Heathkit FM-2 tuner

Heathkit FM-2 tuner

The FM-2 was Heath's first superhet FM tuner. It was introduced in 1950 and replaced Heath's simple FM-1 introduced in 1949 which was, of course, not superhet. The early FM-1 used two tubes, one of which is the rectifier. Like the FM-1, the FM-2 came with a prealigned tuning capacitor with single loop oscillator and antenna coils.

The outward design of the FM-2 is the very similar to the AR-1, Heath's first superhet AM and shortwave receiver.

Heathkit  FM-2 tuner

This tuner was purchased at a swap meet. It did not work at all. The schematic for the FM-2 is available at the Heath schematic site linked from the homepage.

I replaced several out-of-tolerance resistors in the limiter-discriminator circuit. For example, one 100K ohm resistor had climbed to over 200K ohms. However, replacing the resistors did not solve the main problem. Replacing one of the 6SH7 tubes finally solved that. While the tuner worked after the tube replacement, it was obvious that alignment was needed. A quick check of the oscillator showed it to be about 10.1 MHz higher than station markings instead of the expected 10.7 MHz difference. This caused the tuning dial tracking to be off considerably.

Heathkit tuner-receiver ad in Feb. 1951 Radio-Electronics magazine
1951 Heathkit ad

Aligning a Foster-Seely limiter discriminator
I did not have the manual or alignment instructions. A quick review of the chapter on FM alignment in the book, Radio and Television Receiver Trouble Shooting and Repair by Ghirardi and Johnson provided the needed guidance. Following their instructions, I fed an unmodulated 10.7 MHz signal into the IF chain, peaking the voltage on the limiter grid resistor while adjusting the IF transformers. I then adjusted the discriminator transformer input, peaking the voltage across one of two cathode resistors and finally adjusted the output side for a zero volt reading on the audio feed. I also tweaked the oscillator and antenna trimmers.

After alignment, the tuner was able to bring in all the local stations as well as more powerful distant stations. The tuner does not have an RF amp stage so sensitivity is noticeably weak when compared to a modern receiver. However, for 1950 technology, it works reasonably well.

Knight Signal Tracer used as audio amp for the Heathkit FM-2 tuner
Heathkit  FM-2 tuner feeding Knight Signal Tracer

A Hallicrafters S-119 Sky Buddy II receiver was the previous item on the bench.

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