Early in January 2015 I went in to hospital for a second knee replacement and a friend decided it would be good for my recovery if I were to fix this radio he had bought on Ebay. When he received it there was sign of transit damage and although he had been assured by the seller it was in good working condition it did not show any sign of life. My first knee operation in October 2012 took me many months to recover from but the specialist assured me the new proceedure he wa using would have me back "dancing" in less that twelve weeks. Funny about that, I have never been able to dance.
Peter lives in country South Australia and drives to and from the city to work and so after work one afternoon he called by and gave me the radio, saying fix it and its yours. He was impressed to see the progress I had made just two weeks after the operation.
The first thing I found was the Local/DX switch had fallen apart, not uncommon with those little slider switches. Replaced it with a small toggle switch and did a modification to that circuit suggested on the Fox Tango web site. Found a full service manual on the web and a great survival guide by PA0PGA in the same place.
Took the front panel off and cleaned the control knobs in an ultraconic cleaner and added the 12 Volt DC input connector. Much happier running these old radios off a 12 Volt supply.
When I put it back together I could hear background noise but no RF signals until I put a hand near the output end of the RF front end board while tuning the chassis over to look at the other side. I had tuned the dial to 891 AM, a local ABC station, just in case I managed to push a signal through somehow. In came the BC stations and the dial frequency was correct. I soon decided the problem must be in the band pass filter (BPF) switching or the Dual Gate FET first amplifier, not an uncommon failure in this generation of receivers. I guessed the band pass filter (BPF) switching may still working because as I swept through the bands I could hear a click each time I got to the frequency at the ends of each BPF range. There are six BPF's and they cover the full range from 100KHz to 30MHz. There is a one of six decoder driven from the digital circuits that controls BPF switching and after sketching this part of the circuit so I could read it I found that this was working correctly.
I then measured the voltages around the Dual Gate FET and they looked correct. As I put the probe on G1 of this device in came that signal again, this time much stronger giving a good indication on the S Meter. I then resorted to some real high tech test gear, Radio Station 891 and a thin pointed scriber, injecting the signal into the output of each filter. The four highest filters gave the same effect as on the G1 of the device itself but the two lowest frequency filters gave no output. There is a diode on the output of each BPF that couples the signal into a track that runs to the input stage and at first I thought there was break in this track or two faulty diodes, most unlikely but PA0PGA mentions finding several faulty diodes in the FRG-7700's he has fixed.
Time to take the BIG PCB out. After disconnecting a lot of small connectors and removing a lot of screws I was able to slip the board out (taking great care with the display), stand the radio on its front face and the PCB was now easy to access. As I was heating a pad for one end of the first diode I noticed the bent over lead wire from the diode was also comming out of the hole. All six diodes are folded over and mounted verticle. Looked at the other diode and it was the same. I guess If I had connected an antenna when I first stared working on the radio and tried the higher bands the radio would have at least shown more signs of life..
I took out the two broken diodes and installed two new 1n4148', plugged in all the connectors put a few of the screw back and Oh No, it still did not work. Using the previously mentioned high tech test gear, this time around the inputs to the BPF's, I found the two diodes on the inputs to the two lowest filters were also open circuit and this time I was able to have a closer look and the diodes sure looked like they had been cut ! This made me have a closer look at the two diodes I had removed and yes, they had been cut also. Was it an act of sabotage by someone in the past hoping to pick it up for a "song" at an auction or a sale ? Was it someone using a hack a chop method of fault finding ? I doubt the later as there does not seem to be anything else wrong with this radio. The circles in the last photo above indicate the position of the four diodes.
I have found the tuning to be very sharp, there needs to be a higher reduction in the drive between the tuning knob and the VFO. The tuning shaft has a slight bend and perhaps there is another reduction mechanism built into this area ? If you know the answer to this quection please let me in on the secret. There is also a suggested modification on several web sites that adds a Varicap Diode inside the VFO module and uses the "M Fine" control to provide a fine tune function when the memory module is not fitted.
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Written by and Copyright, Phil. Storr © Last updated 8th February 2015