Sunday, May 03, 2015

FT-707 repair (1) 20 and 30m not working

This is part one since it's not finished yet.

Last week I bought not one but two Yaesu FT-707, a full line and another one for spares but stated, aligned by one of the best radio technicians around....

Here it is, part of the set:
Missing in the picture the second radio and the antenna tuner.

Ok, so after power on here's the list of faults by set, reception only, since I didn't TX yet:

Set 1:
-No VFO display ligh
-No complete external VFO connectivity, tough power is there.

Set 2:-
- No 20m
- No 30m
- CB band's on 10A,B,C and D

Obviously unit 1 will be, hopefully, quickly fixed so I started looking on unit 2:

* The problem with 20 and 30m band is the display showing of around 91 Mhz, a common issue on this radios, normally a bad contact on the selector but not on this one.

Let's look on the premix unit, those 11 coils, red is 30m and blue 20m:

After connecting the scope and not getting the overtone frequency, only the fundamental... I started to suspect on fault components at the tank part, that was when I notice that the coils were not the original of the radio...
I removed the coils and placed a "work around" on the 20m one just ot check if I got the overtone:

Basically 10 turn on an T37-6 (the yellow coil in the picture) will resonate on around 28Mhz with 100pf, see top black circle on the schematic bellow, 28.487 Mhz is the premix for the 20m band.  It did resonate with my workaround, good, the coil that I removed would never work.
I didn't tested the resonance, yet, of the removed coils but I suspect that they were placed by the very good technician that take care of the radio just because they were similar, one of then even had the parallel cap in place... oh well, any "butcher" can get a soldering iron!

This is the general idea of the workings of the premix:

And here the two problematic oscillators:

The coils are branded Mitsumi (628 or 608 not sure now) and from what I searched they are a bit of unobtainable... so what to do? Easy, get the two coils from the 10m A and 10m B band since the radio was converted to CB and I don't give a s$%  about. It will work until I can get two new ones along with proper 10m band cristals.

Continuing the saga.
I got after the bandpass filter for the premix a good 30m band LO signal but not a good one for the 20m band, so after some testing I have to check again the switching of the 20m band LO signal bandpass filter.

Regarding the 30m band I was listening to the marker generator but not to air signals in the end of this troubleshooting that took me almost 2 days I got a glimpse of the issue, If I bypass the first bandpass filter I can listen to 30m band so it means either the relay of the input is not operative (the relay or the control lines) or the band pass filter is open or not switching. Some hope!

The 30m band second filter from antenna (in red):

Here's the problematic LO for the 20m band:

 More fun for the next few days...

Have a nice week!

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