Sunday, June 28, 2015

FT-707 repair (10) Finding replacement coils

As stated on a previous post of the FT-707 repair series I had to remove two coils from the 10m A and B local premix oscillator, to use on the 40 and 30m band because there was a previous owner that really made a mess out of it.
Now I'm the the process of finding replacement ones, the coils are a non existing item and there is not much information on their specs so I got some coils from the junk bin and started to find which ones would work by winding them on a trial and error process.
For that I replicated the oscillator part from the technical manual on a small PCB:

..the assembly:

 ..this way it's easier to test multiple coils.

The output of the circuit looks similar to this when the coil is ressonant or near ressonance:

 After some testing I already have one coil finished and in place, it's not perfect but for now fits the bill:

I used some IC terminals to test in the board also and if needed replacing latter on: was easier this way to avoid do much soldering on the board.

 The coil is marked "3392" and comes from an unknown board that my guess was used on a wallkie-talkie (has cristal holders and a side switch), whatever that coil match's in terms of specs or inductance to is unknown to me, I have to build and inductance meter yet.
I run 2 turns for the secondary and 6 for the primary for 10m B band, the removal or increase in the coil number is done by testing with a crystal upper in frequency and also lower of the main one for test and see if the output increases or decreases, eg: if more amplitude on a 50Mhz than on a 35Mhz for a 42Mhz center frequency I assume the higher the amplitude the closer to the resonance frequency so has low inductance and more turns needed (keeping the same parallel cap, 39pf).

If you look closer still missing the first coil for the 10mA, top of the right column of coils, I already have one tested, just not in place, marked "3410 E 0" that works with a 14Mhz crystal (42Mhz overtone), just waiting to find a suitable crystal for 42.5Mhz, the one I previously placed there doesn't want to oscillate on overtone, was placed before proper testing and was removed.

Now it's like this:

 ... except that the band is already on the correct band soon as I have the 10m A in place I might leave 10m C and D with the CB frequencies :)

Have a nice week,

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