Continuing debug of control logic board

I proceeded to push the L/Rew button by temporarily grounding the appropriate input, while watching the state machine for an advance to WAIT before timing out and ending at LD CHECK state. Initially it didn't work, which I traced to the reset of the state machine forced by leaving the IN-LIM-1G0 input high.

This signals that the tape is in the vacuum columns between the low and high points. Once the tape has loaded like that, the load sequence is over and reset is forced. I added a wire to pull it to ground which then allowed the L/REW pushbutton to advanced the state machine from REST to WAIT.  It sat there until the timer expired, then jumped forward to load check state.

In between each such test, I 'push' the Reset button by grounding its input, to restore the state machine to REST and to clear all errors. This process worked fine. I won't document this step any further but will reset between every test attempt.

I also checked that the appropriate outputs such as BACKWRAP were activated while in WAIT. This is an open collector output, thus I had to pull it up with a resistor before probing the output pin. When in the idle (REST) position, this signal is pulled high but upon entering WAIT it is pulled low.

Another initial failure, as the signal was always on (pulled low). I traced through the signals that generate this output to see whether there was another input I needed to force low or high. I analyzed the gates to figure out what would make the output normally high but drop low while in WAIT.

The key signals and their required state, in order for the BACKWRAP to be triggered only in the WAIT state, are:

• CRT-PRS-0X0 should be high, indicating that an autoload cartridge is not present
• TAPE-PRS-1G1 should be low, indicating that the tape is not in the path near the tape head
• P-EOT-SW-1S1 should be low, indicating that the end of tape is not near the entry slot
• WRAP-1+2-1G0 should be low since the state machine is in REST or WAIT
• HUB-V-SW-0S2 should be high indicating that tape is not wound around the take-up reel
• THRD-1+2-1G0 should be low since the state machine is in REST or WAIT

With the conditions above satisfied, the output of the BACKWRAP signal is high (logically off) until the WAIT state signal goes active which drops the output to low to signal to the reel motor preamp board that it should slowly rotate the supply reel counter-clockwise.

I fired up the board, left it in the default REST state and probed all the signals above to determine which ones were incorrect. I immediately found another bad chip (U6E), a 74LS05N hex open collector inverter. When I measured the resistance of the output to +5 (pull-up resistor in the circuit), I got about 500 ohms but the resistor in question was 10K.

I looked at the pullup values of the other five inverter outputs, then calculated the equivalent parallel resistance if all six outputs were shorted together - 556 ohms. I removed the chip from the board, confirming that all outputs were shorted to ground with the chip unconnected. Time to replace it with a good chip and hope that I don't have too many other blown chips waiting to pop their heads up as I check out the logic.