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Neither the MR-71 or the FM-3 have external de-emphasis switchability (as you already know), and the correction can not be accomplished via interconnect. The change to the circuit must be done internally, and with the FM-3 at least, a series of capacitors and resistors must be changed, as shown on the circuit boards above, indicated by the red arrows.

The circuit schematic to change the FM-3 from 75 µS to 50 µS may be available somewhere, but you'd have to know your way around a soldering iron, even if you do find it. I can't speak for the MR-71, which may involve changing a single resistor, but I have my doubts about that.

I think the easiest route would be to do as itman suggested and find yourself an FM tuner with an external switch, such as his Pioneer F90 has.

The good news may be that FM tuners are often available now at nearly-give away prices. Visit the Tuner Information Center (link below) and they may have tuners with externally switched de-emphasis listed, to make your search easier.

Edit: If you have any interest, here's a good explanation of Pre/De-emphasis in FM modulated transmission systems.

"Pre/De-emphasis in FM modulated transmission systems

There is greater noise during FM modulation transmission at higher audio frequencies than at lower audio frequencies. The broadcasting industry developed some guidelines for Pre-emphasis and corresponding De-emphasis to correct for this imbalance and even out the noise across the audio spectrum. Pre-emphasis is used in a modulator to enhance the audio before modulation.

After demodulation, the receiver employs de-emphasis to restore a flat audio frequency response. As a result, the Signal-to-Noise Ratio of any particular FM transmission technology is greatly enhanced. The FM radio de-emphasis standard in Europe is 50 microseconds, while it is 75 microseconds in the USA."





Edits: 11/01/23

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