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!
Having now had a chance to take a closer look at the Creed to see why when Y is sent by the TDMS it prints as Z it is clear why! The following shows why… the TDMS is sending a continual stream of Y’s and then R’s:
This is the system between characters, all is well, but…
…this should be a Y, i.e. MSMSM (M = mark, S = space), but prints a Z, i.e. MSSSM.
And the R? This should be SMSMS, not SMSSS as shown. SMSSS is carriage return, no wonder then why the typehead remains firmly at the left of the platen.
But, checking the waveform output from the TDMS shows it is indeed sending a Y, and with a good square wave. This gets rather mangled when the teleprinter receive coil is connected but still has sufficient peaks to drive the thing. So the issue is between the input to the teleprinter and the selectors.
One item to note though is the three photos above are from a video recorded on my iPhone in ‘slo-mo’ – which proved to be really very useful here!
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 all functional mechanically. The code combinations ‘sound’ reasonable too (if you’ve never heard a teleprinter you won’t know what I mean here) as with the motor off the clacking sound seems reasonable for the codes being sent by the TDMS. Very unscientific I know but it backs up what the ‘scope shows.
Some adjustment needed then… but really as I have yet to have any baseline in any of this it is a bit of a moving target. The TDMS will not send just one character but in theory, and with the motor off, given it sends the same character continually it should set the selectors correctly for me to examine them to see if Y as sent really is Y as received. That’s the next step, only the bench the teleprinters are on is too crowded to work on them.
So I now have a functional Creed 75 teleprinter. The person it came from had done a first rate job at cleaning it up and getting it to function. Thus far all I have done is put it in the workshop waiting for me to make some time and space to have a play. I also need to get my head around it as I have never had a 75 before.
And here it is. It is in good shape given how old it is and I hope eventually to be able to sneak this into the shack, aka the little bedroom. There is just no way the 444 would hide in there but there is almost a 75-sized hole.
The innards do look neat and care has been taken in getting it working.
More to the point though it prints just fine, a stage I have yet to reach with the 444.
The first order of play is to get some volts onto it via the TDMS and make sure it receives as well as sends. Ah, that means I need to fix the TDMS which has stopped working. This also means I need to rearrange all the valve collection as this is taking up half the workshop right now. Snowballing…