It's a GE portable/ electric radio with AM, FM broadcast and FM from 150 to 174MHz (VHF highband). It works reasonably well on FM, but on moderately weak AM stations it sounded like crap. Changed the AM detector diode from silicon to a geranium, not much better. But if I run a small amount of bias current (around 27uA) thru that detector diode, I can make it sound much better. Making the diode knee less tall, so weak IF signals can get thru. I'm also counteracting a back bias, a biasing circuit for an FM IF amp transistor. As the diode's circuit is the grounded IF transformer secondary, then diode, then a 220K resistor to a switchable -6V source. Turns out this radio uses the AM detector circuit resistors for FM IF amp biasing, so I had to make that -6V source switch off in FM mode, else the FM level and quality suffered. So I rigged up a few transistors to level shift an internal switched B+ of about 3.9VDC to switch on or off the -6VDC. The -6VDC I obtained from an extra rectifier diode and filtered off the little power transformer.
03-06-2014, 09:16 PM
Linear Mode
