The paper tape reader on the left side of the model 33 ASR teletype has a power supply that feeds it 160VDC for the magnet that advances the tape. In addition, if the paper tape reader is the automatic type rather than the manual version, it has a 48V DC supply and a control relay included alongside the 160VDC circuits.
While the power supply for the paper tape reader can be mounted up in the body of the teletype, often it was mounted down inside the pedestal stand. That freed up room for the various communications options such as a telephone dialer. It is not uncommon for someone to pick up an ASR-33 for restoration but not receive the pedestal stand. The teletype will work just fine sitting on a tabletop, except that the reader power supply will be missing, disabling the reader capability.
DESIGNED A SUBSTITUTE POWER SUPPLY FOR WHEN THE ORIGINAL ONE IS MISSING
I designed a printed circuit board based on the schematics from Teletype so that it could be connected into an ASR-33 and allow the reader to operate. The circuit is the same, although some of the components are compatible but not identical. I didn't try to match the shape and mounting exactly, since I didn't have good measurements available.
The ASR-33 models come with a cable that has a 15 pin Molex socket on the end, meant to plug into a 15 pin Molex receptacle on the power supply card. I build my card with a 15 pin Molex connector that is hanging from the wiring coming from my card, allowing the owner of the teletype to just plug this in and have it work just like the original.
SOLDERED TOGETHER BOTH TYPES - AUTOMATIC AND MANUAL READER VERSIONS
The manual paper tape reader version of the model 33 is operated by moving a switch on the paper tape reader to the Start position, which begins the process of reading the tape until it runs out, jams or the operator turns the switch to the Stop position. The automatic version supports a version of the switch on the tape reader that has a momentary Start position, a 'Run' position, and a momentary Stop position. In addition, if the teletype receives an XON (DC1) character, it will start reading the tape. If it receives an XOFF (DC3) character or a WHOIS (ENQ) character, it stops the paper tape reading process.
Making a manual reader version of the power supply is done by omiting components necessary for the automatic function, such as a control relay, and wiring up fewer pins on the 15 pin Molex connector. Otherwise, the same board is used. I built two supplies for an automatic reader model and two supplies for a manual reader model, in support of someone restoring teletypes for a museum.
BENCH TESTED THE SUPPLIES
My first test was of the section that implements the relay for the automatic reader version. This is powered by 48VAC, rectifies it with a single diode and that closes the relay. Two contacts on the relay are closed when the coil is activated, which I could check with a continuity tester. The relay began chattering, not fully closing. I finally noticed that I hadn't connected a third wire from the board to one side of the 48VAC.
One side of the incoming AC goes through the rectifier and on to the relay coil. The other side of the relay coil is only connected to the other side of the AC input when the relay is activated. The filter capacitor across the output of the diode, however, is always connected to the other side of the incoming AC, thus there are three connections to 48VAC not two from the board.
That caused the chattering because nothing was smoothing the 30 pulses per second of 48VDC through the coil. I added the third connection, so that one side of 48VAC went to the diode and the other side went to the other two connections; this would activate the relay with a smoother DC voltage. I tried this wiring and the relay reliably activated and released.
I picked up a 115VAC relay to act in place of the distributor trip magnet that fires off the transmission of a character that has been sensed by the paper tape reader. I also used it to act in place of the feed magnet that pulls the tape forward one position since I don't have a teletype on hand for testing.
The process of reading begins with the circuit energizing the distributor trip magnet. The distributor in the teletype converts a 6 bit character from parallel into a serial stream with a start bit, the six data bits and the stop bits. This is done with a rotary mechanism. At the end of the rotation, the distributor activates a switch that closes the circuit to power the feed magnet in the tape reader. This causes the tape to advance one position, after which the distributor trip magnet is again energized unless we are stopping the reader.
I used the inexpensive relay I bought to observe the operation of the circuitry, verifying that closing a switch (simulating the end of a rotation of the distributor) will energize the relay that is simulating the feed magnet. This section is driven by the circuit that rectifies the 115VAC into 115VDC to activate the feed magnet.
Testing the 115VDC circuit, I saw the current limiting resistor emit a burst of light as a thin slice vaporized. This is a 22 ohm resistor in the input line to the bridge rectifier, whose peak load is when the AC mains are first turned on and the filter capacitor of the bridge rectifier is charging up. The surge is very brief but will definitely blow out a normal resistor type which is what I used. Wirewound resistors are typically used as they can absorb the very brief surge without failure.
I will install 5W wirewound resistors on the boards instead of the surface mount part I originally used. There is a good surface mount part that I could have put on there, but it would require a new version of the PCB since it is larger than the footprint of the original part I used. The original was a 16W component with a very large SMD size already, but less than the wirewound SMD part requires.
For the automatic version of the power supply I operated some switches that stand in for the momentary Start, the momentary Stop, as well as the XON, XOFF and WHOIS detection switches. I checked that the relay on the power supply board activates and turns off appropriately based on those simulated switches. I also verified that the distributor trip magnet (the relay simulating it) activates when the control relay is turned on and does not activate when it is turned off.
Once the functions were validated and the voltages and currents verified, and the new resistors installed, it will be time to pack these up and ship them off to the restorer who is waiting for them to make use of the paper tape reader.
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