Can someone identify this component?

[teemuk]

I was cannibalizing this old integrated tuner-amplifier when I came across this mysterious component (see attachment). I don’t have a photo but one would not likely be any help either since there were no type markings of any kind. Here’s a verbal description, though: The component has three pins and two of them are closer to each other like depicted in the (quickly drawn) picture. One pin connects to the ground reference, one pin connects to audio output of the tuner circuit board and one pin connects to audio output of FM stereo demodulator chip (MC1309P) through a coupling capacitor. There were two of these components, one for each channel. The component has a plastic casing (without type markings) and the plastic case was covered by a metal “cage” that was bent from thin sheet metal. Its size is that of an ordinary relay – and relay was in fact the first thing I thought the component was. However, considering the pinout and the circuit configuration that doesn't make any sense.

I am completely puzzled by this - could it be somekind of an auto-transformer?

[poynton]

Originally posted by teemuk
I was cannibalizing this old integrated tuner-amplifier when I came across this mysterious component (see attachment). I don’t have a photo but one would not likely be any help either since there were no type markings of any kind. Here’s a verbal description, though: The component has three pins and two of them are closer to each other like depicted in the (quickly drawn) picture. One pin connects to the ground reference, one pin connects to audio output of the tuner circuit board and one pin connects to audio output of FM stereo demodulator chip (MC1309P) through a coupling capacitor. There were two of these components, one for each channel. The component has a plastic casing (without type markings) and the plastic case was covered by a metal “cage” that was bent from thin sheet metal. Its size is that of an ordinary relay – and relay was in fact the first thing I thought the component was. However, considering the pinout and the circuit configuration that doesn't make any sense.

I am completely puzzled by this - could it be somekind of an auto-transformer?

Band-pass filter

[teemuk]

Now why didn’t I think of that?

Makes sense. Thanks Poynton.

[head_spaz]

It sounds like you're describing a "SAW Device."

A SAW device is basically a capacitive piezo driver mounted on a substrate with a piezo receiver on the other end. It only couples signals in a fairly narrow band from transmitter to receiver. As I recall, the substrate is actually resonating (standing waves). They make awesome filters.
Sometimes they're called SAW Resonators, but are more commonly referred to as SAW Filters.

[Elvee]

Originally posted by poynton


Band-pass filter

More accurately, probably a combination of low-pass filter and notch filter(s). (At 19 and 38KHz).

[Petraman7]

A SAW filter wont output an audio signal. It's most likely a filter. Input and output have a common ground

[oshifis]

Originally posted by Elvee


More accurately, probably a combination of low-pass filter and notch filter(s). (At 19 and 38KHz).

They used it back when tape recordes were more common than nowadays. The bias oscillator created a very annoying audible beat with the 19/38 kHz, the filter was there to prevent this.