Saturday, August 11, 2018

Progress on model 14 Transmitter Distributor restoration and a motor investigation


I disassembled my model 14 and oiled everything on the main shaft in preparation for trying out a potential new gear set. The gear I received from RTTY Electronics needed some holes drilled but was otherwise cut with the necessary number of teeth to gear the synchronous motor down to the 60 wpm rate we wanted.

Marc had some gear sets he received with his teletype, but none of the motor pinions fit my gear properly. It meshes perfectly with the 80166 gear on his TD and that is the part I still need to make this unit work at the intended rate, rather than the faster 75 wpm rate it was geared to produce. 

The clutch on my TD is a later type, a drum clutch, unlike the friction clutches on most model 14 TDs including Marc's unit. This has shoes that expand under spring pressure to rub against the inside of the drum when the clutch is engaged, then they retract when the clutch is disengaged after one rotation of the shaft. 

After lubricating all the parts I tried to reassemble it. There is some trick I have to learn to compress the shoes in order to fit them up inside the drum; I wasn't successful on Friday in figuring out the needed technique. 

I partially reassembled my TD and set it aside for when I have a new pinion gear, at which time I can also complete reassembly of the drum clutch. If I don't find or make a suitable pinion gear for 60 wpm, then I will need to reassemble the old 75 wpm gear set and make use of an alternate method to slow down the TD. 

I have a number of methods that could be used to make my model 14 TD produce teletype serial streams at the 60 wpm rate:

  • buy or make the pinion to mate with my new main shaft gear to operation at 60 wpm
  • use a variable frequency drive to slow the frequency of the mains down and slow the motor
  • build a box that will accept and buffer data at 75 wpm and drive a 60 ma current loop at 60 wpm

Marc worked on his model 14 TD. It had been disassembled to send the base and covers out to be powder coated. With the coated parts on hand, he began assembly of the unit. Most parts went together easily and the wiring was obvious. The wires to the main contact plate, however, don't match the schematics or other TD units, which was where he stopped at the end of the day. 

Ed had bought a synchronous teletype motor to disassemble, as he was curious to see its construction including the motor start winding and its centrifugal switch. He pulled it apart although it was not obvious that the bearing on one end would pull the centrifugal switch mounting bracket off with it. This stripped out the two small screws that hold the switch to the end plate.

Ed picked up a replacement plate and screws from RTTY Electronics. While there, he also picked up a new set of brushes for my TD. The brushes slide on the brass rings on a disc, making contact to route the various pinhole microswitch contacts from the paper tape reader to the serial output line. 

The brush that rides on the segmented ring, the one that breaks a rotation into a start bit, five data bits, and 1.42 stop bits, was worn down quite a bit by the action of the slots between segments. The other ring is continuous and its brush was much less worn. 

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