Wednesday, August 1, 2018

Preparing for VCF West exhibition, disaster strikes on ASR 33, work continues on model 15


As soon as I arrived home from a trip to visit family on the east coast, I worked on the modification to allow the SBC 6120 (PDP 8 clone) to drive the Reader Run Control board on the ASR 33 teletype, allowing the software (e.g. OS/8 and its apps) to start and stop the paper tape reader, avoiding overruns.

I want the inverse of the state of the Data Ready pin of the Harris 6402 UART chip to drive the relay of the Reader Run Control board. The board in the teletype is designed to operate a reed relay when the control line is active with 12V (RS 232 levels). The chip, of course, uses TTL logic levels of 0 or +5V.

My board has to have an inverter to reverse the sense of the pin - since I want the motor to run only when data is NOT ready - and then something to convert to 12V and drive enough current for the teletype board.

I could use a MAX3232 board to convert the TTL level to RS232 voltage, but I am not certain that it can handle the current demands of the reed relay.

The other and simpler choice it to use a small relay board to switch a 12V supply  The relay board doesn't demand much, thus the inverter chip will easily drive it. The only complication for my exhibit is that I need a 12V supply in addition to the 3.3V supply that handles the Altairduino connection. 

The other major work I needed was a replacement for the disintegrated plastic piece in the line feed mechanism of the ASR 33. Apparently this piece which spaces a lever below a stop tab has a very narrow range of heights that will work properly.

The proper part is devilishly hard to spot on the parts catalog, especially because the plastic portion is not an independently ordered unit. Therefore, the metal bracket comes with the plastic in place, under part number 187078 for those who are also trying to locate this.

Too short and the line feed does not occur. Too high and it produces spurious line feeds. I have to get it to about 1mm of the correct size to have the teletype work properly. I decided to use some acrylic and machine it with a Dremel tool. I can start with it a bit high and grind off a bit at a time until it works perfectly.

That proved much harder to execute than in my imagination, particularly because the acrylic melts when being cut, drilled or ground. I had to abandon that and try with other methods, such as my original bit of shoe sole trimmed down a bit or with sets of rubber grommets. The sets are a bit too high, but a single grommet is too low.

While I was trying to work on the line feed issue, I began to smell a toasty electrical smell from the CCU (power supply area on the right of the teletype). Operation began to degrade. When switched to Local mode, typed input isn't being decoded correctly, instead every keystroke acts like a Rubout.

When in Line mode with the Altairduino driving it, print occurs normally and correctly, but my keyboard responses are being corrupted. I thought it would be useful to attempt to read a paper tape in either local or line mode, so I installed one of my demo tapes.

The paper tape reader didn't run when switched to Start. It did appear to be blocking the keyboard input, as if the paper tape reader was jamming in bits on top of the codes coming from my keyboard.

This is fairly disastrous if I can't find and fix the problems before Friday, as the ASR 33 is the center focus of the entire exhibit. I will need to think about the symptoms and study the documentation in the hope that I can narrow this down enough to locate the failure point.

I will need to pick up my model 15 teletype and get it over her, so that everything is staged for an easy move over to the museum on Friday when we set up for VCF. Not sure when I will have time to do that. 


While I was away, Marc and Ken continued to work on the two model 15 and one model 19 teletypes. Marc's model 19 printer is typing clearly and accurately. My model 15 printer types clearly and accurately. Marc's other model 15 printer is typing with mis-selections and it appears that it needed 70ma not 60ma to work.

Marc has built a box to hook together two teletypes and supply 60ma of 120V DC, letting him type on the keyboard of one unit and print on the other unit. It allows him to test the machines. He discovered that his other model 15 had the selector magnets incompetently repaired, with one of the two electromagnets wired in reverse. He fixed this and all three printers now work perfectly.

Work remains on the model 19 reassembly and on the two model 14 Transmitter Distributor boxes (paper tape reader). This won't take long to complete. Since the desk of the model 19 is back from the powercoating and looking great, we do have to replace all the wiring inside and start assembling the entire model 19 system. 

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