Sunday, June 24, 2018

Good progress on the USB to teletype interface debugging, plus restoration work on TTY 15 and ASR33


I received two items last night, an alternative CP2102 USB module and a new USB cable. The replacement module has the same signals accessible, but in a different arrangement so that it can't be mounted on the project PCB with headers. Further, it is longer and has a full size USB A connector. However, with several failed boards behind me, I just wanted to have a working USB to UART board with 45 baud support.

I plugged new new board into my PC and it correctly installed and worked. I could adjust its settings using the Silicon Labs Studio development software, converting the 600 baud entry to 45 baud and increasing the current request to the USB host to 400ma to support the project's requirements.

That seemed to point to the original boards as defective, but this morning I tried using my new cable to connect an original board. It worked fine and was altered to 45 baud and 400ma. It seems that I had two bad USB cables with micro connectors - neither had working data lines I guess. The new cable works well. What a stupid reason to have lost a week of head scratching and failed tests.

The new board does turn on /SUSPEND, but after about 20 seconds it is switched off (suspended) until the COM port is opened causing it to spring back to life. This makes sense. Based on that behavior, I tested the original board to see if it turned on /SUSPEND when connected. It did also. 

Given that, I will solder the original USB module back on the project board and resume testing. I will step through the power and waveform tests, watching to see that the device delivers 5V to our main bus and the circuit seems to respond properly. I am NOT seeing the 120V that should be active on the test points while idle, although this requires the first MARK to SPACE transition on TX before it may start charging. Thus, I may have a problem but may not, see below.

Another test that can be done without a teletype is to feed TTY signals from a PC program over the USB cable, watching the LEDs to see that the board is responding appropriately. I am hooked up to Heavy Metal, configured to 45 baud and sending various test streams, but the TX light is not blinking. RX will only blink if I activate the keyboard, which I am not doing since it is disconnected.

It is likely that I have the configurations wrong in Heavy Metal and am not actually sending data over the USB link. It opens the port properly, as the /SUSPEND goes high and allows me power to my interface. I don't, however, see any SPACE values coming down the line. I need to debug this first, since I need to send characters before the charger circuit will develop the 120V reservoir.

The last test before hooking up a real teletype is to short the output and measure the current being delivered - this should be 60ma. That indicates that the proper voltages and currents are being developed. I might be able to do this during the steady MARK condition even with no data coming over the USB link. A project for tomorrow. 


I brought home the power supplies (the Call Control Unit as named by Teletype) to check it out and restore it if necessary. First up was to use my old Heathkit Condensor Checker, which does leakage tests at varying voltages up to the limit of each capacitor. There were three big electrolytics to test. One of them is marginal and I will replace it later, but all are good enough to run with for now.

First power on tested the main power switch, which has three positions - Local, Off, Line. When in the Line position, a relay loudly activates, as it should, but not in Local mode. I then tried to note all the connector points where I should be measuring power - various AC and DC voltages are developed in this unit.

The documentation is far from clear. No way to easily spot where various voltages or signals are placed on connectors. Finally I came to understand what "R 5C4" and "3A5 or 4A6" meant. The first number is the relative schematic page, titled as "Sheet 3" for example. The remaining characters are the row and column on the page where the wire occurs. 

I found an exceptionally obscure card, looking like it was not made by Teletype, wired into my unit. It takes two wires from the DB-25 computer connector, pins 5 and 6 which are green and brown, to the coil of a reed relay. Thus, some power from those wires will energize the reed relay. 

The contacts of the relay bridge the L1 and L2 poles on the main LOCAL/OFF/LINE switch, thus making the unit both online and local simultaneously. I believe it forces the teletype to be online, even if the rotary knob is set to Local. The modification also provides power while in Local mode to pin 7 of J4 - purpose not yet known but will check when I have the rest of the teletype on hand. 

The only power generated in the Call Control Unit and its modules is:

  • 120V to the motors when in Local or Line mode
  • DC power, 34V to drive the selector magnet for the printer, based on line or keyboard current
  • DC power to drive the current loop line from the automatic tape reader (120V or higher)
  • DC power to drive the internal current loop in local mode (67V) since it is half wave rectified

I verified all but the automatic tape reader supply, using my VOM, but that didn't work for the ASR power since it is driven through a plug on the reader cable that is not at my house at this time. However, I checked all the components with my VOM and diode tester, until I was satisfied that it should work properly when connected. 

At this point, I have finished the checkout and restoration of this second portion of the teletype, the Call Control Unit. The remaining units to test are the tape reader, tape punch and printer units, which will wait until Friday when we get together for the next burst of Teletype restoration.


We lined up all the model 15 and model 19 teletype machines and began with a good cleaning to remove dirt and lubricants. This consisted of mainly scrubbing with Simple Green but some Alcohol and other cleaners were needed for a few special places.

My Model 15 and Marc's Model 19 have similar motor baseplates and we worked on them first. They have two power plugs and two telephone exchange style plugs. One power plug consists of three prongs in a triangular arrangement. It is used to supply 120V DC for the current loop signal lines. The other is a four prong outlet, three that are oriented in the same direction and the fourth rotated 90 degrees, used for 120V AC to run the motor.

We studied the schematics and wiring to determine that the four prong plug is arranged with ground on the one plug that is perpendicular to the others. The three that share an orientation are arranged at the other four corners of a Rhombus shape. The corner opposite the ground pin is the neutral line. The two other pins are both for the hot leg of single phase 120VAC.

There is a difference inside the teletype base. One of the hot pins runs through the ON/OFF switch and motor control relays, while the other is routed directly to the motor. Thus we call them the switched and unswitched lines; we plan to only use the switched hot wire.

The Model 19 desk had several of the four prong receptacles into which this plug will fit. Marc took one and wired it into a handybox to give us a proper outlet to plug in our teletypes. Before we ran them, however, we re-oiled the bearings and worked out stale lubricants. Finally, we could plug in both baseplates, flip on the power switch and watch the motor spin happily away.

I moved on to work on my keyboard mechanism. This is built with five rods (one per bit of the 5 bit teleprinter or Baudot code). The keys on the keyboard each have a rod, oriented perpendicularly to the five encoding rods, with the key rod having notches that will force each of the encoding rods left or right, depending on whether that bit of the character could should be a MARK or a SPACE.

My keys worked well and those encoding rods did slide left or right reliably to encode the character. What did not work was the distributor that should serialize those rods into a sequence of a start bit, the five data bits in order, and a longer stop bit. To do this, a clutch latch has to be triggered by a sixth bar, similar to the fire encoding rods but always depressed - thus called a universal lever.

When the clutch is released, the distributor should rotate 360 degrees under motor power and latch again, waiting for the next keypress. Multiple cams on the axle operate switch contacts at the appropriate point during the rotation. First, breaking the circuit to produce the SPACE value of a start bit. Then, either making or breaking the circuit based on whether each bit in turn should be MARK or SPACE, then generating a MARK signal for the remainder of the rotation as the stop bit.

The cams push on levers that can be allowed to swing out to make contact or blocked, depending on the left or right position of each encoding rod. Those levers pivot on a shaft, or should have, but they were corroded into place.

I had to use 3 in 1 oil and about an hour of time patiently wiggling and rocking each of the six levers (five data bits plus the start/stop contact) until they moved freely. When I was done, I had also lubricated the shaft bearings and had the clutch release working well with the universal lever. Now, I could push any key and trigger a rotation with the right sequence of on and off current flow to encode that character. My keyboard was restored and ready to use.

We were near the end of the workday, permitting just enough time to assess the other parts and prioritize next week's work. Marc's model 15 (19) keyboard had the same frozen levers that mine had, so this will need an hour or so of work to free up. His printer unit main shaft rotates, which is a good sign. My model 15 printer unit has a main shaft that won't turn at all - frozen lubrication strikes again. 

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