After ages of it sitting there taking up bench space and having been moved about the workshop 3 times I finally made time to check the output waveform from the 444 today. Seeing I have a nice new shiny ‘scope what better test? Failed miserably of course…
After 5 minutes reading the ‘scope manual (!) I can now capture the keyboard output from the 444 and apart from a lot of ringing it appears to be generating the correct codes and spacings. No surprises on the distortion given it is switching between + and – 80V in quick succession with no form of conditioning. But, given the teleprinter refuses to print the correct letter for the pressed key it does suggest this is down to adjustment in the receiving setup and not the keyboard transmitter. Here are the characters R and Y:
I also checked the output from the TDMS5BV which is creating a nice, usable stream. Here it is repeating the character R:
Now, feeding the stream of characters from the TDMS5BV into the 444 and checking what it is trying to print out is somewhat enlightening. Y comes out fine, but R comes out as G and I as P. When I checked the bits for R, adding bit 5 makes G; similarly from I to P. Y already has bit 5 set, so it looks like bit 5 is stuck on. I am going out on a limb somewhat here given the first 4 bits of the code appear ok to think this is more a mechanical issue rather than the receiver electromagnet.
Fiddling by hand (a very scientific application of slight finger pressure on the transfer levers) does turn a G into an R when the R key is pressed. Given the mechanical hypothesis it probably just needs a really good clean! Onwards…!