No keyboard operation with FZGT or i-Telex

[JDAngus]

87 v, + on c - on a. Nothing connected to the i-Telex box.

[JKde]

I assume the 2 cables from the i-Telex are inverted (switched polarity).
In idle mode c is - and w2/a is +.
On imcoming call c is + and w2/a is -.
The control unit reacts on polarity!

[FredSonnenrein]

Note regarding Jochen's drawing: The grey Box "Telex" should include the call control unit.

[JDAngus]

I'm almost embarrassed to admit to what the problem was.
At some point in time I added an RS-232 to loop converter across the Signal jack on the Call Control Unit.
It's supposed to be 100% passive with no serial connection.
At some point in time, the MOSFET opto-coupler across the keyboard line shorted.
Well that did a splendid job of not letting me type anything on the Lo-15b.
RS-232 adapter

The Call Control Unit is working correctly now.
Both Local and outside connections. I type, it prints.

Thank you to everyone that helped me chase my tail around in a circle.

Jeff
i-Telex 76470