More typewriter work but progress is made

The typewriter mechanism I am adapting, an IBM Electronic Typewriter model 50, has one problem caused by solidification of the lubricants inside part of the mechanism. The oil and grease used on the Selectrics and on these models become a sludge after years of sitting still. If the machine is used regularly, this tends to re-liquify the lubricants and postpone problems, but it is extremely common for someone to haul a Selectric out of a closet, turn it on and find it jammed or malfunctioning. This is all caused by the lubricant issue. These machines were designed to be cleaned and relubricated yearly, something reasonable in an office environment when the typewriters were leased and under service contract, but not something most owners today get done. Dust and other contaminants add to the problem, of course.

I have relubricated and cleaned the machine, but problems remain in the interior of the "pin block", a mechanical binary to decimal decoder used to activate two plastic rotary cylindrical cams that tilt and rotate the typehead. To cut down on the need for expensive solenoids, and save space, the desired amount of tilt or rotate is commanded by two tilt and three rotate solenoids. The binary value determines the selected amount of tilt or rotate.

Pin block and rotary selection cams

For example, without a tilt solenoid firing the typehead is at tilt 0 position, with T1 fired it moves to tilt 1, T2 solenoid moves it to tilt 2, both T1 and T2 activated (a binary value of 11 - equal to decimal 3) choose the tilt 3 level. The pin block converts the two T values into three pins - decimal values 1, 2 and 3 - that will move the head from the default tilt 0 to tilt levels 1, 2 or 3.

Decoded request for a tilt pin to move to tilt 2 position

The rotate cam takes a binary value from the solenoids R1, R2 and R3 and from them selects one of six valid decimal values (out of 7 nonzero values that could have been decoded). This activates one of six pins, but the machine needed only three solenoids to select the rotate pin. The pin block is the mechanical logic that converts binary input (solenoid activations) into a single tilt pin (or none) and a single rotate pin (or none). The typewriter has a rotate 0 position when no pin is selected, as well as rotate 1, 2, 3, 4, and 5 levels. A separate solenoid (rack shift) sets the direction of rotation, as the rotate can be + or - those values, e.g -3 or +5. Five pins are needed for the five rotate steps, the sixth pin activates a case shift, flipping the ball 180 degrees to switch between upper case and lower case characters.

The pin block sludge means it does not reliably activate pins, particularly for rotation, and the solenoid does not snap back reliably when the activation signal is removed. I was unable to fix them by external oil application and hand movement, thus I had to remove the pin block from the carrier and clean it out by flushing and partial dis-assembly.

Half the problem was oil that had gotten into the barrel of the solenoids, the surface tension of the oil acted as a resistance on the slug sliding in and out. I removed them, wiped them down with alcohol, and repeated a few times until the motion was uninhibited. The pins themselves and levers are now clean, clear and well oiled. I reassembled and readjusted it on the carrier, before performing a few checks.

I seem to have a channel worn into the surface of the rotation cam for pins 4 and 5, from when the mechanism wasn't cleanly resetting the pins. The rotate 5 pin is jumping into that accidental groove, thus not moving the cam over, when it should follow the intended groove which would slide the cam over to cause rotation of the type head. Spare parts are almost impossible to locate, which means my options are limited. I have to either find a way of repairing the cam, find a way to convince the pin not to jump into the false groove, or find another electronic typewriter whose cam and pin block is in good working order; if not, it will be very difficult to make this work properly - certainly it will not be able to type any characters at the +5 rotate or -5 rotate portions of the type ball. Even if the remaining 80 characters are sufficient to fit the ones used by 1130, I would need to create a custom typeball to match the relocated positions.

Since the solenoids and pin block are still a bit fussy, I will make a more complete disassembly and clean it very thoroughly, then remove any burs or other impediments to smooth movement of every single part of the darned mechanism. Perhaps with these changes it will ignore the false groove and that issue will be moot.

Former IBM employees who repaired these models have provided plenty of help - in particular, they note that the cams are manufactured with those shallow false grooves, for some unknown reason. This does help swing the spotlight of suspicion more firmly on the pinblock, whose parts I will microscopically examine, clear up any anomalies, and then reassemble it. My intent is to get a very crisp, fast acting and smooth mechanism, which I can then test in the machine.

I used some waterstones (8000 grit equivalent) to dress the edge that slides along the 'bottom' of the pin block (the face opposite the open area where the solenoids are mounted), lightly oiled and reassembled. Action seems cleaner and crisper, will have to remount to verify.

I have found two sources with replacement cams, thus if the worn cam is any part of the problem I can switch out the bad part. I first need to complete my pinblock work, at least until the cause of the problem is definitively known. If all in the cams, a replacement is easy. If the pin block is bad, I need to investigate availability of replacement pinblocks. Other possibilities include misadjustment and defects in other parts of the carrier mechanism.

On another front, the driver logic for the 1130 display panel that has over 100 lights flashing the contents of registers and state of the machine is finally working perfectly. I had some strange issues including transmission line reflections, sagging power supply lines because my test bench connection to the board used thin wires (doh) whose resistance dropped the effective voltage whenever a bunch of LEDs drew current, and a default output signal voltage for the fpga of 2.5v even though it operates at 3.3, which was too low to be a valid 1 for many of my connected boards. I don't want to remember how many times I restructured my fpga driver logic or did research on the protocol with other devices, or made futile diagnostic changes, when the issues were the unsuspected analog electric effects.

No comments:

Post a Comment