Sunday, November 3, 2019

Restoring operation of Power Window on Telex 8020 drive A


The front door of the tape drive has a glass window that slides down under power to allow the operator to insert or remove a tape from the supply reel, then slides up when the drive loads and uses the tape. It should be triggered by a push of the Unload button, to open, and of the Load/Rewind or Reset button, to close. At power up, the glass is lowered.

The mechanism has switches at the bottom and top of the travel range of the glass, to signal to the logic board that it has fully opened or fully closed. It also has a momentary contact switch on a plate at the top of the glass, which detects if a foreign object, e.g. operators hand, is in contact with the window while it is trying to move.

The PCB in the logic cage, slot 06, looks at the Reset, Unload/Rewind and Load/Rewind buttons, the current state of the window, as well as whether the tape is loaded. It commands closing or opening the door by pulling the appropriate signal line to ground. That line goes to the logic card in the door.

The card in the door looks at the switches at the ends of glass travel, the safety switch on the top of the glass, and the commands coming from the card in the logic cage. It drive the motor one way or the other to accomplish the desired position.

In addition, the door holds the motor and related components that interact with the local logic card. Among these are a full wave bridge rectifier for the logic board.

Power window motor and logic board, plus switches

The wiring is spread across four diagrams in the schematics - an overall tape drive level, the power door level, a control card inside the door, and a control PCB in the main logic cage. I had to take time to map out the interactions - situations such as pin X of one terminal or jack hooked to pin Y on another schematic, which in turn is wired to Z on the door.

Once I had the wiring clear in my head, I could work out a testing strategy that took voltage readings from various signal lines and verified proper operation of the switches. Initially, I was presented with an unasserted command to close the window and an apparently asserted (grounded) command to open the window. Since it is physically sitting in the open position, that is consistent.

The two logic outputs that should indicate whether the window is in the opened or closed state are both at ground. The two command signals didn't change state regardless of keypresses of Reset and Load/Rewind buttons.

I decided to first verify the presence of the input voltages (+45, -45, +12, -12, and +8). If these aren't present the logic and motor aren't going to work.All present and accounted for, after the testing.

Next I pulled the PCB and beeped out the two range switches to be sure they are in the proper position and indicating the correct status. Each switch is a DPDT momentary type, which switches both poles to its normally open (N/O) position when the glass hits the lever, otherwise the poles are at the normally closed (N/C) positions at intermediate glass positions.

Aha! One of the two switches had a bad contact for the N/C position. It was for the upper range (window closed) position, however no movement can occur if the N/C contact isn't made while the window is below that point. It was extremely bad so I removed the switch to try to deoxidize the contact.


I used my trusty Deoxit spray, although it took some doing since the switch was all-but-sealed, with no entry points for the spray. I found that there were very tiny gaps where the electrodes entered the plastic body, over which I could put the applicator straw and then force the fluid inside. After a couple of rounds of this, my switch was restored to like-new condition.
DPDT switch with bad N/C contact (top right)
My backup plan was to swap connections, since one pole only has a N/C connection and the other pole only has a N/O connection. As long as one side had a good N/C and the other a good N/O connectivity, rewirind would do the trick. Fortunately, this wasn't necessary.

The switch was reinstalled, the PCB put back in place and I powered up for the test. Voila! The door now closes on its own when the Reset or Load/Rewind button is pushed and opens on its own with Unload button activation.

No comments:

Post a Comment