Saturday, August 11, 2018

My Model 14 paper tape reader is the wrong speed, but we make continuing restoration process on the Teletypes


I was given a transmitter distributor, which is a paper tape reader for the 11/16" 5 bit baudot tape used with older teletypes. At our work session with teletypes we worked on two of these units, one that came with Marc's model 19 system and one that I was given. As well, Marc made some adjustments to his model 15 teletype to correct two flaws we had discovered.

The model 15 has a feature that will stop the motor based on a character sent to it (motor stop is the H key in Figures mode), then restart the motor if new data begins arriving. One function of this feature is to flip the switch from SEND mode to RECEIVE ONLY mode during a stop. 

We had found that it was bouncing the switch nearly out of SEND mode on every typed character, and from time to time it would move it all the way to RECEIVE. This was frustrating to users trying to type messages on the keyboard as it would suddenly stop sending their keystrokes. 

Marc did some adjusting and it now works perfectly. The lever doesn't bounce at all except when seeing an H character (H key in Letters mode), where it moves just a bit but never forces the lever to RECEIVE ONLY. When a stop is sent (H key in Figures mode) it will flip the lever to RECEIVE and stop the teletype motor. Worked correctly every time. 

The other flaw we had discovered was with the autorepeat feature for the spacebar on his keyboard. This is designed so that a normal tap on the space bar produces exactly one space character, but if you press down hard on the bar and hold it, a string of spaces are emitted until you release the bar.

On this keyboard, it would autorepeat on any tap, so that single spaces were usually turned into 2 or 3 in spite of how briefly the typist hit the bar. Marc adjusted the mechanism until it was performing perfectly, providing single spaces unless you intentionally pushed down the bar and held it. 

Marc's TD (Transmitter Distributor) is disassembled, since he had the cover and baseplate powdercoated along with the rest of the model 19 external metal surfaces. He spent the time cleaning up dirt and corrosion from his parts, adjusting it properly and cleaning all the contacts to remove old oxidation. Someday soon he will begin reassembly of the TD. 

My TD needed cleaning and relubrication before I could test it. I spent time freeing up mechanisms, scrubbing off dirt and cleaning most of the corrosion on metal surfaces. I then cleaned the contacts and inspected the unit carefully. It appeared ready for a test, but we had to provide connectivity first.

The model 14 TD has a set of metal contact fingers on the back which are intended to slide into a baseplate such as the desk from Marc's model 19 system. We worked out the required wiring to provide 110V AC to the TD and hooked up the current loop for the output signal. 

I crimped ring terminals on some wires including a phone jack cable for the data output and a household plug cable for delivering power. These were hooked to the appropriate fingers of the contact mechanism. 110V is provided to two fingers to power the AC motor and to two other fingers to power solenoids that control function. The phone jack hooks to two fingers for the line common and MARK output side.

The TD is designed for both the normal NEUTRAL signaling method and POLAR signaling. NEUTRAL signaling delivers a 60ma 120V current for MARK bit values and no current to indicate SPACE. POLAR signaling delivers current in two different directions, one for MARK and the other for NEUTRAL. We wire our TDs for the NEUTRAL method we are using, but could use them to drive a POLAR line with a bit of rewiring. 

I fired up the TD and verified that the motor spins properly, a tape advances through the reader properly until it either runs out (sensed by one microswitch) or the tape becomes too taut and pulls up a safety switch bar. The result on the signal output line seemed correct as I pushed or released each of the five pins that sense/read the paper tape holes.

It was time to hook this in a current loop and see if the tape we read will be properly typed on one of the model 15 printers. Marc had produced a few test tapes with his model 19 keyboard, which has a paper tape punch called a perforator that transfers keystrokes into tape characters. I put in the first tape, holding a one line message, and transmitted it.

The results on the printer were gibberish. Having a suspicion that this might be a speed mismatch, we hooked up an oscilloscope to the line and observed the duration of the bits being produced. Our normal 60 word per minute units have a bit cell time of 22 ms and the stop bit, being 1.5 bits long, is 33ms. The scope showed a stop bit that was only 26 ms long.

We produced another tape, this one with a string of R characters, which would allow us to easily see and measure data bits. That confirmed that the bit cell was a bit more than 17ms, reflecting a transmission rate of 75 words per minute. The steady string of R characters produced gibberish on the printer, but a repeating pattern that further proved this was a speed mismatch problem.

I will need to replace the motor pinion and main shaft gears with a set designed for 60 wpm operation. We plan to visit RTTY Electronics to look around and buy spare parts on Monday, so that would be a good time to pick up the new gears I need. 

There is the complication that model 14 TDs had three different sets of gears for 60 wpm from a 60Hz AC synchronous motor. The motor pinion is always the same, but the main shift gear is one of three different part numbers. I am not sure which I should be buying, but hopefully I can spot the correct one when I look at them on Monday. 

We put the huge thyratron based model 30 power supply into service powering our teletype current loops as well as the perforator solenoids on the model 19 keyboard. Ken and Ed took lots of measurements of parts and watched the operation to understand this beast more thoroughly. 

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