Tuesday, October 10, 2017

Working on HP 1000 peripherals - disk, tape and terminal keyboard cables



I discovered that I terminated the 9 track tape write incorrectly, in my earlier hand code. When sending data words from register A or B to the tape unit, when done, the programmer must reset the control bit of the data channel. Instead I was releasing the tape drive from the controller via a CLR command to the control channel.

I fixed my hand code and wrote the tape over again with my two words of o123456 and o050505, I would get some tape movement but then the program would sit in a loop waiting for the data channel flag to be set before outputting data words. Sometimes it hung waiting for the first, sometimes waiting for the second.

I am not sure what is going on. Until I can get this to happily write the two word record, there is no point attempting to read it back in. This may be a flaw in my understanding, a timing issue keeping up with the tape or it could be something wrong in the controller card or drive.


The sound of my 7906 disk drive as it spins is probably an issue with the motor spindle - either the bearings themselves which are NOT user replaceable components, or if I am lucky, the ground brush and spring that rotates under the spindle.

To determine which it is, I had to turn the drive over and open the bottom of the spindle assembly. I found lots of fine gray dust inside, which appeared to be the source of the screeching sound. It could still be true that the spindle motor bearings themselves are shot, but I can hope that when I fix this issue my problems are solved.

The bottom of the spindle shaft tapers to a point, onto which a springy metal plate presses from the bottom spindle cover. This is an anti-static connection. The metal plate has grinding marks on it and there was the fine powder throughout the area.

To repair this I have to move the contact spring away from the area that ground down and test to verify the noise went away. If not, I have a main bearing issue which will be complex to resolve, involving quite a bit of disassembly and bearings which were not designed to be replaceable.

I tried a reassembly to listen to the sound while it spins up, but I now get a "Drive Fault" indicator lamp and it won't try to spin. Some diagnosis needed, obviously. I began with the drive fault indicators inside, a set of four colored LEDs that signal the approximate area of the fault.

I saw the green LED illuminated, which is an interlock fault. One of the causes is a failure of one of several power supply voltages: 5, +12, -12, and 25 among others.  There are other causes as well. My first step was to check the fuses for each of the voltages from the power supply. None were blown.

It was time to locate test points for all the voltages and verify they were present. Unfortunately, they all are good so the problem must be one of the other issues that raises the interlock condition.  To do this required the card cage to be opened and suitable probes clipped to various IC pins on the control board, in order to trace this to the condition triggering the problem.

2262 terminals

The second 2622 terminal arrived today - without a keyboard - but it seems to have an even better screen image, plus a bottom and stand. I will probably use this one, with the keyboard, keeping the original as a ready spare unless I find another keyboard somewhere.

I received the DA15 cable and the battery today. The battery will retain configuration settings in RAM on the monitor PCB, although I have to hook up the keyboard before I can change them from the default. I began wiring up the cable between keyboard and monitor.

I have pinout diagrams for both sides, but it is essential to orient correctly, so that I don't create a mirror image left to right of the proper wiring. I did some studying of both diagrams and the actual PCBs to assure myself of the correct orientation.

I soldered together the wires on one of the header sockets, then put did the same for the other socket. After a break, I went back and double checked the connections the cable made between the monitor and the keyboard. Tomorrow I will try the terminal using my new cable.

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