Friday, June 29, 2018

Good progress with model 15/19 teletypes, plus restoration work on IBM 1401 card reader


Our German 1401 system has been down for weeks because its card reader (1402) is unable to read cards without spurious read checks or other errors. It was determined that a set of cams were chipped and broken, causing cards to be fed inaccurately. 

The reader has a hopper of incoming cards, with a set of picker knives at the bottom which have a lip that is less than the thickness of a punched card. These knives push the card toward the entry into the machine, the throat, which has a carefully adjusted slot that is also a bit less than one card thick. This causes one and only one card to enter on each clutched cycle of the machine.

To move the two picker knives at the proper time in a clutch cycle, a pair of offset cams are on a shaft inside the machine. These are constructed of Bakelite, an old, hard and brittle plastic. Two cams are molded together as a unit, but the cams are offset in rotation. One cam will push the knives forward to feed a card and the other cam will push the knives back out of the way.

The plastic cam assembly is chipped, with big enough gouges that it doesn't move the knives at the proper time anymore. The cam assembly is fixed onto the steel shaft with a tapered metal pin. We didn't have a spare cam assembly on hand.

However, the reader mechanism in a 1402 was leveraged into three different machines by IBM. In addition to the 1402, the IBM 360 generation card reader, the 2540, is a somewhat modernized version. The 088 card collator, used in pre-computer accounting machine installations, also uses this mechanism; actually, it has two of the readers, feeding from each side of the machine to common central stackers.

Besides the official collection of the museum, artifacts carefully stored and preserved, there can be units available for public demonstration and education purposes, as long as they are excess to the collection needs of the museum. We are fortunate to have multiple tape drives in this category, for example, from which we can take parts. We also had an 088 collator available for use as parts.

We visited our 088 carcass and examined the two reader mechanisms. The right side feed had some chips on the Bakelite cams, but the left side feed cam assembly was in great shape.

We found that each cam and shaft is pinned differently; each was hand reamed at the factory and sometimes the wide end of the taper was inserted from the back of the cam assembly instead of the front. Nothing was interchangeable without reaming the shaft plus cam assembly.

Mike, one of the newer restoration team members, did the reaming, installed a new tapered pin and cut it to length. He then replaced the shaft in the card reader and made the adjustments for the picker knife timing.

Because we disturbed the multiple CBs (cams with microswitches) on the same shaft, we must re-time all of them to the appropriate range of a rotation of the shaft. The logic in the card reader samples the state of the machine and activates various functions at specific times during the rotation of that clutched shaft. If the times are too far off, the machine detects the error and stops.

Next week we will start doing the timing, adjusting up to a dozen CB microswitches until everything is within specifications. We hope that this will restore the card reader to proper operation.


I decided to build a second instance of the John Nagle designed USB to teletype box. I had two spare PCBs built, but needed another set of the components to build it up. I placed the order and received almost everything. I soldered in the components tonight and will put it in the case once that arrives; otherwise it is complete.


We met at Marc's and resumed work on the model 15 systems. I showed Marc what he needed to do for his keyboard to free up the frozen levers on the keyboard serializer/distributor. He began cleaning and addressing issues with his keyboard.

Having cleaned and tested my ASR 33 Call Control Unit (power supplies and interconnection point for cables), I returned it and instead placed the model 33 printer unit in my car for work at home during the week. Once I have this unit working, my aim is to hook it up to my Altairduino (Altair clone) and to produce copies of key software on paper tape and then boot software and interact through the teletype.

My model 15 printer head has so much solidified grease that the main shaft woudn't turn by hand This one will take lots of time to flush out the gunk and free up the mechanisms. I worked on it and have already restored about 20 degrees of rotation to it.

I have pivot levers that drive the function and print mechanisms which are stuck. The clutches on the shaft are also sticky and need lubrication. The big question to me is whether I will need to remove the shaft to fix it or can work on the machine with it in place.

I put the spring back on the model 15 printer - the one that popped off when I was moving the printer when it hooked on my work gloves and pulled loose. It pulls a pivot lever back after the carriage has moved away from the left edge. The lever is connected to a 'dashpot', an air chamber shock absorber which cushions the carriage as it slams to the left edge during a carriage return operation.

My lever and dashpot were frozen in place, but with a bit of lubrication and lots of gentle manipulation, it is working properly and ready to protect during CR operations. My carriage itself, when released, only moves about 15-20 columns over before it sticks. I will be hunting for the cause of the sticking and repairing it.

We focused on the REC30 power supply that came with the model 19 military teletype system. This supply plugs into the 120V mains and produces a regulated 120VDC, supplying up to 600ma of current It is a massive system, much larger than you would expect for a 80W power supply.

Part of the size and weight is a huge autotransformer on the front end which allows use of the supply with a range of power inputs from 25V to 250V and multiple frequencies. It also provides 120VAC output for a motor regardless of the input voltage.

The supply circuitry uses a neon tube to establish a voltage reference, a pair of mercury filled thyratron tubes to produce the DC output, and a pentode tube to control the grids and regulate the output. It also has many chokes in place to block the switching noise of the thyratrons.

The thyratron tubes should have liquid mercury droplets inside which vaporize when the tube is hot, but instead we saw many loose grey flakes inside. That didn't give us confidence in the condition of the tubes but we pressed ahead.

I used my old Heathkit C3 condensor (capacitor) tester, which can check leakage at a range of voltages from 25V up to 450V. Often a capacitor seems fine under low voltage, as checked with a modern capacitor measurement device, but will develop excessive leakage only up at higher voltages. This unit also minimizes the inrush current as you check out these old capacitors.

We found all the capacitors to be good enough to use, acceptably low leakage even up at full voltage. The input to the autotransformer had a reasonable DC resistance, thus we prepared for a power-on test and possible release of magic smoke or worse.

We applied power and saw no signs of failure as the tube filaments heated and the time delay relay heated its bimetallic contacts to allow the thyratrons to vaporize their mercury before power was applied The relay snapped on and the thyratrons developed a lovely blue glow. More importantly, the output was 140VDC and nothing was overheating or smoking.

Initial power on of the supply
We set up a small load to let us measure the ripple and power supply noise as well as adjust the output to its target of 120V. The ripple was only a few hundred millivolts, cleaner than we expected. A quick rotation of the potentiometer and our supply was delivering exactly 120V DC.

I brought my teletype interface box, which we then connected to Marc's other model 15 teletype which appeared to be working properly, based on hand cycling of the machine. I fired up the BaudotRSS program through the John Nagle designed interface and we had the teletype chattering away typing text.

The ribbon is old and dry, but we could read just enough of the text to see that it was getting most of the characters right. We hadn't adjusted the rangefinder, a device that varies the 'sampling' point within each bit cell of the incoming serial stream. We can probably get this typing flawlessly without much more effort.

The Break button worked properly, as registered by the BaudotRSS program, but keypresses were not recognized at all. This might be a problem with oxidized contacts on the Send/Receive/Break switch on the unit. This will be investigated more during our next work session. It was time to head to homes for dinner with our families. 

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