The Heathkit AJ-1510 is an FM stereo tuner dating from 1972, utilising a Phase-Locked-Loop tuning system built from hard-wired logic with SSI/MSI ICs.

The Sherwood Micro/CPU-100 from 1976 is presented in another article as an early example of embedding a microprocessor in a common home-use or consumer item. The encroachment of digital technology into this arena began a few years earlier. The Heathkit AJ-1510 is an example of this, without the microprocessor. The digital display, push-button keyboard entry, and lack of rotary knobs was very futuristic in 1972.

Some of the features of the AJ-1510:

The RCA Numitron display tubes used in the AJ-1510 can be problematic. As hot-filament incandescent displays the segments do burn out. These displays were introduced around 1969, shortly before 7-segment LED and vacuum-flourescent displays, and were obsolete almost from the day they were introduced. In addition, the AJ-1510 uses the smaller and rarer of the two sizes of Numitron made by RCA. See the Metrotech digital clock for an example of use of the larger version of Numitron. Numitrons were not extensively used and can now be difficult to source.


A number of controls are hidden behind a push-to-open door at the bottom of the front panel. Up to 3 station-selection cards plug in on the right side.

Close-up of the frequency display. The display is 4 incandescent RCA Numitron tubes situated behind a green filter. A lamp-test button is included in the controls to check the segment filaments.

The Numitrons exposed. The shrouds around the tubes reduce reflections of adjacent digits from the glass envelopes.

A station-selection card. The cards are programmed by notching out the 1-bits in BCD encoding of the frequency.

Most circuitry is held on seven fairly low-density plug-in circuit boards.

The front panel is connected to the backplane via a large conventional-for-the-time wiring harness.

Power Supply circuit board.
Discrete regulators for +5V, +15V and +30V. The +30V is required for the tuning voltage applied to the vari-caps in the front-end.

Programmer circuit board.
Latch to hold the selected station frequency in BCD representation, and logic to control the frequency selection. The frequency latch also functions as a down counter in the auto-sweep mode.

Preload Decoder board.
Logic to convert the selected station frequency to a division factor (also in BCD) for the tuning PLL.

Generator-Oscillator-Divider board.
Crystal reference oscillator & loadable divider for the tuning PLL, along with a pulse generator for stepping through stations in auto-sweep mode.

Tuner-PLL board.
The tin cover has been removed here from the shielded front-end module. In the shielded box are the VCO for the tuning PLL, the RF amplifier & the mixer. Circuitry outside the metal box is the phase detector & initial divide-by-8 logic for the tuning PLL.

IF board.
IF amplifiers, FM pulse-width demodulator using a TTL monostable, AGC detector, signal-level and multipath sensing.

MPX-Audio board.
Stereo multiplex decoder based around an MC1310 PLL decoder IC, audio line amplifiers, sweep-stop logic and squelch logic.

  Unit Log
Heathkit AJ-1510
2015 Jun