UK Vintage Radio Repair and Restoration Discussion Forum - View Single Post - Help with FRG-7700 tuning problem needed

Bob_KA1GT · 12th Jan 2021, 5:58 am

My FRG-7700 has been sitting unused for a couple of years. It was fine the last time I used it. When I turned it back on today it would not tune. The display counter does not change and the frequency does not change.

I checked the VFO output with a scope. Looks like it tunes from around 2.5 to 3.6MHZ and the output is around 0.5v peak to peak. So I suspect the VFO is fine and the frequency display is probably fine. The inside of the radio is very clean with no sign of corrosion. I re-seated all the multi-pin connectors, but that mad no difference.

What I see is as follows, where xxx indicates rapidly fluctuation digits

At 0-5 Mhz it displays 3.xxx at 0, 5.xxx at 1 and 6.038 at 2,3,4and 5 MHz
At 6-12 MHz the display is 0. xxx
At 13-20 MHz the display is 21.345
At 21-29 MHz the display is 30.554

The radio is working because it picks up an AM station at 6.038 MHz!

I'm not really up to speed on what's going on inside the 7700 but I think the VFO output is mixed with various PLL oscillators to generate the desired Rx frequency. Do the values displayed above (21.345, 30.554) give any clue as to what's going on? Are there any known "weak points" in the PLL circuitry that should be looked at first? Is there a PLL unit troubleshooting guide? I've looked on the web but not found anything very useful yet.

Any hints/tips from those familiar with the radio would be much appreciated.

12th Jan 2021, 5:58 am	#1
Bob_KA1GT Triode Join Date: Jan 2021 Location: Bar Harbor, Maine, USA. Posts: 11	Help with FRG-7700 tuning problem needed My FRG-7700 has been sitting unused for a couple of years. It was fine the last time I used it. When I turned it back on today it would not tune. The display counter does not change and the frequency does not change. I checked the VFO output with a scope. Looks like it tunes from around 2.5 to 3.6MHZ and the output is around 0.5v peak to peak. So I suspect the VFO is fine and the frequency display is probably fine. The inside of the radio is very clean with no sign of corrosion. I re-seated all the multi-pin connectors, but that mad no difference. What I see is as follows, where xxx indicates rapidly fluctuation digits At 0-5 Mhz it displays 3.xxx at 0, 5.xxx at 1 and 6.038 at 2,3,4and 5 MHz At 6-12 MHz the display is 0. xxx At 13-20 MHz the display is 21.345 At 21-29 MHz the display is 30.554 The radio is working because it picks up an AM station at 6.038 MHz! I'm not really up to speed on what's going on inside the 7700 but I think the VFO output is mixed with various PLL oscillators to generate the desired Rx frequency. Do the values displayed above (21.345, 30.554) give any clue as to what's going on? Are there any known "weak points" in the PLL circuitry that should be looked at first? Is there a PLL unit troubleshooting guide? I've looked on the web but not found anything very useful yet. Any hints/tips from those familiar with the radio would be much appreciated.