Chasing faults in second drive Control Logic board

HOW BACKWRAP SIGNAL IS GENERATED

Gates that produce the BACKWRAP signal

As we study the schematic above, it shows that the BACKWRAP signal is produced by both inputs to AND gate 215 being high. Essentially this is active in the WAIT state of the sequencer when the autoload cartridge is not detected, but can be broken by sensing the physical end of tape and by a few other error conditions.

VERIFYING THAT BACKWRAP IS ACTIVE

I set up the bad control logic board in the tape drive, with probes hooked up to pin 07, the BACKWRAP signal, as well as to one pin of the binary-decimal decoder chip that indicates whether the sequencer is in the WAIT state.

At power-up the sequencer should be at the REST state and therefore we should not be producing BACKWRAP. My tests showed that BACKWRAP was floating low and that the REST state was NOT active! At this point I need to do more debugging to figure out what is happening.

PREPARING TO TEST OUT OF THE MACHINE

I pushed on my 70 pin connector to the board in preparation for wiring up the power rails and beginning testing. I happened to check connectivity between the VCC pins of the TTL logic and the +5V input and immediately found my (first) problem. The choke inductor L1 that connects the +5V from the backplane to the rest of the board is missing!

100uh inductor goes here

I took a quick run to Anchor Electronics for the inductor, then soldered it onto the board. I took the time to wire up the power and input lines from the connector to the terminal strips I also bought. This gives me a 1x20 and 1x15 for each side to cover all 70 positions (eventually).

Initially I chose only 35 wires to solder, still a tedious task. Making matters worse, one of the pins broke off on the connector. I was able to tack solder that wire back, just in case I need to simulate the End of Tape (EOT) sensor input which is assigned to that pin number. .

With everything in place, I hauled out my triple power supply to provide the +5 and -12V DC voltages to the board. Another power supply provided 12.6VAC which let me fully power the board.

Most of the inputs have pull-up resistors on the board and depend on a sensor or switch connecting the line to ground in order to activate it. That is convenient because I could leave most lines unconnected and still get reasonable behavior.

BENCH CHECKOUT OF THE CONTROL LOGIC BOARD

First up was power application and a sanity check of the state of various outputs and internal gates. I wanted to ensure that none of the movement command outputs were active (low) and that the loading state machine was sitting at the REST state initially.

Indeed the state machine was at REST and none of the REV, FWD, WRAP and other movement oriented signals were active. However, when I 'pushed' the Load/Rewind button the sequence didn't advance. I looked at the circuitry that should trigger the step input to advance the load state machine.

I found that the logic asserts that the L/Rew pushbutton is active, so I traced back to see why this is happening. The switch itself activates to ground with the line pulled up to 5V when inactive. It passes through an inverter so that the low logic level of the active pushbutton turns into a high level for use to advance the state machine.

Both the input and the output of the inverter are at logic high level! Since the button isn't pushed, the input makes sense but the output should be low. I checked the operation when I grounded the input to simulate the pushbutton. The input to the inverter does move properly but the output is bad.

I decided to check the other inverters on that chip to see if there are any other gates in the chip showing incompatible logic levels. Indeed, the very next one I checked was also showing high-high so the chip is blown. I located another 74LS14 chip on the board and found that its gates had compatible values on the inputs and outputs.

Before I replace this one, I will do a sanity check on all the chips on the board, looking for illogical combinations of inputs and output. I would prefer to assess the scale of the problems on this board and identify if any other chips need replacing so I can do them all at the same time.

I need another trip to Anchor to pick up replacement chips for any that I don't have in stock.