Heathkit AJ-1510A (1975)
The AJ-1510A is a digital FM stereo tuner. Heathkit's own literature calls it "one of the first synthesized FM Stereo tuners to be introduced in the 1970's," and the catalog copy brags that Heathkit had "scooped the world's manufacturers - including the Japanese." HiFI-Stereo (November 1972, p.106) was just as enthusiastic about the related Heathkit AR-1500 receiver, praising its 90 watts per channel and 1.8 µV sensitivity. The AJ-1510A sold as a kit for $499.95 per the 1975 Heathkit catalog listing. The research record carries $579.95 from another source, so treat the kit price as unsettled. It was Heathkit's flagship tuner, sold on digital frequency synthesis instead of a conventional variable capacitor, with tuning accuracy quoted at 0.005%.
The manual lays out what made it unusual. Tuning was digitally synthesized, and there were three ways to get to a station. You could type the frequency on the front-panel keyboard. You could let Auto-Sweep count down the FM band and stop at each listenable station (or only at stereo ones). Or you could hit one of the A, B, and C buttons to recall stations programmed onto computer-type punch cards. Heath supplied extra blanks so every favorite could have its own. A front-panel meter showed relative signal strength and doubled as a test meter during kit assembly and checkout. Multipath outputs for DC-coupled scopes were provided, along with adjustable AGC and squelch. The manual's spec block lists 1.8 µV sensitivity, greater than 95 dB alternate-channel selectivity, 1.5 dB capture ratio, 0.3% mono harmonic distortion, and stereo separation of 40 dB at 1 kHz falling to 20 dB at 15 kHz. The 1975 catalog adds frequency response of 20-15,000 Hz ±1 dB and 75/300-ohm antenna inputs. The schematic shows the guts: a phase/frequency detector and PLL divider chain, MC1350/MC1357 IF and demodulator ICs, an MC1310 multiplex decoder, and a keyboard/readout section built around 7400-series TTL and a seven-segment decoder driver. The cabinet measured 6" high by 16¾" wide by 14¾" deep.
Not everything is settled. The catalog spec block is headed "111-1510" even though it appears under the AJ-1510A listing. The schematic's revision notes flag that the earlier AJ-1510 differs in several board assemblies. The IF/demodulator board in particular was reworked, replacing one IC with discrete parts. Collectors should confirm which version they're looking at. The known weak points are the original power supply capacitor, which should be replaced preemptively, and the obsolete PLL IC, which is hard to source. A display upgrade is still sold for the model. One restored unit reportedly changed hands for $1,200 in 2020, which is a single data point and not a market average. Selectivity figures beyond the manual's alternate-channel number, and dimensions beyond the catalog's, are not documented in the sources we hold.
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