Creed 75 getting serious...

Ok so I adjusted the teleprinter's transmitter to the correct tolerances and made matters worse! It was at least trying to send characters out in response to key presses even though they were all wrong. Now it sends fewer, i.e. some keys send nothing at all, but still equally wrong! I can't see how I made it worse by correcting it. But it's interesting delving into this beast. The transmitter itself is simply a series of contacts. The selector mechanism pushes on pins which push or leave the code bits, and these are then read by another set of contacts which fire in sequence to send the relevant word. But it would be nice if it worked... So I thought it might be an idea to get a meter on to each code contact to see if it was connected to mark or space when a key was pressed and the mechanism moved appropriately. Nothing. Ok... take the transmitter out again. It does not match the wiring in the book. The book / manual says that each code contact comes out on pins A1 to 5 but they aren't. There is no wire on A5 and in each case the…

Having been unable to find any documentation at all for the CT100 TU I made a better effort at tracing the circuit out and I now think I know what connects to where. The magnet output is obvious, as is the audio input (TX output). I have now worked out the audio output (RX input) and traced enough of the circuit to figure out the keyboard input which has three connections for mark, space and tongue, the latter being connected to earth. The mark and space inputs pull down the inputs to matching nand gates and thence into the rest of the circuit. The keyboard connections were the last bit of information needed. Attaching headphones to the audio output I can trigger both tones by shorting each of the mark and space inputs. So far, so good. But... ...the keyboard outputs from the 75 are open circuit, yet at rest the 'stop' contact should be made and thus one of the keyboard connections should be connected to it. it isn't. A slight pressure on the contact makes it. Similarly, each of the 5 code contacts never 'makes' when operated (i.e. pressing a key and turning the motor by hand). But…

Work on the Creed 75 is slow at present and not helped by the fact the workshop is a mess since I moved all the valves in there and thus had to relocate all the other junk. Having recently reminded myself that 1/4 watt resistors do not generally like being expected to sink 3 to 4W (!) I have some decent resistors to ensure the PSU gives the required 20mA. So I have 80-0-80V from the PSU attached to the 444, I have 20mA (actually 23), the 75 types to itself locally, which is mechanical, but does not receive correctly. I am generating code from the TDMS but Y comes out as Z and most other code combinations are unrelated to reality. I wondered if this was the result of the typehead not lifting correctly but Y and Z are on the same level. The ‘scope shows a reasonable waveform coming out of the TDMS. So, currently I have two teleprinters and neither one reliably prints out what is sent to it. Nor do they talk any sense to each other. I can accept that with the 444 as I still need to strip it down but the 75 is…