Saturday, July 7, 2018

ASR 33 together and mostly working properly; progress on model 15 teletype keyboards


I brought the printer unit to Marc's basement and put the entire unit together. Initially I found that the local loop from keyboard to printer wasn't working, but that ended up as a configuration error on the rear panel of the Call Control Unit.

When switched to Line mode, the printer chatters (as it should) because the line is in a SPACE condition which is interpreted as a continuous stream of null characters. Switched to Local mode and it quiets right down, waiting for a keystroke, Answer Back operation or paper tape reader input.

When I hit the Here Is key, it triggers the Answer Back drum, firing off the characters on the drum. Since it is a new drum with nothing set up, it produces a series of null characters, making the printer chatter while it runs.

When I push down on the tape testing pin on the tape reader and set it to Start mode, it interprets the tape as an invalid character, treated as a null causing chatter. Once I produce a tape or find some existing paper tape code to read, I can verify the operation of the reader.

I began pushing keys which printed the selected character on the paper. CR, LF and BELL worked fine as well. The problem I faced was sporadic failure to respond, a period when no key would trigger the machine to serialize the key value. After a bit of time, the keyboard again responded.

I did find that slight tapes on the right front side of the teletype cover would often prod the keyboard to respond again. This may be an issue with the alignment of the keyboard module relative to the printer unit, something I can play with tomorrow.

I can't test the paper tape punch yet because of the broken arm that would be used to tension the spring which puts pressure on the incoming tape. No tension means the tape won't be moved forward as it punches. This is a high priority to repair, first with a temporary workaround and later with a correct replacement part. 


Marc's model 19 has a keyboard with a built in tape perforator and character counter. The operator could punch tape from keystrokes, without seeing it echoed back on the printer. The column counter will show how far across the line one is, allowing judicious CR and LF to be added once the line is too long.

After a careful cleanup and oiling, Marc set to adjusting the punch. Several adjustments had to be tweaked to cause the perforator to punch correct patterns and advance the tape smoothly. These machines used oiled paper; the oil helps keep the punches lubricated.

To check out the operation one needs the keyboard mechanism to be rotated by a motor at a reasonable speed to produce characters at 45.5 baud. The base unit of the teletype has a motor built in and the keyboard slides into the base, but the gears don't mesh. The printer unit must be placed on the base, as the main axle of the printer includes gears to engage both the motor and the keyboard.

We didn't have a printer mechanism that is ready to run under power, as they are all nearly frozen or maladjusted. Marc put together a temporary stand and coupling to spin the keyboard shaft with a small motor he could power with a variable voltage power supply. This allowed the keyboard mechanism to rotate at the proper speed, waiting keystrokes to fire the clutch and serialize each character.

The perforator is attached to the keyboard and the same code bars that set up the five bits for serializing and transmitting will also set up the interposers on the punch unit. If a code bar is in the MARK condition, the interposer is in between the punch rod and the armature of a solenoid. Those with SPACE conditions have no interposer. When the armature moves upward, it pushes the punch rod up into the die for every position where a hole is needed, thanks to the interposer.

The solenoid that rams the punch rods upwards requires about half an amp of current - it must be powered with a relatively high DC voltage. Marc used a 100V DC power supply with current limiting to drive the solenoid. The solenoid is triggered by a microswitch on the rotating keyboard encoder shaft.

To start the tape, one has to punch the small sprocket holes that sit between columns 2 and 3 - this is used to pull tape forward on both the punch and on readers. The bottom right key, with nothing printed on it, is the BLANK key and it punches a character with all five bits set to SPACE.

The BLANK produces just the sprocket hole on the tape and a series of these produces the leader before you begin punching the actual output message. The opposite character to BLANK is the LTRS (letters) key, whose code is five bits of MARK. This punches every hole in the tape.

After the adjustment to punch all holes with LTRS and no indentations at all for the 5 bits with BLANK, we tried some other characters to check the proper encoding. The ITA2 or USTTY encoding used with these 5 bit machines has two useful characters - R and Y - whose patterns are 10101 and 01010. We got those correctly, as well as general typing rendered properly.

Th column counter is not working yet. We didn't have time to debug it. Another microswitch on the keyboard encoder axle should advance the counter, while it should reset to zero when a CR is encoded (01000). We haven't looked at the mechanism yet to learn how it should work, nor to diagnose its current fault.

Our oscilloscope was hooked up to monitor the serialized output from the keyboard encoder. We saw that our bit timing is very close to the nominal 22 ms produced by 45.5 baud operation. Further, we saw properly encoded characters for everything we tried. BLANK has five SPACE bits, LTRS had five MARK bits, while R and Y alternated the patterns of 10101 and 01010.

Marc and I visited the spray shop that is sandblasting all the external metal parts of the model 19 and then powder coating them. Powder Coating bakes on a color finish in an oven after the color powder is sprayed and sticks electrostatically to the metal part. A very high voltage charge is applied to the spray gun and the metal parts are grounded.

The parts looked superb after the sand blasting - it removed the dings and holes which were essentially all in the layers of paint that were accreted during the machine's years of Navy service. Marc chose a black with a fine texture. We are looking forward to completion of the power coating on the 39 separate metal items that are part of this model 19. 

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