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.