TA-838/TT Telephone Set

©Brooke Clarke, N6GCE

TA-838/TT Military
            Telephone


Background 
Received DOA
Military Touch Tone Key Pad
Modes of Operation
Manual
Construction

Background

I think this is the last of the analog field phones.  Prior to this there was the TA-312 which could be upgraded to touch tone operation using the TA-955 touch tone adapter.  The stock TA-312 and all prior analog field phones have used a hand cranked ring tone generator to signal the switchboard or ring the other phone in a point to point configuration.

Plain Old Telephone System Operation

This analog phone can be connected to the Plain Old Telephone System (POTS) by setting the Mode Select switch to 2-wire DC/CB (DC signaling and Common Battery).  Connect a modular phone cord to the two black terminals and plug into a POTS modular socket.  Ring volume max or to your ear.  If ring volume is at min then turn on Ring Indicator switch to LED on ring. No local batteries are needed because the phone company supplies the common battery.

Received phone DOA

See What Goes Wrong (Hints & Tips) for info on how it was fixed.

This was my garage phone for a year or more then quit working.  I don't know if it's just this particular phone that has a problem or it it's something related to the design of the phone.  If you know about the reliability of the TA-838 let me know.  It's strange since the internal connector has gold plated contacts on both sides and cycling it was the only thing I did the first time.  Maybe it needs some type of chemical applied to the connector contacts?

Military Touch Tone Keypad

The keypad layout on the TA-955 touch tone adapter, the TA-838 and the TA-1042 all have the same appearance, but the digital TA-1042 sends different signals.

The Public Switched Telephone Network uses the following signaling tones, but consumer telephones do not have the 1633Hz functions.
 
 
1209 Hz
1336 Hz
1477 Hz
1633 Hz
697 Hz
1
2
3
A
770 Hz
4
5
6
B
852 Hz
7
8
9
C
941 Hz
*
0
#
D

I think the TA-955 and TA-838 both send the same analog signals as follows:
 
 
1209 Hz
1336 Hz
1477 Hz
1633 Hz
697 Hz
1
2
3
FO
770 Hz
4
5
6
F
852 Hz
7
8
9
I
941 Hz
R
0
C
P

The 1633 Hz functions come from the military Autovon phone network (and probably the R&C keys are part of the Autovon system that turned into the civilian * and # keys) and for compatability the military touch tone phones have this functionality.

I haven't tried to use the above special function keys on the Public Switched Phone Network so don't know what they might do.

Modes of Operatoin

There are two modes of 2-wire operation:
Note:  There is no AC/xx mode.  This phone does not have a magneto/hand generator for signaling a switchboard like the SB-22 and so can not call another phone, like a TA-312 that needs to see AC magneto type signaling. 

The TA-838 can however be used as an operator's phone set on the SB-22 and in that function will provide Touch Tone dialing to the POTS using a Trunk Pack.  But in this application it is laking the ability, that the TA-312 does have, of using an external headset.
There are 5 modes of 4-wire operation:

Instructions on bottom of TA-838A/TT

4W-DC/CB, DC/LB, AC/CD, AC/LB: 2W-DC/CB, DC/LB Opn
--------------------------------------------------------------------
A.    Outgoing calls - pick up handset.  Upon dial tone, key digits firmly.
B.    Operator aid before call - key 0
C.    Operator aid during call - key R
D.    Conference call 0 key C and listen for 2nd dial tone.  At 2nd dial tone, key conferee no.  Repeat for additional confers.  If conferee no. is busy or does not answer, key R to Release line.
E.    Release - Replace handset; insure proper seating.  For AC operation handset must remain seated for aprox. one second before placing next call.

Point to Point Operation
-------------------------
A.    Mode select switch in PT - PT position
B.    To signal., pick up handset.  To repeat signal depress and release hookswitch.

Caution - Remove batteries before servicing or storing telephone.

Manual

TM 11-5305-650-12 (AF TO 31W2-2TT-11), 25 January 1985 includes Change 1 but has a lot of English errors.

Construction

Attached to the bottom cover is the main Printed Circuit Board.  This board has a NEC 80C48 microcontroller and a number if 4000 series CMOS digital ICs and on the opposite end of the PCB there are discrete transistors and components for the analog circuitry.  There are a couple of crystals near the digital circuits so I assume the digital suff is for generating all the signaling tones as well as recognizing incoming AC signaling.

Note that if a phone like this one or the TA-1042 has the precedence keys (FO, F, I and P) then if the phone is off hook on a call and a precedence call is incoming, the phone will respond by generating a warning tone in the receiver and connecting to the incoming call after disconnecting the call in progress.  How is this signaled and accomplished on this analog phone?


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