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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: G503.com (removed) and sigmund.tveito.com 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.


Ben said...

Do you know what wire would replace the larger 1/4" wire? Even if it was the same wire or same diameter wire, just looking for something i can order to replace the wires on about 5 of our phones. Not sure where to find wire with the stress rope inside and where to find the rubber ends that keep the wire safe where it meets the phone and meets the handset. It's like a little gasket/stress reliever that the wire goes into. Trying to order these parts. :/

Gadget Caver said...

Hi Ben, good question. If you're ready to try DIY repairs, find some old desktop computer keyboards and cut off the cords. Sometimes you can find old keyboards cheap at thrift stores. It's usually a fairly robust and flexible 5-conductor cable. The more common straight cords work better than curly ones. There's no strain relief cord, but you can use tiny zip ties inside the handset housing and phone housing to secure them. These cords tend to be a bit thinner than the 1/4 inch originals, so you'll have to use heat shrink to fill the space where the cord goes through the gasket at each end. Adding some Gear Aid Seam Grip works well to seal up where the cable goes through the gasket, and the newer UV-cured stuff is easy to work with. Hope this helps.

Gadget Caver said...

Also, try using a pair of needle-nose pliers or medical artery clamps to insert into the holes of the plastic plate inside the gasket. This plate compresses the cable into the gasket, and unscrewing it (counter-clockwise) will enable you to remove the old cable. Reverse the process when you install your replacement cable through the gasket.

Daniel said...

Gadget Caver,

Do you have a 3D printer file, or build instructions, for the AA adapter? Are there any accessories that are worth buying for the TP-6N phones? Any other modifications that you would recommend to improve the phone's durability? What kind of lifespan/problems are you seeing using the phones for S&R missions?

Thank you for the post!

Gary said...

Hi Gadget Caver,

I am interested in the AA battery adaptor. Do you sell them? If so where and for how much? Cheers,

Gadget Caver said...

Hi Daniel, thanks for your interest and I'm sorry for the slow reply. I was apparently not getting notified of comments here. I do not have the 3D print file for the battery adapters. I've been collaborating with others in the past to get them built. I will find out if this is something we can share a build file on in the future.

Besides WD-1A/TT phone wire and spools, I've also built "phone patches" that connect the field telephone line into 2-way radios or cellphones, an amplified speaker, and a miniature field telephone. I've built small numbers of these from time to time, as people have need them. See https://cavephone.blogspot.com/ for more info. Depending on your needs, some folks have successfully connected the TP-6N phones to field telephone switchboards like the SB-22. Hope this helps.

Gadget Caver said...

Hi Gary, thanks for your interest. I have been selling the battery adapters directly in the past, but don't currently have extras besides what are already in phones. How many were you needing?

Anonymous said...

You can purchase battery adapters on Amazon that allow you to use AA batteries in the place of D cells. The smaller batterys slide into the adapater and lock in place.

Gadget Caver said...

You could try using battery adapters from Amazon. Individual AA to D size adapters have been available for some time. We tried stacking some of these adapters years ago and found they don't always provide reliable contact with rough handling in harsh environments.

Anonymous said...

Do you know the part number that will fit on those pins?

Gadget Caver said...

I do not know the part number that will fit on the pins.

Sam said...

Do you have a better resolution picture of the fig 16 schematic?

Gadget Caver said...

Sam, unfortunately I have no better resolution pictures. These were pulled from another website long ago, which has apparently now gone away.

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