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Step 3Modifying the receiver
The receiver should not respond to the commands of the modified remote control because the input filter is still tuned to a different frequency. There are two ways to make the receiver detect the signal.
1. You take a very small screw driver and turn the ferrite inside the original coil to adjust the filter to the new frequency.
2. You build a new filter using a filter set and some small insulated copper wire.
The first option fails most of the time, because trying to turn the ferrite core, breaks it. The ferrite core is fixed in place with wax. It is very, very unlikely thet you manage to get the wax out of the core and then be able to turn the ferrite core. You can try to get the wax out by carefully warming the coil with a hairdryer. You have to be very careful not to melt the plastics of the core. I tried to adjust filters this way some times and nearly every time I failed. Once the ferrite core is broken it is stuck in the plastic core and you can't turn it in any direction anymore.
If the first method fails you need to take the second because you filter is broken. You have to do a little bit math to calculate the ammount fo windings needed for the coil. In this case I did the following math.
Find out the inductance of the original coil. The diameter is 5mm. The wire is 0.3mm thick. There are 7 turns on the coil. The operating frequency is 27.415Mhz. The length of the coil is 3mm.
Using the formula L=N2*D2/l (L=inductance, N=number of windings, D=diameter of coil, l=length of coil) I get an inductance of 0.408uH. I assume that we do not change the value of the filter capacitor. I got an AL value of the new filter from the data sheet. The inductance can be calculated with L=AL*N2 . Using this I get: N=sqrt(L/AL), leading to N=9.
Our new coil also uses a core with 5mm diameter. We also use the same wire (taken from a wire wrapping tool).
I also simply could have taken the 7 turns from the original coil. The two cores are very similar. The values should not differ too much. The calculation is not exact, but gives a good estimate how to wind the coil. Using 9 windings gives us the possibility to take some windings back if we are not able to adjust the new coil to the new frequency. Even if you try to use the old coild it could be necessary to rewind it in case the adjustment does not work. We lowered the frequency, therefore you have to add windings to the coil to make it work.
In my case the 9 turns worked brilliant! ;-)
1. You take a very small screw driver and turn the ferrite inside the original coil to adjust the filter to the new frequency.
2. You build a new filter using a filter set and some small insulated copper wire.
The first option fails most of the time, because trying to turn the ferrite core, breaks it. The ferrite core is fixed in place with wax. It is very, very unlikely thet you manage to get the wax out of the core and then be able to turn the ferrite core. You can try to get the wax out by carefully warming the coil with a hairdryer. You have to be very careful not to melt the plastics of the core. I tried to adjust filters this way some times and nearly every time I failed. Once the ferrite core is broken it is stuck in the plastic core and you can't turn it in any direction anymore.
If the first method fails you need to take the second because you filter is broken. You have to do a little bit math to calculate the ammount fo windings needed for the coil. In this case I did the following math.
Find out the inductance of the original coil. The diameter is 5mm. The wire is 0.3mm thick. There are 7 turns on the coil. The operating frequency is 27.415Mhz. The length of the coil is 3mm.
Using the formula L=N2*D2/l (L=inductance, N=number of windings, D=diameter of coil, l=length of coil) I get an inductance of 0.408uH. I assume that we do not change the value of the filter capacitor. I got an AL value of the new filter from the data sheet. The inductance can be calculated with L=AL*N2 . Using this I get: N=sqrt(L/AL), leading to N=9.
Our new coil also uses a core with 5mm diameter. We also use the same wire (taken from a wire wrapping tool).
I also simply could have taken the 7 turns from the original coil. The two cores are very similar. The values should not differ too much. The calculation is not exact, but gives a good estimate how to wind the coil. Using 9 windings gives us the possibility to take some windings back if we are not able to adjust the new coil to the new frequency. Even if you try to use the old coild it could be necessary to rewind it in case the adjustment does not work. We lowered the frequency, therefore you have to add windings to the coil to make it work.
In my case the 9 turns worked brilliant! ;-)
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Dec 19, 2008. 2:18 PMstonefisk
says:
Never ever use a normal screwdriver to adjust ferrite cores, they will surely always break! That's a cowboy mistake. Use proper ferrite core trimmer tool for the slot size, usually made of plastic. Vintage radio repair publications state that age hardened locking wax can be softened using ear wax busting products like "Earex" worth a try before going to the extent of winding own coils.
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Dec 20, 2008. 7:16 AM
frickelkram (author)
says:
frickelkram (author)
says:
Right, there are special tools available. Using standard screw drivers is a bad idea because the blades are to thin. They twist and put to much pressure on the edges of the ferrit core. I made some tools on my own, using plastic rods. I filed a tip that was formed a little bit smaller than the ferrite cores hole. But even with the correct tools the wax is still a problem. Never heared of "Earex" beeing used for removing the wax. Nice Idea, I'll give it a try. Thank you for the comment!
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