Both of my drives are failing when attempting an autoload. When I hand thread the tape through the path and onto the take-up reel, it fails to dump the tape into the vacuum columns properly.
In both cases, I am not sure exactly where problems are arising because the tape path and vacuum columns are covered by a metal plate. The Telex repair people had a clear plastic plate they could substitute for the metal one, thus observing any defects in loading.
INTERIM APPROACH DEVISED WHILE WAITING FOR CLEAR PLASTIC COVER
I designed a clear plastic plate and sent it out for fabrication yesterday, but won't have it back for about a week. In the interim, I am looking for clues I can gather to help narrow down the area where the load process is going wrong.
If I can access signals that tell me the location of the tape, particularly in the two vacuum columns, it might tell me what kind of failure is occuring. The tape might not enter the column at all, or it might bottom out without the servo loop properly controlling the reels.
I looked at the schematics and was pleased to see that all the signals I want to view are conveniently present on J1 near the bottom of the logic card backplane. Each vacuum column has eight LED-photocell combinations, four at the top of the column and four at the bottom. These signal the current position of the tape column.
The servo mechanism for each tape reel, while in dump or run mode, should turn the reel motor at increasingly high rates as the tape loop moves through the four photocell locations - turning in one direction, to feed in more tape when the loop is at the top of the column, and in the other direction to pull some out if the loop is approaching the bottom.
I set up my Digilent Analog Discovery as a logic analyzer, capturing the state of the sixteen signals over time. Most jacks hooked to the logic cage have a set of wire-wrap pins sticking out for convenient access to the signals on various socket pins. Not so with J1, unfortunately.
I then had to seek an alternate way of capturing the signal. If a signal arrives on one of the sockets on the back of the logic cage, I assumed it must be connected to at least one of the backplane pins behind the PCBs. The only other routing would be socket to socket, bypassing the logic cage.
I found the signals on card slot 4, pins 23, 25, 27, 29, 31, 33, 35, 37. 39. 41, 43, 45, 47, 49, 51 and 53. Nicely sequential and easy to wire up. With the Analog Discovery cable wired up, I brought out the device and hooked it to my laptop, where I configured it as a 16 channel logic analyzer. With that done, I powered up the tape drive and then plugged in the cable to the device.
THINKING ABOUT AUTOLOADING FAILURE
The autoloader depends on air pressure at a number of points to guide the tape around the perimeter of the tape patch and out toward the take-up reel. If any of those are not producing flow, or have inadequate flow, the tape will droop into the upper vacuum column which is what I presume is occuring.
I know there is enough flow to separate the end of the tape on the reel and to blow it up onto the PEOT sensor that detects the end of the tape covering a vacuum port. There also must be air flow inside the chamber so that as the end of the tape enters the covered area it is blown upwards towards the top edge.
Once the tape is pushed along the top edge to the top-left corner, more air pressure should drive it downward through the tape head. Finally, at the bottom left there is pressure to force the tape rightward towards the exit slot where it continues to the take-up reel.
The process of loading spins the supply reel until the end of tape is sensed at PEOT, then it reverses and threads the tape which should be forced by air pressure across the top, left and bottom edges of the tape path. After the tape exits the bottom slot and reaches the take-up reel hub, vacuum holes in the hub grab the end of the tape and a sensor switch detects the Hub Vacuum condition.
The tape winds a bit more around the take-up reel before a timer expires and begins the dump sequence. The supply reel feeds tape in and the take-up reel unwinds some tape. The solenoid based valve should have switched to provide full vacuum to the two vacuum columns, so that when tape is released by the two reels it is sucked down into the upper vacuum column and sucked up into the lower column.
A servo loop between the detected tape position, using photosensors, and the reel motor will feed out or pull back some take to each reel to maintain the tape loop in the middle zone of each vacuum column. When the tape is in position and a timer expires, the drive begins to seek forward looking for the BOT reflective spot that signals the beginning of tape. It stops the motors and illuminates the Load Point lamp. This completes an autoload sequence.
With tape being manually threaded through the path and wound on the take-up reel, the drive is smart enough to detect the Hub Vacuum condition and switch immediately into the dump state. It should switch on vacuum to the chambers and cause each reel motor to feed out some tape until the loops are in the middles of the two chambers.
While the drive currently won't complete the self-threading, it used to. Back then, it would fail in the dump state. This may be the same failure that occurs when I pre-thread the tape, or a different one.
COVER ARRIVES - NOT PERFECT BUT WORKS FOR MY PURPOSE
The picture I took and used to trace out the outline of the metal cover was obviously photographed at an angle and suffered parallax errors. The resulting cover is close but is particularly out of alignment near the top.
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| Acrylic cover over tape path and vacuum columns |
DUMP FAILURE FOR MANUALLY THREADED TAPE ON DRIVE A
I could see the dump process well with the acrylic cover in place. The take-up reel released tape forming a loop right in the middle of the lower vacuum column, exactly where it should stop. The supply reel released tape which fell all the way to the bottom of the upper vacuum column and caused the tape check.
WHERE I BELIEVE THE DRIVE IS MALFUNCTIONING
Based on the testing I did with the clear cover, the failure is with the servo loop that lowers the tape into the upper vacuum column. Specifically, it is not stopping the supply reel motion when the tape hits the middle of the column nor reversing direction as it should when the tape loop passes by the lower four photocells.
The failure could be:
- LEDs and/or photocell board failure causing logic board to miss the tape loop
- Logic board failure so that it doesn't properly respond to the tape loop in slowing or reversing the supply reel
- Servo loop failure so that commands to slow, stop and reverse the supply reel motor are not properly handled
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