WOT, transistor radio question....

[orange_glow_fan]

(At least it's 'older'...)

Working on an old AM/FM transistor radio that has had both the FM RF and convertor transistor short out. (I suspect a near miss lightning strike)

Also, L3 checks stone open out of circuit.  I have never seen a coil like this and don't understand it's operation. ASSUMING it's drawn right...

My questions is, is this coil, labeled as a 10.7mc trap, drawn right? Does it also function as the ground path for the convertor transistor's emitter?

There are only two leads on the coil.

Ordinarily I would just replace the transistors BUT they are a royal pain in the behind to get to and I really don't want to invest the effort if there's no chance of success...

[gregebert]

L3 is before the mixer, so it's still 88-108Mhz raw VHF (FM). My best guess is that L3 and the resistor make up a crude high-pass filter, especially if this is an AM/FM radio, to prevent AM (and other signals) from interfering with the mixer. I dont understand why the other side of the coil has only 1 connection. Perhaps it's a clever/bizarre way to minimize noise.

You are correct about the resistor; it provides a DC path for the bias current of Q2.

Do you have an RF oscillator and scope ? You could inject a 100Mhz signal at the antenna terminals and use a scope to find out where things die.

[GastonP]

L3 is as described by the OP, a 10.7 MHz trap. It avoids feedback from the IF (10.7 MHz) to make the whole radio to oscillate at that frequency. It works by making the stage an "open circuit", then making gain 0 at the IF frequency.

The "one connection" is simply the way to use the interwinding capacitance as the "C" in the LC tuned tank. The resistor in there is just the emitter bias resistor for the common base stage, and is not a part of the IF trap.

[orange_glow_fan]

Well I had an Ah HA! moment.. after a significant amount of research online, I found a Zenith radio with a similar chassis and a MUCH better schematic. This one shows L3 that makes sense!

Thanks for all the input...

Kerry