TA-1042 A/U Military Phone
When visiting the JKL Museum of Telephony, I spotted this military phone in their collection. I thought that would make a nice phone for the lab. It needs a thorough conversion - some might say a huge downgrade - from its proprietary military electronics to commercial phone innards for it to work on a the Plain Old Telephone System.
This was done fairly quickly as a side project, so I just jotted down a few notes on my lab book, just the minimum necessary for me to rewire the keyboard. I first reverse engineered the original keyboard connection, which used dual diodes ICs to pull two lines at a time. I then reverse engineered the layout of my RCA donor phone. Finally, I rewired the military keyboard to match the RCA phone. My lab book notes are included below.
This is the original keyboard wiring, seen from the back. Each key pulls two of the keypad connector pins down via a dual diode surface mount component. This is probably a direct DTMF dialing arrangement.
All the diodes had to be desoldered before the rewiring.
This is my $10 RCA phone keyboard. It is wired in a traditional matrix.
This is how I rewired the military keyboard. This was done the brute force way, by cutting PCB traces and adding some wires until it matched this electrical circuit.
I first had to remove all the diodes. I just desoldered them cleanly.
Then I made cuts in the PCB traces on the button side. You can see them all here. Before I made the cuts, I opened little square windows in the protective plastic membrane around the trace I wanted to cut.
In this picture, you can also see how I made small holes next to the trace, had a wire poke from the back and soldered it to the trace.
In this image, I drill the little via holes right beside the traces I want to connect.
Here I cut a tiny square of the film right next to the hole over the trace, where the soldering will be. I wanted to preserve as much of the original protective film, but you might opt to just get rid of the whole film instead.
Here you can see me soldering a wire that pokes through the hole next to the button trace I want to reconnect.
The wire after soldering.
View of the backside showing the wires going through the little holes. Note that I all the little diodes are gone.
Here is the fully rewired keyboard. The columns are the green wires, soldered to the other side through the holes. The rows are the blue wires, all soldered on the back traces.
The new electronics come from an RCA Slim Line Corded donor phone.
Here is the fully rewired phone. In addition to the red wires connecting the military keyboard to the RCA keyboard chip, you have to wire the input lines, the ringer, the microphone and the speaker.
On mine, I went a step further and connected the ringer volume button and the ring LED. I did not connect the volume button, but I should really do it, probably for a speaker amplifier.