Tuesday, November 5, 2019

Debugging of Telex 8020 cabinet B drive


After hooking up all the wiring and hoses, I powered up the drive with a tape on the supply reel and pushed the Load/Rewind button. The vacuum powered up but no reel movement occured. I watched the diverter valve solenoid to see it pull in to select load mode versus its default run mode.

No solenoid movement. I put the meter across it and no voltage was delivered to the coil. I then began probing the connections to the driver board. It is provided with +12V and -12V plus a signal control line. The line is pulled up by the driver board and grounded by an open collector inverter on the control logic board.

Testing the lines quickly showed me that the +12V rail is not active. I know that the power supply will shut that off if there is a short circuit, such as I had with the bad capacitor on the dump card when restoring the power supply.

I decided to yank cards out of the machine until I figured out which one is shorting the +12V bus, without harm to the rest of the machine. I switched off the +45V and -45V so they don't provide power to the motors, pulled the power amplifier and pre-amp cards, then monitored +12V.

The voltage was zero again, so I yanked all the cards except for the control logic to ensure I had valid +12V as a starting point. I installed the read, write and AGC boards next had the voltage disappeared. A bit of a binary search led me to the write card which had a dead short across the +12V rail.

It was another of those black polarized capacitors, a 15uf 20V molded unit, similar to the one that failed shorted on the dump card. At this point I distrust all of these, but only the one is bad at this time. If I were to keep the drives in service, I would go through and swap out all of these for new tantalum axial capacitors. As it is, I am ordered a batch of 10 as replacements since these are so troublesome.

Shorted 15uf filter capacitor on the Write board
Now the diverter valve works, the supply reel turns to try to find the end of the tape on the PEOT vacuum sensor, but I seem to be missing the blowing air that forced the tape end up in the air. This may be because of the broken plastic part where the forklift crunched the tape drive.

Indeed, it is because of the damage. Further, the allen bolt that holds one end of the plastic down is bent, thus unable to get the part to seat firmly enough for the rubber gasket to keep the blown air channeled to the proper place.

Damaged plastic part with air hole visible

Channel for air to blow the end of the tape off the reel

I should make a rubber or plastic right angle diverter that sits in the air outlet hole and blows it towards the edge of the tape to make it lift up. It may be possible to buy a new allen bolt and flatten the area to make a proper seal but I am doubtful, so auto cartridge load is ruled out for drive B.

Damaged surface where o-ring and plastic part need to seal

I realized that the control logic will see tape in the path and a vacuum on the take-up reel hub, understand that tape is already wound on the drive, and simply attempt to dump the tape into the vacuum columns. I hand threaded the tape and hit Load/Rewind, but the tape in one of the columns bottomed out causing a Load Check. One of the servos or one of the LED sensors is malfunctioning - it appears to be the upper column.

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