Thursday, October 31, 2019

Continuing diagnosis and repair of tape load on drive A of my Telex 8020 system


I pulled the hose off the differential pressure switch for the take-up reel hub and held my finger over the end. When the tape threads around the reel I should feel the vacuum suck in on my fingertip slightly. This test determines whether the fault is in the switch or in the vacuum piping.

Vacuum hose on rear of take-up reel motor

Vacuum hold-down holes in take-up reel hub

Tape above on supply reel and take-up reel below
When I powered up and pushed the Load button, the tape loaded and wound around the bottom hub. I could feel the vacuum on the hose end that I had removed from the differential switch. This suggests that the defect is in the switch itself, or the cabling, since it should activate with the vacuum.


The switch closes to connect a signal wire to ground. Other switches on the same holder are properly connecting to ground, thus my problem has to be one of these:
  • Defective switch, won't activate at the vacuum level
  • Blocked switch due to dirt in the passage (variant of the above condition)
  • Wire lacks continuity from the switch back to J03 pin 11 where it enters the control logic
  • Vacuum level too low, sufficient for PEOT switch but not for Hub Vacuum switch
  • Switch activates but vacuum switched off as diverter solenoid changes to run mode
Four vacuum/pressure differential switches - hub vacuum switch on bottom
I put my scope on the switch line and set it to trigger once if the switch activates. After pushing Load and watching the tape try to load, I determined that the vacuum hub switch never closed. It is definite that the load sequence is stalling in Thread waiting for the Hub Vacuum Switch to close as the trigger to move onto Dump state.

In Dump, the diverter solenoid releases to Run condition, putting the vacuum in the tape columns. The feed reel will rotate clockwise to feed in tape while the take-up reel turns counter-clockwise so it too feeds tape into the columns. Once the loops of tape are in the proper spot in the columns, the tape would begin moving slowly forward until the reflective spot comes under the phototransistor. This is the Beginning of Tape (BOT) marker and then load sequence completes.

I am not getting into Dump because of the failing switch. I decided to test the switch with a stronger human powered vacuum. I yanked the vacuum hose from the reel hub end, waited during a load until the tape went around the take-up reel and then sucked in to try to activate the switch.

In spite of my inhaling strongly the switch never registered so the load operation again failed. I noticed that there is a fairly high flow when I suck on the hose which stops if I block the back end of the switch.

Remaining possibilities as I see it:
  • Differential switch is broken inside
  • Connection from switch to backplane connector J3 is broken
  • Something in circuit within the logic cage is holding the signal to +5 in spite of the switch grounding
Next step was to disconnect the plug from the differential switches and pull the board. With that done I could perform a binary search. From the pin for the switch, I should see a path to ground when I inhale on the hose; that means the switch is good and the cabling or logic cage is bad.

Differential switch board with three set for vacuum, one set for pressure

Micro Pneumatic Logic, In. MPL503 part
The switch is clearly bad. I have more resistance inhaling on the hose to other switches, plus they do activate but the suspect switch does not. I removed the board and will take the switch apart to see if it has "user serviceable components inside". If not, I can buy replacements on eBay.


With a bad switch, I have four options:
  1. First is to grab a good switch from my drive B; that eliminates the chance I can get the second drive operational. 
  2. Second is to swap the role of the Cartridge Detect and the Hub Vacuum switches, by moving both wiring and hoses; this eliminates the ability to use autoloader cartridges and forces me to manually remove the tape seal band. Assuming the other switch is good, this will allow me to continue further on the load cycle. 
  3. Third is to buy a suitable vacuum operated switch, hook it up to the hose and wire it to the signal line bypassing the bad differential switch; this adds non-original parts to the drive but allows for both autoloading cartridges and a potential working second drive. 
  4. Fourth is to find a replacement of the exact same switch component and install it. 
I removed the switch from the board, which was easier than I initially assumed. I began by desoldering the pins that fit onto the PCB, also removing the screws for the plastic holder on the top side of the board. After removal, I realized that the soldered pins where lugs that slide into the switch body, and the plastic holder remains in place.

The side of the switch had a small screw, which I tried adjusting. I discovered that when rotated fully clockwise, the switch was closed. I then backed it out about a third of a turn from when the switch opened and tested with vacuum. It worked perfectly! Somehow this was misadjusted at the Telex factory when assembled onto the PCB.

I don't have to buy any replacement switches or disable any functions of either drive. Good news indeed. I had to reinstall the board onto the tape drive and insert the four hoses correctly. This took a bit of time because of limited access, but eventually it was done.


Pushing Load resulted in the tape starting to turn until it hit the PEOT sensor but not threading into the tape path, immediately signaling a Load Check condition. Time to check my hose connections to the differential pressure board because it seems that one of the switches is operating or failing to operate when it should.

Monitoring the switch that I repaired, I watched the voltmeter on the signal line as I pushed Load. As soon as vacuum came up, but before the tape had even began to advance into the tape path, the switch closed. My setting to was too sensitive now. I need to pop the switch out and adjust it for less sensitivity, iterating until I have it switch on only when the tape wraps round the hub of the take-up reel.

Wow, that switch setting is sensitive. There is a narrow zone where it works. Too sensitive and it triggers when the vacuum comes on. A bit less sensitive and it triggers when the PEOT sensor sees the end of tape early in the load cycle. A magic zone that is about 10 or 15 degrees of rotation of the screw where it works properly, but less sensitive than this it will never see the tape on the hub.

Once I had the Hub Vacuum Switch working properly, the load sequence moved to the 'dump' state where it lowers tape into the vacuum columns in preparation for moving forward to the reflective BOT marker. The diverter value switches off, routing vacuum to the tape columns.

The take-up reel winds counterclockwise allowing the tape to be sucked into the lower column. The supply reel winds clockwise feeding in more tape to the upper column. LED sensors in the tape columns will detect the tape level and stop rotating the reels when the tape is in the proper position.

The load goes wrong in a new way. Now, I hear a weird vacuum and scraping sound, the end of the tape on the takeup reel is completely pulled off the hub and we go into a Load Check condition. My debugging moves on to figuring out what is happening inside the covered tape path with the vacuum columns.

My suspicion at the start is that the load process should wind the tape further on the take-up reel before it begins the dump state. If too little is on the hub, it will slip off during loading of the vacuum column. Time to dig through the maintenance manual and begin scoping signals and timing events.


In the maintenance manual, they indicate that the air blower must produce over 50 inches of pressure and the vacuum pump needs to run at around 30 inches. Test procedures at specific points in the drive, with a tape loaded, so 49+ for blower and 26 inches minimum for the vacuum.

I don't know the CFM requirement for the pumps in order to buy substitutes to power the second tape drive. I can probably estimate from air-holes and the pressure to sort out the probably flow rates. The cross section area of all the holes combined is probably less than .20 square inch.


That is .0014 square foot thus to get one CFM we need air to move around 733 feet in a minute or 12 feet per second or almost 90 miles per hour. If it flowed at anywhere near this rate we would hear some serious whistling and tooting. From this I expect that any pump or blower with just a few CFM will be adequate.

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