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A very long shot.

Has anyone here come across these before?

As far as I can tell they are for converting from the later UK PSTN type connection (Possible V.21) to Double Current.

Any information will be helpful

Yes, it looks like something similar to our German (and Austrian) AGTs.
It looks like a double current to V.21 transverter/modem to use older DC machines
with the V.21 FSK network.

Handy, this might be a simpler way to get older machines onto the network if the V21 problem can be resolved.

I guess this will also need to be used without FSG, so perhaps a FW change will be needed

Use of an FSG (dialling unit) is always the better solution.
I am unsure if the British DC machines have FSG functionality implemented.
Maybe you can find some old Telecomms technican who have knowledge and
perhaps even old manuals of that stuff.

I am working on the V.21 problem.
This weekend is a "long weekend" in Germany with two public holidays and
I will try to test the modified V.21 board.

Hi Henning, really no rush, I have lots to do.

I am trying to find the correct manual for this device - it looks like this switches the power to the teleprinter as well, but I cannot be certain.

The Cheetah Plus has no provision for FSG type equipment - it just plugs into the phone line. Perhaps there is some kind of internal device - you dial from the keyboard, and the machine drops into standby after 20 mins idle time.

FSK (V.21/ED1000) line connection is like a direct point to point connection, no FSG functionality.
Signalling is made by switching the correct tones.

Ive taken a closer look inside the box - The mains connection to the machine is permanent.

So I think any switching is done from inside the teleprinter.

I have posted a query on the Telecommunications Heritage Croup website to see if anything is known about the actual operation of these systems.

I've just read the technical handbook and training notes I have for the Super Cheetah - there is no reference to the need for any form of "FSG" so I can only assume that the V.21 enabled machines do not work like that.

So I think in reality, these units are only to convert double current machines to V.21.

This means that the machine is powered all the time, but in standby mode, when a call is incoming, the change of state is signalled in the usual way of current reversal.