TP-6N Field Telephone

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Introduction and Operation

The TP-6N is a Norwegian-built felttelefon (field telephone) produced for the Norwegian and Dutch armed forces. The Dutch call this the TA4881 veldtelefoon. It was designed by Elektrisk Bureau in Billingstad, Norway, and won an award for design excellence in 1973. It earned the nickname "cricket phone" because of the tone generated by the electronic ringer. Later the Chinese copied this design and produced their own version, the HDX-1. 

In 2008 we purchased some TP-6N phones from a surplus supplier in the US. Our intention was to evaluate and integrate them with older field phones already being used for search and rescue communication. These phones proved very effective and reliable, particularly in harsh cave environments. This page is provided as an informational source for owners, users and other interested persons.

While most rescue communication relies on portable radios for convenience, wired telephones are one alternative which avoids radio interference and offers better privacy. Plus they function underground where regular radios may not. For this reason the military-style field telephone system is a popular standard for cave rescue communication in North America.


TP-6N Overview:

This design offers substantial improvements over earlier field telephone models.

  • The TP-6N is about one-third the size and weight of the TA-312/PT field phone, and a similar weight to the TA-1/PT sound-powered phone
  • It will interface directly with older field phones using standard 2-conductor WD-1A/TT wire or equivalent
  • Voice clarity is significantly better than earlier field phone models 
  • The TP-6N is robust, waterproof, and uses reliable solid-state modular construction with very low power consumption
  • An electronic ringing generator alerts other phones, and an indicator light and handset buzzer signals incoming calls and diagnoses line fault conditions
  • If the batteries are dead or missing, the TP-6N can still be used as a sound-powered phone over shorter distances
  • Body size is approximately 10 x 4 x 2 inches, handset 9 x 3 x 2, satchel 10 x 7 x 2
  • Weight is 3 lbs 14 oz including carry satchel and 3 x D cell alkaline batteries, or a total of 3 lbs when using an innovative adapter with 3 x AA batteries

Built-in Troubleshooting Feature:

A useful feature of the phone is the ability to diagnose a few basic conditions simply by pressing the call button. If the red indicator lamp blinks rapidly and a faint clicking/chirping tone is heard in the handset, the phone is connected to another TP-6N (or some other field telephone).
If there's no light and only a faint (or no) tone, a phone is either disconnected or there is a break in the phone wire. If there's light but no tone, this indicates the wires are shorted together somewhere. 

With no battery power you should still be able to receive incoming calls and operate the phone over a limited range by speaking loudly into the earpiece instead of the mouthpiece. You cannot ring other phones without battery power.

Signalling Limitations:

Users who choose to connect TP-6N phones to other types of field phones should be aware of limitations with the ringer circuit. The TP-6N ringing generator can sometimes fail to sound the mechanical clacker on TA-312/PT field phones and other older models.

This happens because the mechanical ringer on older phones requires more current than the TP-6N electronic ringer. Therefore one TP-6N can easily alert numerous other TP-6N phones, but in some cases it may not produce enough current to signal multiple older model phones.

If TP-6N phones are being used and ringer signalling is a priority, the simplest solution is to limit the number of older field phones connected to the same line. Voice communication between old and new phones is not affected by this limitation.

AA Battery Adapter:

We designed and produced an AA battery adapter for the TP-6N. It weighs just over an ounce and can either be kept inside the phone while AA batteries are swapped in and out, or easily removed to allow the use of regular D cells.

The TP-6N was originally designed to be powered by three BA-30 carbon-zinc batteries (the military equivalent of the old Heavy Duty "D" cell), or by three nickel-cadmium rechargeable cells or two lithium cells. AA batteries were not very efficient back in the 1970's, but today their capacity rivals the older D cells and they make an ideal substitute. In practical terms, modern AA batteries are smaller, lighter, cheaper, and more convenient for carrying spares.

Use alkaline or lithium cells, or high capacity rechargeable nickel-metal hydride cells. Alkalines are cheap and readily-available, but tend to leak and corrode when stored in devices for extended periods. Lithiums are more expensive, but offer greater capacity, less weight, better reliability in lower temperatures, and have a 10 year shelf life. Quality rechargeable nickel-metal hydrides also work well, and can be reused many times. Make sure they are fully charged prior to use.

Older technology rechargeable AA batteries such as nickel-cadmium cells are not recommended for rescue use because of limited capacity and reliability.

Do not mix together different types of batteries. Use all alkaline, or all lithium, or all NiMH.

When installing batteries, make sure the spring on the rear cover of the phone aligns with the battery contact.

Avoid excessive use of the ringer because this will deplete the batteries.


Line Clips:

The line terminals on the TP-6N are designed for connecting the phone to the END of a length of wire. This differs from open-sided terminals on other field phones which can connect to any point along the wire.

Attaching a pair of leads with alligator clips to the TP-6N will allow it to be clipped in anywhere on the phone line. These are easily constructed from jumper lead sets or 18 gauge twin insulated lamp cord. Strip away a half-inch of insulation, twist the wire and apply solder to prevent fraying. The wires are best soldered and crimped to the clips for better reliability. Add a short sleeve of heat shrink tubing to bind the leads together, or simply tie them together with a knot.

Keep the clips stored with the phone to allow for quick connections. For a simple method of tapping into military WD-1 phone wire, use a sharp blade to cut several small slices and peel the insulation away from one conductor. A few inches away, repeat the process with the other conductor. Attach the clips, making sure each one makes contact with the bare wire. You do not need to separate the two conductors in the middle, but be careful the clips do not twist around and lose contact. After disconnecting the clips, wrap the wire with insulation tape.


Connecting Multiple Phones:

All field phones sharing a common line should be connected in parallel, with both sides of the line hooked into every phone. This provides the best reliability and functionality when using a local-battery system with no central switchboard.

Do not connect TP-6N field phones in series using the old military "Platoon Hot Loop" method. If you do, any break in the wire or single phone failure will cause ALL phones to lose communication. Also this method changes the internal resistance of the loop more as each phone is added, reducing the effective volume of both voice and ringer. 


Maintenance, Modification, and Technical Data

The manufacturer states the TP-6N has an "estimated MTBF" (Mean Time Between Failures) of at least 7 years. Of course this assumes military use, not the rigors of cave rescue!

Preventative maintenance is mostly straightforward. Before storage, remove the batteries and ensure the phone is clean and dry inside and out. Check the rear cover, making sure the screws thread properly and the seal inside is clean and unbroken. Check the battery compartment spring and circuit board terminal block are both secure and lined up properly. Check the call button, PTT switch, wire terminals and handset cable for any damage. Connect and test all phones before use and again before placing in storage, and encourage users to label any phones they find defective.

The handset cable on the TP-6N is a physical weak point, as with most field phones. Some TP-6N phones have curly cords, but most use a straight cable - either the older 7mm (1/4 inch) diameter, or the newer and more flexible 5mm (3/16 inch). All cables deteriorate naturally over time, especially the older versions. They become stiff and the outer insulation can eventually flake off while the inner core slowly disintegrates. If an older handset cable is causing problems, it is probably not reliable enough to repair and should be replaced. 

Damage to the newer style thin handset cables can usually be repaired. If the end pulls out of the handset or phone housing, carefully work the cable back through the gasket after lubricating with  Aquaseal (which later sets and seals inside the gasket). Secure the cable by tying the white nylon strain relief cord around the attachment post inside the phone or handset. If there is no white cord inside the cable, use small cable ties instead. If the cable is cut or damaged near one end, follow the same directions after creating a new gasket seal using rubber splicing tape over the cable. Solder the old end connectors onto the shortened wires and insulate them with heat shrink. 

Inside the body of the phone, a series of wires plug onto small pins on the circuit board. These wires can work loose, and sometimes they break away from the call button terminals and need to be soldered back on. Check the terminals aren't touching together and the call button is operating properly. For most other technical problems, use a second working phone to help with fault finding and attempt to identify what functions and what doesn't. Isolate potentially defective parts, and replace them if possible. If not, keep the non-functioning phone for spare parts (clearly labeled) or contact me for assistance.

Circuit Board Terminal Connections:

The TA4881 (Dutch version) phone has 13 wires connecting to the circuit board. Earlier circuit boards have 13 pins, but some later versions have 16 pins (the extra pins are not used). The TP-6N-A (Norwegian version) phone has 10 wires connecting to 10 pins on the circuit board. 
|                        6  YEL to handset
|                                    3  WHT or YEL to call button
|                        5  BRN to call button
|                                    9  BLK to handset
|                        2  BRN to handset
|                                    11  BLK to terminals
|                        12  RED to terminals
|                        13  BLU to call button
|                        8  GRN to call button
|            1  ORN to call button
|                        4  RED to handset via diode block
|            10  YEL to terminals
|                        7  BLU to handset

Press-To-Talk Switch Modification:

The standard TP-6N-C handset uses a combination press-to-talk and press-to-listen switch. Unlike with other field phones, you need to press this switch to receive voice signals. This can be counter-intuitive and frustrating to users.

Historically the original TP-6N-A version used a regular press-to-talk switch and was able to receive voice signals at all times. However the military discovered this compromised their communication security because the earphone acts as a 'bug' picking up nearby voice conversation when the phone is not in use. So the C-version was quickly developed and these are the phones we're now working with. Fortunately it's easy to modify them back.

Changing the handset switch wiring as shown will allow voice signals to be received at any time, even when the batteries in the phone are dead or missing. This modification causes no additional current draw on the batteries in the phone.

1. Remove the 4 plastic caps at the corners of the earphone. The caps are usually sealed in tight and may be difficult to remove. You can try using the point of a knife or a small flat-blade screwdriver to pick them out, or it may be quicker and easier just to puncture the middle of each cap and pop it out.

2. Unscrew the 4 screws and remove the cover from the earphone. Carefully lift the earphone out and turn it over, noting the position of the rubber gasket and O-ring seal.

3. You should see 2 screw terminals with wires attached, and a DIAC wired across the terminals and covered with a sleeve. This protects the earphone from alternating current noise. Unscrew the terminals and reconnect the BLUE and BROWN wires together on one side, and the BLACK wire on the other side. There is no polarity so it doesn't matter which side is which. Tighten the terminals again with a small wrench. The new connections should look like this.

4. Carefully reassemble the handset. Make sure the wires aren't pinched anywhere, the gasket is in place, and the inner O-ring is aligned between the earphone and the cover. Replace the old plastic caps if they are still intact, or carefully apply a hot glue (or similar) to fill the screw holes. This helps prevent dirt and moisture from corroding the screw heads. If the handset needs to be opened again, the material can easily be picked out with a small screwdriver.

TP-6N Manufacturer's Brochure:

This was kindly provided by DSK from NorwayClick on each page to enlarge the image. The technical data from page 4 is shown enlarged at the bottom.  

Field Telephone Pocket Guides (in Dutch):

Royal Netherlands Army - Instructions for Field Telephone TA4881

1. General Data
Aim: Field or office telephone set in two wire LB network.
Composition: Field telephone set case with shoulder strap
Electric data: Feed 4.5 V (3 x NBA030)
Mechanical data: Telephone set dimensions 250x190x60mm Telephone set weight 16.66 Newton

2. Preparation
Remove the battery lid. (1) Place the batteries. Positive side to (+). (2) Replace the lid.
Test the battery. Connect terminals together (3) and press the call button (5) Call indicator (4) blinks. Replace batteries when indicator blinks slowly.
Connect the telephone line. Remove the insulation for approximately 2cm. Press down on the terminals (3) and insert the bare ends of the line in the holes on the side of the telephone apparatus.

3. Service
Outgoing call: Press the call button for 2 to 3 seconds and the call indicator lights up.
Incoming call: The call indicator lights up and a tone is audible in the microphone.
Speaking/Listening: Press the talk key (6).
Note - Never wind the cord around the handset because it could drain the batteries.

4. Line Test
Indicator does not light up when the call button is pressed = line break.
Indicator blinks slowly when the call button is pressed = line short.

5. First Echelon Maintenance
Store with the outside of the telephone apparatus dry, clean and dust-free.
Account for the telephone apparatus when damaged.
Check with the DL if all components and parts are present.
Determine the operations for the telephone apparatus.
Remove the batteries from the telephone apparatus.

Schematic Diagrams of the TP-6N-C and TP-6N-A:

Sources: (removed) and here and here

TP-6N Description from Jane's Military Communications:

(This description simply paraphrases the manufacturer's brochure.)

The TP-6N field telephone set was developed in co-operation with the Norwegian armed forces. It is a lightweight, waterproof electronic unit available with push-button or rotary dial unit. It consists of an olive-green H-67N-type handset and an olive-green case with printed circuit board and batteries. The handset is moulded in reinforced plastic and the case is aluminium alloy.

The handset includes a microphone, earphone and press-to-talk switch. A belt-clip can be snapped on. The earphone end of the handset is shaped to fit under a field helmet. The microphone and the earphone are identical insets. An incoming call will be heard as a wobble-tone.

Space is provided within the case for three BA-30 battery cells, three rechargeable Ni/Cd cells, or two lithium cells. When the telephone is used 8 hours a day at a rate of six calls per hour, of which half are incoming calls, the battery life will be approximately three and a half months if the average conversation lasts for 2½ minutes and the outgoing call signal lasts for 3 seconds. It is fully compatible with traditional magneto and CB telephone systems.