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Page last modified on March 18, 2019

 
 
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Repairing the YAESU "FT-7" transceiver

The FT-7 was an entry level 10Watt transceiver in the late 70’s in Australia (VK) and I had one of them at the time, bought new. It’s so long ago I don’t recall why it was sold but about 10 years ago I picked up a non-working FT-7 from a Ham fest in VK3 for a very low cost. Reason being, it wasn't working and the seller told me this and said he thought it had a “relay fault”. Taken with a grain of salt at the time I might add. It did turn out that a relay was a symptom of the fault, but not the cause. Read on for why.

There's also an FT-7B model which has a higher power output PA.




I'm a home brewer, making all of my 'rigs' and accessories. This FT-7 was the only commercial HF rig I owned.

See also FT-7 internal digital display and vfo


Fault finding:


Fast forward to 2019 and it was finally taken out of the box for a look, to see what the problem was. Quickly, it was discovered that the receiver was “deaf”. I.e. low sensitivity. It took about 1 mV (not uV) to hear anything at all. Strangely, after a PTT, it was even worse.
After some investigation, it was found that relay RL2102, in the antenna switching area, did not drop out after a PTT function. In fact there was still around 6volts on the coil of the relay in Rx mode, effectively holding it in Tx mode. Clearly not correct! Ie it opens the Rx antenna path.

Figure 1 Antenna changeover relay


The FT-7 has a large number of plug-in modules to make servicing “simpler” as it says in the FT-7 handbook. That may well be, but it assumes you have the two different extender modules to do this. So, fault finding initially consisted of soldering wires onto a module in strategic areas, then plugging them back in and signal tracing using the signal generator and oscilloscope. I had to disable the AGC to do this of course as shown in the handbook. (Page 23)

Figure 2 Top of the FT-7 showing plug-in modules. (The 10W PA cover has been removed, left side.)


To cut a long story short, it was found that the area that seemed to be the problem was D2104 which is coupled to the Rx input via a 10nF capacitor. The capacitor checked out Ok but I couldn't find the actual diode! It was finally discovered fitted underneath a tag-strip which does not appear in the handbook pictures. The diode measured around 400ohms in both directions. It was replaced with a 1N4148, being the nearest thing I had to the specified 1S555 diode.

The fault area, D2104.

Figure 3 The tag-strip where the new diode was fitted. Its now visible under the disc ceramic capacitor. Before it was totally under the tag-strip.


Voila! Sensitivity was now close to that specified and the relay no longer stayed on. Oh joy!


Another issue was the input coil for 40M was missing. Strange that it was removed, perhaps to repair another FT-7. No information is available for the coils, apart from a part number. Good luck finding of those in 2019.



Figure 4 The missing 40M coil.

I decided to wind a TOKO 10K former from the junk box with 3 turns on the primary and 16 turns on the secondary with a 100pF to approximately resonate it on the 40M band, along with the tune capacitor in the rig which is of unknown capacitance. The new coil is shown below, just below the crystal. 40M then sprung into life although the Tune cap was very broad. No doubt some changes to the 100pF cap value would fix this, but testing showed sensitivity was as good as the other bands, so left alone.

The new hand wound Toko coil fitted.


The S’meter also showed signs of problems, it would stick about 1/3rd of full scale and it didn't fall back unless tapped. The front panel was removed to get at the meter and the meter itself removed and the cover opened. The needle was bent and hitting the scale.

I had a very similar meter in the junk box, although of different sensitivity, and contemplated using it by moving the scale from the sticking meter over. But I thought I should try to repair the original if possible, nothing to be lost. Some dexterous reshaping of the needle cured that problem also.

The transmitter was tested for power on all bands which resulted in about 12-14Watts output. I should add that I didn't get a microphone with the FT-7, but did have an old ICOM microphone from an ICOM IC-22s. A slight re-wiring of the microphone to suit the FT-7 Microphone socket was needed.

While apart, the front panel was cleaned up from of years of grime. Yet to do, clean the knobs also.

Not sure what I will do with this little rig, but very satisfying to get it up and running again.

THE FUTURE?

Plans are now afoot to slightly 'modernise' the FT-7. I am contemplating removing the analogue VFO and display and replacing it with a more up to date digital VFO with either an OLED, colour TFT or the old standby, 7segment LED displays although current hungry, probably driven with an Arduino NANO board. Why? Because it's possible.........

UPDATE:- A very interesting digital dial which emulates the FT-7's analogue dial can be seen below.

It uses an ESP-32/ESP32s development board (eBay) and a colour 1.8" TFT display. I have not yet checked carefully, but it might be possible to fit the display into the FT-7 by removing the existing mechanics and VFO. An encoder is used to replace the VFO knob itself and the VFO output uses an Si5351 board.

 

 

See https://tj-lab.org/2019/02/17/vfo5/ also.

 

Created by VK3PE March, 2019

Disclaimer

The information above is presented in good faith and reflects the fault as found in my FT-7. Similar symptoms in other FT-7's may be caused by other faults.