The Teletype ASR-33 comes with a paper tape reader attached to the left side of the body. There are two flavors of the tape reader - manual and automatic. The manual one is started by the operator flipping a lever to start and stopped the same way unless it runs out of tape first. The automatic one can also be started by characters sent over the serial line from a remote system. The XON character, called DC1 by Teletype, starts the tape reader and receipt of an XOFF, called DC3, stops the process. The ENQ character (named WHOIS by Teletype) will also stop the tape reader since it triggers the machine to transmit a sequence that is hard coded into a drum inside the body.
The power supply for the manual version of the tape reader has to power the feed magnet, a coil in the reader that drives the tape forward one sprocket hole each time it is energized and released. The power supply rectifies the 115VAC building power and creates 150+VDC to activate the feed magnet. A switch inside the body of the teletype is triggered each time the distributor is released and makes one revolution. The distributor converts parallel data, from the keyboard or the reader, into a serial bit stream for transmission.
The start switch on the manual reader activates another magnet which triggers the distributor to turn, serializing and outputing the character that the reader just sensed. When the switch is turned to stop, or errors such as end of tape or tape too tight occur, the magnet stops activating the distributor and once it finishes its last rotation, the feed magnet stops advancing the paper tape.
The automatic reader implements a relay driven by rectifying the 48VAC supply inside the teletype. The reader power supply takes in the 48VAC from the other power supply, makes it DC and limits the current. Contacts on the relay form part of the logic that controls when the reader will run or stop.
The relay can be activated by the switch on the reader, much like the manual reader operation, but can also be activated by the XON character being decoded by the printing part of the teletype because it received that character from the remote system. The relay keeps itself latched until one of a few things break the circuit to deactivate it. The switch on the reader can be moved to stop. The XOFF or ENQ character can be received by the printer. Various errors such as tape too tight or tape running out can also stop the reader.
ASR-33 Teletypes come in many configurations in addition to the chose of automatic or manual reader. They can have phone dials, interfaces to advanced communications systems, or just hook up to a current loop serial interface. Depending on those other options, the power supply for the paper tape reader can be installed in the space that holds those other functions, or it can be placed down inside the stand on which the body of the Teletype sits.
The units that are being restored came without the stands. They will work just fine sitting on a tabletop, but the automatic reader one lacks the reader power supply that was in the missing stand. The restorer was happy to look for additional teletypes or parts that might have the reader power supply up in the body, but that was currently holding up completion of the restoration.
I DESIGNED A REPLACEMENT POWER SUPPLY FOR THE AUTOMATIC READER ASR-33
The schematics from Teletype show the power supply board and all the components, with their values. It was easy to whip up the schematic in KiCAD and then design a PCB to implement the same circuit. I located all the components needed and put them in a cart on Digikey.com, then looked at the cost to fabricate the PCB using JLCPCB.
The PCB order comes with five boards and that means I could build up to five power supplies. The manual reader version leaves out a number of components, including the relay, but can otherwise be built using the exact same PCB I had manufactured. The per board cost for components run from $22 for a manual reader to $31 for an automatic reader type, with the shipping from Digikey amortized over the number of boards I build.
I am waiting to order components while the restorer assesses how many of each he might need. Within a couple of weeks I will be able to complete these and get his restoration back on track.
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| Automatic Reader power supply board |
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