Can this FM detector even work?
 

I'm not getting any audio on my Delco experimental car TV. (The audio output amp works fine with an injected signal.) After tracing out the schematic of the FM detector, I'm wondering if it can even work. It doesn't match circuits I've seen for either the Ratio Detector or the Foster Seeley Discriminator. There is no center tap on the secondary of the transformer. Has anyone seen a detector like this? BTW, pin 7 of the 12AU6 goes to ground.

http://antiqueradios.com/gallery/mai...serialNumber=1

Bob D.

That's a strange circuit, but may be functional as a "slope detector." That means it is simply an AM detector, but the resonance of the tuned circuit is tuned slightly off the IF peak. That way, as the frequency of the carrier is modulated, it moves closer to and farther away from the tuned circuit resonance, causing the amplitude to rise and fall.

Normally you don't need two diodes for a slope detector, and I'm not quite sure what is being accomplished by having two diodes here.

It might be a good idea to use an oscilloscope to check if there is an actual audio IF signal going to the 12AU6. There is no point messing with the detector circuit in an undocumented set if it has no signal coming into it.

Could there be a center tap on the secondary that is tied to ground inside the can?

Good point. There might even be additional components hiding in the can. I'll try injecting a 4.5 MHz FM signal into the grid of the 12AU6. As Tom C. pointed out, the circuit may not be getting a signal to demodulate.

(Just a thought, when did the sound carrier switch from AM to FM?)

Bob D.

FM sound was adopted in 1941.

Looks like an early flux capacitor to me....

At first glance I thought this resembled a (push-pull) voltage doubler more than any FM detector that I'd ever worked with.

I thought I'd compare it with a known pre-WWII TV AM sound detector and looked at the RCA TRK-5 and got a surprise:

http://www.earlytelevision.org/pdf/r...-rider-tv1.pdf

In that set, V-10 the 6B8 pentode, is the 2nd sound IF and its two plates on pins 4 & 5 are the AM detector.

You will notice a circuit similarity (although the TRK-5 detector cathodes are common, and yours are not), and it is drawn quite differently.

It is a known fact that some of these 1939-1940 sets detector transformers were intentionally mis-tuned (for AM) to keep them running in the early days of FM sound when replacement parts were not available.

I'd say that this circuit could probably work as either an AM detector or an FM detector depending on the tuning.

Notice the other components inside the transformer in the TRK-5 circuit.

Also, notice that one of the diodes has both sides connected to ground, therefore it is not used.

In your circuit, I think they were trying to increase gain by capturing both polarities of the slope to increase level.

Jas.

The last slope detectors were used in 1946 and 1947 kit sets. None were used in any production set after that. In addition slope detectors won't work in intercarrier sound designs, which I'm sure is the case with this set. This isn't a slope detector.

There is a slope detector description at the bottom of that TRK-5 file you uploaded,
describing the mis-tuned detector circuit.... Seems to me that actual tuning is more critical because of where on the slope the detection needs to take place.....

Something missing in that hand written schematic...? did you double check it.....?


Still looks like a flux capacitor.....

Skip injecting a 4.5, Sweep it and see what those two sides are tuned to...

Is it a single core ?

Hi Steve,

Is the reason a slope detector wouldn't work with intercarrier sound because there is amplitude modulation present on the 4.5 MHz carrier from the video carrier? Just curious.

Tom, that is correct.

The polarity of the diodes suggests Foster-Seeley. Also, there must be some caps in the can to tune the coil.

Its clearly a discriminator. There's a center tap on the secondary
and a coupling cap inside the transformer, in addition to
tuning caps.