is not closing. This is hooked with a hose to the take-up reel hub which has three holes that are hooked to the vacuum line. When the tape winds around the hub, it blocks the holes, producing the vacuum that should trip the switch on.

My action plan tomorrow is to verify that vacuum is applied to the hub, then verify that we see a vacuum when tape covers the hub. If the vacuum is felt then, the problem is in the switch itself.

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