wa2ise
Super Member
We are always happy to tell and describe successful modifications and restorations of tuners and amplifiers, but not so much when it doesn't work out.
Background: Years ago I struggled with 2 FM radios, one a GE SS table radio, and another a cheapie SS FM stereo receiver, both using a bipolar transistor as an FM RF amplifier. No input tuned circuit. So the entire FM band (and TV channel 5 and 6, and some aviation above 108) is hitting these transistors. And living near New York City the band is completely packed with stations. And lots of interference from I think intermod products, probably due to lousy 3rd order intercept http://en.wikipedia.org/wiki/Third-order_intercept_point. A fellow ham mentioned that bipolar transistors in FM front ends had poor dynamic range, and FETs would be better. And that tube front ends didn't have this problem. Managed to replace the front end bipolar transistors in both of the above sets, and got it to work, much less interference. (you need to change the biasing from what the base had to what the FET wants (I used an MPF102 JFET) the FETs looking like a triode tube circuit, gate at ground potential, and the "cathode" drain? has a resistor, likely the same the old transistor had, and the plate goes where the collector went, usually the first tuned RF circuit). FETs being about the same physical size as the old bipolar transistor it usually worked for me.
Well, tried it with a triode subminiature tube (a 5703) in a junk tuner (no big loss if it didn't work out), and it didn't work out. The tube would go into oscillation at around the frequency of the FM stations being tuned, blocking reception. Shielding the outside of the tube helped some, not enough. Couldn't get enough multiple shield wires from the tube shild to various radio RF grounds. This was a single sided circuit board I was trying this on. At this point I declared defeat
and decided that there's a good reason why FM tuner kits (like my father's Eico HFT90) always supplied the FM front end as an already built module, as 100MHz and tubes are just too squirlley... Maybe if I had a completely enclosed metal box with internal partitions, but that was too much agony...
Background: Years ago I struggled with 2 FM radios, one a GE SS table radio, and another a cheapie SS FM stereo receiver, both using a bipolar transistor as an FM RF amplifier. No input tuned circuit. So the entire FM band (and TV channel 5 and 6, and some aviation above 108) is hitting these transistors. And living near New York City the band is completely packed with stations. And lots of interference from I think intermod products, probably due to lousy 3rd order intercept http://en.wikipedia.org/wiki/Third-order_intercept_point. A fellow ham mentioned that bipolar transistors in FM front ends had poor dynamic range, and FETs would be better. And that tube front ends didn't have this problem. Managed to replace the front end bipolar transistors in both of the above sets, and got it to work, much less interference. (you need to change the biasing from what the base had to what the FET wants (I used an MPF102 JFET) the FETs looking like a triode tube circuit, gate at ground potential, and the "cathode" drain? has a resistor, likely the same the old transistor had, and the plate goes where the collector went, usually the first tuned RF circuit). FETs being about the same physical size as the old bipolar transistor it usually worked for me.
Well, tried it with a triode subminiature tube (a 5703) in a junk tuner (no big loss if it didn't work out), and it didn't work out. The tube would go into oscillation at around the frequency of the FM stations being tuned, blocking reception. Shielding the outside of the tube helped some, not enough. Couldn't get enough multiple shield wires from the tube shild to various radio RF grounds. This was a single sided circuit board I was trying this on. At this point I declared defeat
and decided that there's a good reason why FM tuner kits (like my father's Eico HFT90) always supplied the FM front end as an already built module, as 100MHz and tubes are just too squirlley... Maybe if I had a completely enclosed metal box with internal partitions, but that was too much agony...