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

Bob_KA1GT · 14th Jan 2021, 9:06 pm

Well, I'm glad it's working, but sorry it's not actually "fixed". One day It will happen again, and when it does I'm now better equipped to track down the actual issue. I know the 47.6 MHz oscillator has been working all the time, but there could have been a bad buffer stage or mixer. Now it's all working I can take some additional measurement so I know what to expect when looking at things like the VCO and mixer outputs.

Slightly off topic, but does anyone know why the bridge rectifier gets hot enough to fry an egg on! With a small heat sink attached it's still at over 90C which seems excessive. Lots of people report this, so it appears to be "normal" for this radio, but it seems odd it should get so hot when the whole radio is supposed to draw <40W total. So that's maybe 4A or less and the bridge is, I believe, rated at 8A. That's hard to verify though as I can't find the official spec shoot on it. I haven't measured that, but I'm temped to replace the bridge rectifier with a larger unit (15 or 25A) and a larger heat sink. It just doesn't seen right it should get that hot, even though many bridge rectifiers are rated for operation at a case temperature of 100C.

14th Jan 2021, 9:06 pm	#13
Bob_KA1GT Triode Join Date: Jan 2021 Location: Bar Harbor, Maine, USA. Posts: 11	Re: Help with FRG-7700 tuning problem needed Well, I'm glad it's working, but sorry it's not actually "fixed". One day It will happen again, and when it does I'm now better equipped to track down the actual issue. I know the 47.6 MHz oscillator has been working all the time, but there could have been a bad buffer stage or mixer. Now it's all working I can take some additional measurement so I know what to expect when looking at things like the VCO and mixer outputs. Slightly off topic, but does anyone know why the bridge rectifier gets hot enough to fry an egg on! With a small heat sink attached it's still at over 90C which seems excessive. Lots of people report this, so it appears to be "normal" for this radio, but it seems odd it should get so hot when the whole radio is supposed to draw <40W total. So that's maybe 4A or less and the bridge is, I believe, rated at 8A. That's hard to verify though as I can't find the official spec shoot on it. I haven't measured that, but I'm temped to replace the bridge rectifier with a larger unit (15 or 25A) and a larger heat sink. It just doesn't seen right it should get that hot, even though many bridge rectifiers are rated for operation at a case temperature of 100C.