Restoration of a Zenith K725 (chassis 7K01) AM and FM Broadcast Band Radio

What a great sounding radio!  This one features negative feedback class A audio output section with tone control, making it by far one of the best sounding radios of its era and holds its weight well with modern music.

This radio belonged to my great-grandfather.  I was told he would listen to it while staring out at the lake from his cottage.  When I was a kid, i used this radio to tune in to my favorite FM stations until it stopped working one day and was buzzing too loudly.

Summary

• This is a dual AM and FM broadcast band receiver.

• It features a tone control

• Class A audio output power amplifier with negative feedback

• Civil Defense (CD) indications on the AM dial, which presumably must have been for tuning in special radio stations in case of world war three.

• High quality build, easy to service inside.

• I replaced all of the paper capacitors and the power supply electrolytic multi-tap canned capacitor.

• The dial cord was also broken and had to be re-done.

• The power/volume control was seized up and had to be replaced, although the replacement was not as long and not designed to mate with the original knob.  I had to use my dremel to machine out the new power/volume control so that it would work with the original vintage knob from the radio.

• All of the internal wiring was in good shape.

• None of the resistors had to be replaced.

• The large speaker allows this radio to really blast out the tunes.

• This radio sounds amazing when listening to the local jazz program.  It provides a warm tube sound to FM broadcast stations.  Most notably are Hammond B3 organs and jazz guitar.

• It was a true joy to restore this radio and hopefully it will be in use for many more decades.

Engineering Notes

Zenith K725 schematic