Wednesday, August 1, 2018

ASR 33 repaired and ready for show; testing all demos to be ready for exhibition


With my line feed and other issues fixed, I ran through the Altairduino (Altair 8800 clone) demonstrations to verify they all work. I then ran the non-tape based SBC 6120 (PDP-8 clone) demonstrations to validate them.

If I complete the reader control modification to the SBC6120 and teletype, I will test a paper tape based demo for that machine after the modification is working.


I shaved down the line feed repair part I made from a rubber deck shoe sole and put it onto the teletype. It works perfectly! Linefeed is back in operation and I could turn my attention to the oddball behavior associated with a toasting electronics smell.

I did some differential diagnosis and discovered that the problems all stem from the paper tape reader. The reader engages to try to read a character, even if the reader control is set to Stop or Free, pushing up the pins and generating bits based on the hole pattern detected by the pins.

With no tape in the reader, the pins are all up and this is detected as a character 11111111 which is a rubout. If I put in a blank section of tape, the pins are all down and it is detected as an idle character 00000000. The reader pushes the pins up when the teletype is plugged in, even without turning the power switch to either Local or Line.

Whether I flip the switch to Start or Stop, the reader sits partway through a cycle with the pins pushing up. With the switch in Start, if I hit any key on the keyboard to trip the distributor, that also advances the paper tape. Thus, the paper tape reader is engaging when it shouldn't with power switch OFF, it doesn't trip the distributor clutch to complete its cycle, and this ORs in bits with valid keyboard encodings.

Since the paper tape reader is an important part of my planned demonstrations, I had to get this fixed.  The operational description in the model 33 documentation was a good starting point, where I could look over how the mechanisms related to paper tape reading work.

From that background, the symptoms led me right to the failing area. There is a microswitch that is closed when the paper tape reader trips the distributor clutch to encode the character sensed by the reader. The microswitch then energizes a big solenoid that drives the pins up through holes in the paper tape and after release will move the tape forward one position.

To have the pins sticking up and the holes jamming in data bits, the big solenoid had to be on permanently. It switched on when the plug was inserted into the wall outlet, without turning on the teletype to Local or Line. That meant the switch was closed. The last symptom, that the paper tape reader didn't advance until keyboard keys were pressed, showed that the clutch trip coil was not tripping the distributor clutch.

I looked at the clutch trip coil and related microswitch. The armature of the trip coil was out of position, so it would not move to trip the clutch. Further, it was allowing the microswitch to close. I moved it back into position and tested the machine again.

Everything now works! The toasty smell was heated resistors in the power supply, since the big solenoid in the reader is intended to be powered intermittently, not steadily as occurred with this defect. No more heat or toasty smell now. 

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