Fritz!Fon C4 Front Case

My DECT phone, a Fritz!Fon C4 from the Berlin company AVM, had the front housing broken in several places. In further use, then also the speaker failed and you could only call through the speaker for hands-free, which is located on the back. The reason for this is that the contact between the loudspeaker (two spring contacts) and the circuit board was lost. This was a bit annoying in everyday use.

NOTE: By the way, the display still worked perfectly, even in the broken state. The supposed picture disturbance, which can be seen on the cover picture, was only caused by the way the picture was taken - don't panic.

In the meantime, I had tried to glue the broken frame with superglue. From a certain point, however, this was no longer sufficient. This was then the trigger for me to think about another solution.

I was able to find several reports in forums that describe the same problem. In some cases, the entire bezel fell apart, making the phone less functional than a brick.

What was my motivation?

Well, I love to repair things to bring joy to others, not only to myself. In addition, this saves the finite resources of our planet. The product lifetime could and should often be much longer, but repairs are often artificially made more difficult by the manufacturers. And that sucks. If a device is solidly constructed, then it can do its job for 20 years or more, just as it did in the past.

For the Fritz!Fon successor model C5, someone had already taken the time and modeled a 3D model. Due to the flat design of the Front face, the C5 model is even easier to print and the rework, i.e. sanding, is not necessary here. At least I was able to save my Fritz!Fon C5 from premature scrapping. Many thanks for this. This was my motivation to breathe new life into my broken C4 model.

So I designed a new C4 front case in FreeCAD and printed it with white ASA filament. It fits almost perfectly, as you can see on the right side of the cover photo.

Improvement to the original case:

• The material thickness in the keypad area was designed to be more solid. A material fatigue fracture in this area, thus becomes less likely.

How do you turn a brick-like thing into a DECT phone as good as new?

Quite simply! You own a 3D printer or you know someone who owns one or you have a Maker Space or the like in your area?
Then just download the 3D model. You only need to print one part and it doesn't require any support structures. If you have a little patience, you will be able to do the repair.

How much does the printout cost and is it worth it at all?

The total cost is about 0.23 to 0.31 euros.

• I needed just 7.62 g of filament.

  • For an inexpensive ASA filament, which costs only 24.99 EUR, for example, it would be just 0.19 EUR.

  • The somewhat more expensive PolyLite ASA filament (for 34.99/1kg) cost me 0.27 EUR.

• In terms of energy, the printer required a rough estimate of 100 Wh.

  • With an energy price of 0.40 EUR/kWh, that is 0.04 EUR.

A really inexpensive repair in my opinion, considering that a new Fritz!Fon C6 would currently have cost at least 72 EURO.

Printing instructions

Which filament is suitable?

I recommend printing the case in ABS or ASA. The original case is made of ABS and there is a reason for that. The thin-walled structures into which the screws are screwed must also be somewhat flexible. PLA could, in my experience, prove to be too brittle. In addition, ABS or ASA can also be sanded very well.
I printed it myself in Polymaker PolyLite ASA white, which worked fine for me.
If I had to buy ASA for this, my first choice would be Prusament ASA. According to the manufacturer, this one doesn't shrink as much, and the resulting fumes are probably less noticeable.

NOTE: Nevertheless, please always make sure that there is sufficient air exchange in the room, especially when printing with ABS or ASA. For example, when printing ASA, I do not stay in the same room whenever possible during the printing process. Once the printing is finished, the first thing I do is make sure there is fresh air in the room.

Notes on the printer

For printing in ABS/ASA filament, a closed print chamber is advantageous because the material usually tends to warp. I printed the case on a K1 Max, which has a closed print chamber.

You don't have a printer with a closed print space available?
Then this shouldn't be a problem, because two things indicate that the print should also succeed with an open print chamber.
These are:

1. the area of the print object is still relatively small and 

2. the print object is not high.

I also have a Prusa MK3 and think that the printout with ABS/ASA would also have worked with it. If several people are unsure here and there is a need, then I could also test it afterwards.

Print bed:
PEI coated printing plates are very suitable. Should the ABS or ASA adhere too well in your case, then a film of thinly applied glue stick may help you to get the print object off the print bed afterwards. However, this was not necessary with my K1 Max 3D printer.

Print settings

Since the print settings generally depend on the printer and the filament, it is not easy to make a general statement here.

For the printout on the K1 Max, I used the Orca Slicer, which as you know is a fork of the Bambu Studio, which in turn is a fork of the Prusa Slicer, and which is a fork of Slic3r. And so I refer to the Orca Slicer at one point or another.

Print recommendations:

• Nozzle diameter: 0.4 mm

• Pressure layer: 0.20 mm

• Perimeter: 2

• Infill: 100%

• Support: No

• Brim: Brim ears (It may work without a brim. I used Brim Ears for the corners. If this is not supported and the printout shows warping, you can also activate the normal brim)

• Bottom shell layers: 3

• Top shell layers: 4

• Nozzle temperature: 260 °C (follow the manufacturer's instructions for the filament you are using)

• Print bed temperature: 105 °C (follow the manufacturer's instructions for the filament you are using)

Print speed:

• In Orca Slicer, I chose the Quality setting. I prefer to print a little slower, especially with print objects where quality is the main concern.

• Nevertheless, the print was finished in 39 minutes on my K1 Max. Qualitatively without complaint.

Cooling:

• Themoplastic ASA (Acrylonitrile styrene acrylate) likes a warm printing environment. Use the fan speed sparingly.

• For me, the model fan ran at 80% for the outer perimeters and the rest at 10% of maximum speed.

• My K1 Max has a case exhaust fan (back fan). I recommend deactivating it, if available - this is possible with the K1 Max even after the print job has started. The background is that this would reduce the case's interior temperature (in my case, this would then have reached a maximum of 33 °C). We remember, ABS or ASA likes it to be toasty warm ;-) . With the back fan deactivated, I was able to achieve an interior temperature of 43 °C.

Postprocessing

The printout is done, you proudly hold your housing in your hand?
Then it's time for post-processing.

1. If you have printed an additional border, it is time to remove it.

2. The next step is to grind the curvature of the case.
   In contrast to the Fritz!Fon C5 model (which is not curved), you should now take a few minutes to sand the curvature on the front of the case, along the two long sides, as on the original. The original front surface of the Fritz!Fon C4 is slightly curved, so is the keypad of the phone. You want to be able to press the keys later.

   1. For this I used a sanding block and 100 grit sandpaper. You can also use finer sandpaper, then it will take a little longer.

   2. For each of the two sides I needed only a few minutes for sanding.

   3. For stabilization while sanding, would recommend pinning the case back to the printed front. Be a little careful when sanding so you don't sand the silver colored case back with it. It is advisable that the sandpaper lies flat or is clamped flat on a sanding block, then it should work.

   4. Check whether the ground-in curve shape fits. Please try to insert the keyboard pad carefully. If the keys protrude evenly (approx. 0.3 mm to 1 mm), then the shape is perfect.

   5. If you like, you can of course still smooth and polish the surface a bit with finer wet sandpaper. However, since the ASA I used is matte anyway, I did not do this.

   6. Then rinse again with water and then it can go to the assembly.

Disassembly of the old housing

We still need all the components from the broken enclosure.
These are:

1. Light guide for the status LED

2. Loudspeaker - in general, make sure that you do not damage the small loudspeaker!

3. Loudspeaker gasket/seal and the mesh under the loudspeaker

4. Window with rupper seal

5. Keypad

6. Light diffuser plate

7. Light blocking pads

Fitting the printed housing

The 3D model, however, I have already created very accurately and at least adapted to the printout with the K1 Max and the PolyLite filament used. However, since each printout on the different printers will probably turn out a little different, it may be that you still have to do fine rework so that the gap dimension is correct.
The main work will probably be to fit the keyboard pad. I recommend using a small key file and a cutter knife for this.

Pay attention to the following:

1. First, please try to insert the keyboard pad carefully and check if all keys are free and not stuck. When printing the first layer on a 100°C hot print bed, it can happen that the openings are a bit narrower than desired. You would then have to adjust the openings for each key with a small file. If this does not work, you could increase the Elephants Foot compensation in the slicer of your choice and start a second printout.

2. After that I would adjust the opening of the window if necessary.

3. Then check the rest of the openings, for example the microphone and speaker openings.

4. Now check if the front of the case fits flush with the back of the case. If everything fits, then it can go to the assembly.

Assembly

1. First, I would put in the keyboard pad.

2. After that, I would fit the window. The window has a spring on the left and right side that should snap into a slot in the case.

3. Now fit the light guide for the status LED, into the opening next to the speaker.

4. Then insert the speaker mesh, after that you glue the speaker gasket, with its adhesive side, into the case - on the bar on which the speaker later rests.

5. Then insert the speaker. Pay attention to the correct placement, see photos.

6. Optionally, you can insert the two black rectangular pads that will block the light from the keyboard backlighting.

7. Finally, place the light diffuser on the keypad. Make sure it is in the right position, there is a slight depression in the middle of each key. This depression must face inward into the case and not outward.

8. Clean the window, which is probably made of polycarbonate, before screwing the case. It should be dust-free. For cleaning use a cloth that does not lint, e.g. a cloth for cleaning glasses or a microfiber cloth. I used isopropanol as cleaning fluid, but a glass cleaner should also do the job.

9. When everything is back in its proper place, marry the two halves of the housing together again.

   1. Leave the assembled front half of the case at the bottom.

   2. Insert the mainboard into the back of the case, with the two tray contacts inserted through the corresponding case openings.

   3. Hold the mainboard sideways so that it doesn't fall out during the assembly step. Then carefully place the back of the case onto the front half of the case.

   4. If everything fits flush, you can now screw the case together with the eight small screws. It is best to start at the top, at the speaker. So that this does not slip. The screws should not be tightened too much!

10. Now check the function of the keyboard again.

11. If everything fits, you can insert the battery and put on the case's back cover.

Now try to turn on the phone.

If the phone works properly again, then congratulations.
You did it! I hope you can enjoy your self-repaired Fritz!Fon C4 for many years to come.

Additional details about the different Fritz!Fon models

How good are the other Fritz!Fon models compared to the C4?

Over the years, I have had the opportunity to gain experience with several Fritz!Fon models.
The even older MT-F, as well as the successor model of the C4, the C5, all have this structural housing flaw. A design-related flaw, which in my opinion is due to the fact that AVM unnecessarily wanted to build a smaller case than, for example, the DECT phones of the competitors Gigaset or Panasonic. It is noticeable that their casing is larger, which makes it easier for a designer to construct the casing robustly.
AVM unfortunately made the wrong decision in this regard, at least from my point of view. They sacrificed robustness, and thus product life, for a slim, pleasing design, which is not necessarily more ergonomic, however. I have somewhat larger hands and for me the case of the C4 model is also a bit too slim, it is therefore not as secure in the hand. All a matter of taste undeniably. However, this makes the crossbars in the front frame, which is made of ABS/PC plastic, structurally too narrow and weak. Also, the previously mentioned posts, with a hole into which the screws are screwed, have a wall thickness of only about 1.1 mm (or 0.0433 inches) as a result - this was not a good decision AVM. This is the case with the MT-F, as well as the C4 and the C5.

Since mid-2023, AVM has released the X6 model. Based on the product photos, however, I'm sorry to say that AVM has again chosen a slim design here, a mix of C6 as far as the keyboard is concerned (which is positive) and the ancient MT-F model, with a surrounding frame that is too thin for my taste. It seems at least, in contrast to the slightly curved C4 case, a flat design... that could then be printed better without additional grinding work ;-)

What makes the product so appealing?

A very strong plus for buying AVM devices is that AVM provides its routers and DECT phones with software updates and even new features for a very long time. Using the example of the Fritz!Router 7490, which probably appeared on the market in 2013, as well as the Fritz!Fon C4 (end of 2013), which both received an update in 2023, this is now 10 years of software update support. In terms of sustainability and the avoidance of electronic waste, I think that is certainly a big plus.

A repair is not an option for you?

It's a pity, but if you can't or don't want to repair your C4 anymore, I recommend the C6 model with the somewhat more rounded design. You can currently (as of September 2023) buy it for around 72 Euros. Why should one spend 17 Euros more for the new X6 model (cheapest street price currently 89 euros)? However, I would make a protective silicone cover or print one out of TPU as a precaution... and then add the DIY charging cradle... you can do, but you do not have to :-)

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