Wednesday, October 11, 2017

Good day at CHM with 1401, plus a wee bit of work on the HP 1000 peripherals


We have had a lingering problem with the card punch side of the 1402 on our "Connecticut" system, where we experienced spurious punch checks as the card which has just previously been punched is now verified by passing through a block of read brushes. 

The brushes are installed in a brush block, eighty brushes one for each column of a punched card. As a card moves through the brush block, each row passes under the brushes and is detected starting with the 12 row and ending with row 9. The number of holes in each column is saved and checked against the value which is expected, due to the hole pattern that was punched. 

Each brush consists of multiple metal whiskers that drag along the top surface of the card, making contact to a conductive roller underneath whenever there is a hole punched in the card at that spot. Over time, brushes can become bent, both as a group tilting towards one of the adjacent columns, and individual whiskers can pop up and bend over to short against another brush. 

The brushes in the block were quite ratty looking, many tilting and plenty of lone whiskers askew. While it would be possible to work on each brush to get its whiskers together and bent properly, perhaps trimming the straggler whiskers, it is a very, very time consuming process. 

One of the team looked at our stock of spare parts, where we have nine brush blocks and a number of replacement brushes. He found one whose brushes looked as good as the day they rolled out of the factory. All were perpendicular, grouped together and spaced evenly from one end to the other of the block.

The block had some hardware mounted to it - a bottom rail and a side rail - that were incompatible with the punch check position, but the corresponding side rail of the ratty block could be moved over. We simply discarded the bottom rail which was superfluous. 

Then, we transferred the wiring plugs that connected 80 wires to the holders for 80 brushes, pulling them off the ratty block and pressing them in place on the replacement. That done, we were still getting punch checks because the 'timing' of the brushes was not yet set. 

Timing means that the position of the brushes in the brush block is slide frontward or readward, relative to the punched card motion, to align the brush with the point on the card where the hole should be sensed. This point is determined by a set of contacts on cams that produce 12 pulses - think of these as sampling pulses - which will record the state of the hole at that instant in time. 

The long line of brushes can slide back and forth from the two ends - at column 1 and at column 80 - thus the timing involves getting both ends at the best position possible. We set up the scope to record the sampling pulses on one trace and the brush output, having replaced the wire for a chosen column with our scope probe lead. 

We could see that our first chosen column, number 9, had the hole position too far to one side on the trace, so that the sampling pulse arrived just as the hole we detected was ending with the brush riding past it. This was true for both the 12 row and 9 row holes since we were punching the letter A in that spot, which is encoded as holes in both rows 12 and 9. 

We took the brush block out, moved the brush to read earlier in the card path, and watched again. The sampling pulse was now right at the other side of the detected hole. We had overshot the adjustment we wanted.

I took it out and split the difference, putting the brush position about halfway between the original and current spots. Now, we saw good results with the sampling pulse well within the hole sensed for both rows 12 and 9 in that column. 

To ensure that the other end of the brush block, near column 80, is also in a good position, we replaced the wire on column 9 and moved our scope probe over to watch column 76. This side was off a bit in timing, but we could swing just that end of the brush block a bit until we were happy with the pulses on this side too.

Restoring the wires to column 76 and shutting off the scope, we then ran a program to duplicate a deck of cards, taking each card as it is read and punching a copy on the punch side. This ran without a single punch error, so we declared victory and handed the systems over to the demonstration team in preparation for their scheduled 3PM demonstration to museum visitors. 

Another team member was working on the tape drives for the "German" system, continuing to adjust the relays that determine the duration of key intervals such as the interrecord gap or length of a tapemark. He used a special tape that ensures this is referenced against a standardized output that we try to match as closely as possible. 

This done, he mounted a regular tape reel but found that the drive would no longer even load the tape. This was a pure coincidence, with a contact on a small incandescent bulb becoming loose, that bulb used to detect the beginning of tape reflector during the loading process. He quickly found this and corrected it, giving us a full set of working tape drives on both 1401 systems. 

One of our restoration team members, Joe Preston, passed away yesterday morning after a many week battle with a bone infection caused by an antibiotic resistant bacterium. This is the second team member to pass away this year, following the death a few months ago of Ron Crane. We are planning a lunch next Wednesday to remember Joe. 


7970E Tape Drive

I received my replacement circlips (retaining rings) that will allow me to remove and clean all the rollers that are a bit sluggish on the drive, I believe due to dust gumming up lubricants. I didn't have enough time to do anything else with the drive today. 

7906 Disk Drive

I did do a bit of research and printed out the schematics and board layout for the control PCB which recognizes the disk fault condition. With this I can set probes tomorrow and from that determine what specific error condition the drive believes it is detecting. 

2622A Terminal

I completed my triple checking of the wiring of my cable, then insulated and taped it up ready for the live test between the monitor and keyboard. When I switched on the terminal, I did get some characters when I pressed certain keys but definitely not the appropriate ones. 

There is something wrong in my cable wiring (or in the monitor or keyboard), but my first candidate is my understanding of the wiring of the cable. One one side of the cable, the lines are called Key Address 1 to Key Address 7, but on the other side they are labeled Key Data 0 through Key Data 6. 

If it is simply the case that one side uses origin 0 and the other uses origin 1, I should have been okay. Perhaps, however, the assignment is reversed, where Key Address 1 is Key Data 6, odd as that seems. I think I should independently power the keyboard and do some debugging of the values returned. 

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