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 those code contacts are quite a long way off and I don’t want to adjust them until I read up on that part of the mechanism. Amazon just delivered a set of feeler gauges (my set from motorsport days are very rusty!) and spring balances so I can begin to work through the adjustments.

My general experience of teleprinters dates back to when I was still at school and all Creed 7E’s… the 75 is quite different.

Getting serious now. I managed to sneak the 75 into the shack aka little bedroom. It is sitting on a bit of plywood to stop oil getting onto the desk. At least it is near the radios now.

The device to the right is a Catronics CT103 RTTY terminal unit, but I currently have no information at all about it. It says CT100 on the front but the PCB is marked CT103 and the little information I found via old adverts suggests CT103 is a fuller version of the range.

It does generate around 90V off load so is a step in the right direction. 4 DIN sockets on the rear have been marked by hand as Magnet, Keyboard, RX/TX and VDU. There are 5 switches inside, one row of 4 and one DPDT which appears to switch in a pair of NPN 300V transistors so I wonder if that converts between single- and double-current operation. I am trying to trace the circuit to make sense of the connections before I go further.

Search engines have not been kind here, throwing up only advertisements for the TU and no actual information. Enquiries are ongoing!

Update: Some progress made by trying to trace the circuit out by eye. There are two switches on the PCB, one 4-way and one DPDT. The DPDT brings in or isolates two power transistors and I figure this is to switch between single- and double-current operation. The 4 switches appear to switch in or isolate power resistors and I am assuming this is to set the loop current. As set, the CT100 produces 20mA when connected to the 75’s magnet. This is with switch 1 and 4 on. Switching 2 on as well makes this 24mA.

Audio in is via two pins on a 5-pin DIN, one having a ferrite bead and going via a resistor to antiparallel diodes, the other is earth. Feeding audio in from a known good RTTY signal in the 10MHz band does result in the signal and mark LEDs flashing and, when the station is sending RYRYs the 75’s magnet makes that typical RYRYRY sound. So, so far, so good. All I need now is to figure out why the teleprinter does not respond properly to an input signal.

I really need to tidy up as well… but to make things worse I really think I need to bring the TDMS in so I have a reliable code pattern to set the teleprinter up against. I may need to go weight training first!