ASR/33 NUMBER ONE - MOSTLY WORKING NOW
David brought his teletypes over to my workshop where we could jointly work on them to get them working properly. He can attach them to his vintage DEC equipment once this happens. Turns out he has multiple units, which greatly increases the chances he can get one or two fully working machines.
The system he designated number one was given the most attention by us. It took some time to get the keyboard adjusted properly. On a teletype, when you push down a key, it rotate a lever which connects through an H shaped plate to another lever that eventually trips the clutch to fire off one cycle of the distributor.
The distributor is a parallel to serial converter. The keyboard emits eight signals - bits - which encode an ASCII character. The distributor converts those eight bits into a stream of a start bit, seven data bits, one parity bit and a stop bit. The clutch needs to be tripped for every key press in order to send out that data serially. At the end of the distributor cycle it resets the keyboard so the next key can be depressed.
The mechanism has to be carefully adjusted so that the clutch is reliably tripped but only once per keypress. That involves moving a bracket carefully with two screwdrivers until the desired behavior ensures. I tweaked this on the keyboard.
However, several things have to happen correctly. The keyboard encoder, a set of wire contacts and levers which make or break the contact with fixed terminals, has to produce the right bit values for the key selected. We used a continuity checker to test this and found that it was not working well.
After some tweaking of contacts and adjustment of the position of the terminal block, the parallel data bits from the keyboard were correct. The next step is that the contacts on the distributor have to work properly so that the serial output matches that ASCII character. Fortunately that worked well. With the power supply generating the 20ma current source, we had a teletype that sends keypresses out correctly.
Putting the machine in 'Local' mode will connect the sent data to the receive circuit, ultimately driving the machinery to type characters or perform functions like line feed. In order for that to work properly, many things must perform correctly. The receiver driver board has to drive 48V to the solenoid based on the incoming current (or the looped data from the send circuit).
The reception clutch sits at the stop position while the current is flowing - a condition called Mark in teletype circuits. As soon as the start bit arrives, a Space condition meaning interrupted current, the clutch is tripped and begins rotating. As each bit arrives serially, the solenoid is either attracted in the Mark condition or loose in the Space condition. That sets up levers in the decoder machinery, one lever for each of the bits arriving. Finally, the stop bit is a Mark and causes the clutch to hit the stop tab and end the reception cycle for that character.
After the bits are set up, they move a number of long side to side levers which have notches in some locations. Those levers also are connected up to the typehead where they control the rotation and elevation of the cylindrical print drum. If the code is a printable character, the typehead, having rotated and elevated to position, is fired forward to slam into the inked ribbon and onto the paper behind it.
If the notches in the long levers line up under a particular front to back lever, it drops down and engages some function. That might be carriage return, linefeed, bell, space or something else. Thus the long levers encode functions and the front to back levers are installed for each function that is implemented on this particular teletype.
Functions like line feed, space or bell trip another latch and cause power to produce the intended action. Thus, for the output side to work properly, we need the solenoid, decoding, levers, typehead movement, function bar selection and the eventual powered action to all work properly.
In addition to the above, an ASR/33 has paper tape punch and reader components. The punch side is mechanically connected to the decoder, thus every cycle tripped by an incoming start bit will set up the long levers but also the punch levers, at the end of which the pins that are selected push through the paper to form the eight bit pattern for the same character that was just received.
Paper tape can be threaded onto the reader, which is why this model is called Automatic Send Receive (ASR) compared to the simpler Keyboard Send Receive models that don't have a reader. The reader is fed 150V DC to a solenoid. It pulls the tape forward one position and electrically trips the distributor clutch (the same one that is tripped by a keyboard press. The eight pins on the reader are wired in parallel to the keyboard, so that what is on the tape at this position is sent to the distributor, serialized and send out by the teletype.
By the end of the weekend, here is the condition of teletype unit one:
- Keyboard is working almost perfectly except for the Break key which binds and won't work
- Send unit works perfectly and sends out proper serial ASCII
- Current loop works properly at 20ma
- The receive unit and its solenoid work properly decoding each character correctly
- The typehead rotates and elevates to the correct character
- The ribbon advances after each press
- Functions like bell, space, line feed and carriage return work properly
- The paper tape punch works correctly
The carriage return is powered by a spring wound up as the carriage is spaced to the right during typing of a line. When released, the spring yanks the carrier back to the left side of the machine. A piston enters a cylinder where the trapped air cushions and slows the carrier to a gentle stop. On this machine, the piston is still a bit sticky and sometimes does not fully enter the cylinder. If the carrier doesn't get all the way to the left, it doesn't release the return and re-engage the carrier for spacing. Some oiling will correct this, to be done next time we meet.
The Break key is sticking in the cover and not moving up and down smoothly. It moves fine until the cover is fully in place, at which point the binding occurs. I believe this requires some rebending of the key and will be corrected next time we meet. All the other keyboard functions are correct. The keytops are a bit worn and we probably will swap them all with better keytops from the donor (spare) keyboard.
The typed character is not distinct, with only the right edge fully impressing through the ribbon. This requires a minor adjustment of the rotation of the type cylinder and will be addressed next time we meet.
Finally, the reader was not working. We found that the power supply for the reader had a blown .75A fuse. We inspected the components all of which seemed to test fine. I used my BK capacitor meter, ESR meter and the VOM on everything. All appeared good with the capacitors, diodes, and resistors on the board.
However, when we put in a new fuse, it dramatically blew. To figure out where the problem lay, we isolated sections of the circuit. There is a large resistor dissipating voltage and limiting current, between the big filter capacitor after the diode bridge and the other side of the circuit. It was connected by spade lugs and easily disconnected.
When we put in a new fuse and powered just that section, the fuse again blew. We took off the filter capacitor, used a new fuse and it no longer tripped. The 200uf section that measured at 300uf and low ESR using a tester failed at full voltage. This can happen and is troubling since you don't see it on the tester. The only safe way to test a capacitor is to dial it up to the full working voltage, but my gear wasn't capable of that.
David took the power supply unit home and will validate the rest is good and repair it prior to our next session. When it is working we can hopefully get the reader working and verify that it is sending out proper characters, tripping the distributor clutch and therefore fully functional. That will wait for the next session.
ASR/33 NUMBER TWO - KEYBOARD GREMLINS
Machine number two looks to be in pretty decent shape, although the carrier is stuck in position because of accumulated dried grease. This machine will need cleaning and clock oil to free up the mechanisms on the reception side. The distributor clutch fires but the keyboard wasn't working reliably. Keypresses didn't trigger the clutch except sporadically. Adjustment of the H shaped clip linkage didn't help.
I disassembled the keyboard several times and tried to spot the problems. The cover of the keyboard was missing its locator and retainer pins, but we swapped in a spare cover that was mostly intact. The keyboard unit has a front to back lever that in the rear pivots and connects to the H shaped clip to trip the distributor clutch.
The lever is locked so that the front end is down when the keyboard is waiting for a keypress. When a key is pushed down, the key moves a 'universal lever' that will release the front-back lever. It pivots up under spring tension, trips the clutch and is pushed back down to lock by the action of the H clip at the end of the distributor cycle.
The keyboard was not tripping the clutch! Keys would go down, the universal lever will pivot but it doesn't release the front-back lever. For the longest time I thought I had just reassembled it incorrectly but eventually I spotted the issue.
There is a brass lever that is spring loaded to pop into position over the front-back lever when it is all the way down - thus latching it. This was working. The universal lever pivots a clip that pushes the brass lever out of the way, thus unlatching the front-back lever and allowing it to pivot up.
Unfortunately, on this keyboard the brass latching lever can wobble and tilt, so that it moves away from the tab that should push it aside. The keypress moves the universal lever but it fails to push the latch lever away. That means the long lever doesn't pivot the H clip and the distributor doesn't trip.
It is still unknown why this wobbles or what should prevent that. Since this is buried inside the keyboard mechanism I can't see what a properly working keyboard is like. I will open one of the other keyboards during the next session and try to figure out how it should work. Hopefully that will inform the proper repair to undertake.
ADDITIONAL ASR/33 SECTIONS AND PARTS
David also brought a keyboard disassembled in a box, a typing unit in rough shape, a spare motor and some other sections. We will use these as necessary to repair at least two units to full operation, but a third unit would be a stretch goal.
PREPARING TO BUILD IOB6120 TO ADD TO MY SBC6120 (PDP/8 REPLICA)
I have a PDP/8 replica which consists of a single board computer version (SBC6120) and a front panel that mimics a PDP/8 (FP6120). The machine is built around a single microprocessor that implemented a PDP/8 for use in building the DECmate products. The design had an expansion board that could be added, the IOB6120, which adds emulation of quite a few input-output devices including a VT terminal.
At the time that I bought my FP6120 and SBC6120, I couldn't find any IOB6120 kits or completed boards. However, the documentation is online including the gerber files (what is sent to a PCB fab to specify the board to build). I just ordered the board from JLCPCB.com and will soon source the components in order to build this and attach it to my SBC6120 system.
|IOB6120 PCB layout|
IMPATIENTLY WAITING FOR GERBER FILES TO BUILD A FEW OF 'THE ANALOG THING'
Analog computing is an interesting area, where electrical quantities are used to model some physical process, directly implementation mathematical operations like integration, addition and multiplication to view the behavior of the modelled system.
An analog computer with enough elements to model a meaty process such as the orbit and velocity changes of a spacecraft, thus needing quite a few integrators, coefficients, multipliers and so forth, would be unacceptably expensive. At least, too expensive for a casual interest such as mine.
Amazingly, some analog computing superstars from Germany have created an open source system that is modular and extensible. You can read about it at https://the-analog-thing.org/wiki/Main_Page Each module is built on a sandwich of two PCBs and features eight coefficient pots, five integrators, four summers, four inverters, two multipliers, two comparators, two input expanders, two +1 and two -1 machine unit sources, five capacitors, six diodes, four outputs, a trigger output, and a panel voltmeter. It also supports modes like coefficient setting, initial conditions, run, halt and repeat, at normal and slow speed. All in all, the module is a pretty capable analog computer.
The modules are chained in master-minion mode so that the mode switch on the master module will drive all the chained modules simultaneously. With the five PCBs that come in a minimum order from a fab, I can have 25 integrators, 10 multipliers, 20 summers, 40 coefficients and so forth, which would implement a pretty complex mathematical model.
The project is so new that the builders are assembling and testing their design right now, after which they will release the gerber files. I expect within two weeks I should have the gerber files and can commission the boards to make five modules. I will probably only buy components to assemble two initially, then when I am sure it can do what I wish, I can fill it out.