1. Restoration of a Zenith K725 AM/FM radio

  2. Built February 22, 1954 (stamped inside of radio)

  3. AM and FM broadcast band receiver

  4. Information provided:

  5. summary of restoration

  6. schematic

  7. Summary of the restoration (see schematic and photos):

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

  9. It features a tone control

  10. Class A audio output power amplifier with negative feedback

  11. 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.

  12. High quality build, easy to service inside.

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

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

  15. 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.

  16. All of the internal wiring was in good shape.

  17. None of the resistors had to be replaced.

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

  19. 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.

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

Engineering Notes

  1. Zenith K725 schematic

  2. G. L. Charvat, “Repair and Restoration of Antique Radio Equipment,” MIT Haystack Observatory, October 21, 2009.

  3. The author is not responsible for any injury or death that may occur due to working on antique radio equipment.  Antique radios are dangerous because of the high voltages inside of the radio equipment.  Do not work on antique radios unless you have proper training or experience.  Serious injury or death can occur.  These briefings are for reference only.

  4. Repair and Restoration of Antique Radio Equipment (.pdf)

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

Photos of the restoration

Zenith K725 inside and out

November 2009