Wednesday, December 13, 2017

HP 2645A tape units tested, IBM 1401 memory problem diagnosed


Testing 2645A CTUs with RTE

I brought up my system this morning to test out the ability to access my minitape cartridges (CTUs) on the 2645A terminal. They were generated as logical units 16 (left) and 17 (right). I successfully wrote a file from RTE down to the tape unit then was able to read it in local mode on the terminal.

Terminal displaying downloaded file from left minitape unit
For some reason, my right CTU is stalling when I insert a tape. It won't respond to rewinds or reads, instead flashing a 'STALL' error message on the terminal. I will need to look into this at a later date.

I tested my 2622A terminal from the tape diagnostics and verified that it worked properly, but when I tried to use the second terminal under RTE the I/O hung. I suspect it is something like ENQ or other protocol issues that are not configured properly (yet) on the terminal.

As I have mentioned before, RS232 based communications are an enormous pain in the rear because of the many permutations of signals and protocols that can be used and the inadequate documentation in most manuals about which options were chosen.


Armed with my diagrams and other research on how the memory system worked I arrived to work on the problem on the Connecticut 1401 system. The first 4000 characters of memory appear as all zeroes, with a parity error, regardless of the address within the range.

The likely area of failure was in the logic that drives pulses through the matrix switches in order to generate pulses on the X and Y wires to a core on each core plane. We checked out the timing and control pulses that are inputs to the drivers, then the performance of the bias circuit that adjusts drive current to fit the temperature in the core stack. All was good.

Next up was to look at the voltages on the terminating resistors at the other end of the driver circuit. The driver runs through a row or column of the 50 to 80 transformers in a matrix switch, but the other end of the winding on those transformers are completed to ground for only the one transformer that matches the current address.

Overall there are four drivers in the memory. Two are associated with the 50 transformers that produce the X select lines for the core planes, the other two are associated with the 80 transformers that produce the Y select lines.

We watched the pulses showing up at the terminating resistor for the specific address we were accessing, comparing the good German machine against the failing Connecticut system. We found that one of the drivers was not working. That means that an X or Y select line was never activated, because a row or column of a matrix switch had no current. The transformers in the matrix switch only work if both the row and column of that transformer have current flowing.

We verified this by borrowing the four driver cards from the German machine and demonstrating that the Connecticut machine now worked properly. We replaced the original Connecticut cards one by one until we were sure we spotted the one failing card.

Ken Shirriff dug into the card and found an open inductor that caused it to fail. He found a spare coil on a donor card and moved it over to repair the failed card. In the interim, other members of the team found a few spare cards for the driver card.

Oddly, the spares are had only a single transistor while the cards in the machines have a pair on each card. The card type is AQW or AKA, the two names are synonyms for identical cards. The schematics for the AQW/AKA card matches the single transistor versions, not the cards in the machine.

The team decided to insert one of the new versions of AQW/AKA into the CT machine, while Ken was repairing the original type card. With the card in place, the power on button was pushed but the machine refused to sequence up.

We pulled out the new type card and put in Ken's repaired card, but the machine still refused to power up. No circuit breakers were tripped on the many power supplies in the machine. We used a multimeter and found that the -6V power supply was putting out less than a volt.

The power supply was pulled and tested in the workroom. We found one clearly shorted transistor and a couple others that were suspicious. These power supplies have six power transistors operating in parallel to handle the 12A maximum load this unit can handle. While it was open, we decided to replace all six transistors with new equivalents.

We found it produced -6V when we hooked it to the power mains, but we didn't put it under any load. Instead we put it back in the 1401 and tried to power up again. Still no luck and the -6V supply is again indicating about a volt. We ran out of time as the public demonstration of the systems was starting. Next week we will test the power supply more and hopefully get the Connecticut system fully back in operation. 

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