Heathkit AJ-1600 (1976–1977)
A $469.95 kit that will pull in stations nothing else will touch - and then make you regret ever opening the case.
Overview
Heathkit's 1976 answer to what a tuner should be was the AJ-1600: solid-state, digital and expensive. A period catalog listing in High-Fidelity (1981, p.134) notes the pushbutton up/down tuning, adjustable muting threshold, and switchable narrow/wide IF bandwidth alongside the 4-digit readout. The tuning scale is digital and the bands are FM and MW. The chassis is a low-profile tabletop design, 16-1/2" wide by 5-1/8" high by 12-1/2" deep, and it weighs 14 lbs. That's a lot of aluminum and circuit board for a tuner. Inside you get a phase-locked-loop multiplex demodulator, a MOS-FET front end, ceramic IF filters, and a crystal-controlled decoder.
The front panel carries signal-strength and center-tuning meters, interstation muting with an adjustable threshold, and pushbutton up/down tuning. High-Fidelity cataloged the pushbutton tuning, the adjustable muting threshold, and the switchable narrow/wide IF bandwidth alongside the 4-digit readout. The display is a vacuum fluorescent tube with a dimmer. There are built-in AM and FM antennas, plus a symmetric screw terminal and a 75-ohm F-type antenna input for those who want to do it properly.
The spec sheet is where the AJ-1600 makes its case. FM sensitivity is rated at 1.8 µV for 30 dB quieting, signal-to-noise at 65 dB, and THD at 0.3% at 65 dBf input. Capture ratio is 1.5 dB, selectivity 70 dB at ±400 kHz, and image rejection 75 dB. The FM frequency response runs 20 Hz to 15 kHz, output level is 1.0 V, and the FM IF bandwidth is 280 kHz at −3 dB. The AM section is rated at 30 µV with the internal antenna and 10 µV with an external one. Those are respectable numbers for a kit tuner of the period, and they explain why the AJ-1600 has a small but devoted following among people who want weak stations.
Key Features
Phase-Locked-Loop Multiplex Demodulator
The multiplex section is the heart of any stereo FM tuner, and the AJ-1600 uses a phase-locked-loop multiplex demodulator. The manual's parts list identifies the multiplex IC as an HA11223, a chip that handles pilot cancellation, stereo indication, and PLL phase correction in one package. A well-aligned PLL decoder gives clean stereo separation. A poorly aligned one gives mushy, phasey sound that sends you back to mono. The AJ-1600's decoder is only as good as its alignment, and alignment is the recurring problem with this model.
MOS-FET Front End and Ceramic IF Filters
The front end uses MOS-FET devices, and the IF strip relies on ceramic filters. Ceramic filters give a fixed, stable passband that doesn't drift with temperature or age. They're the reason the AJ-1600 can offer switchable narrow and wide IF bandwidth without a mechanical switch drama. The narrow setting is for crowded dials and weak signals. The wide setting is for strong local stations where you want the full audio bandwidth. The manual lists the IF and demodulator ICs as an HA1137 and a pair of MC1355s, with UA703s handling decoupling duties. None of this is exotic by modern standards, but in 1976 it was the right way to build a tuner.
Vacuum Fluorescent Display with Dimmer
The 4-digit readout is a vacuum fluorescent tube, not an LED or a mechanical counter, and it has a dimmer. A bright VFD in a dark listening room is a distraction, and Heathkit gave the user a way to turn it down. The display tube is driven by an A438112 controller with a separate MC14049 handling the segment drivers, and the tuning microcontroller is a D\$8629. The display is one of the more reliable parts of the tuner.
Signal-Strength and Center-Tuning Meters
Two analog meters sit on the front panel, one for signal strength and one for center tuning. They're the old-school way to know whether you're locked onto a station, and when they work they're more informative than a row of LEDs. They don't always work. A tested unit listed on Reverb had non-functional signal meters while everything else - dial, display, lights, switches - worked perfectly. That's a known failure mode, and it's worth checking before you hand over money.
Switchable Narrow/Wide IF Bandwidth
Switchable narrow and wide IF bandwidth is the feature that makes the AJ-1600 a serious DX tool. Wide mode gives you the full 280 kHz passband for strong local stations. Narrow mode tightens things up for weak or adjacent-channel situations.
Collectibility & Value
The AJ-1600 sits in an awkward spot in the market. It's a capable tuner with a loyal following, but when it breaks it is, in the words of one owner-reviewer, "one of the most notorious door stops in the history of audio components," because servicing is a headache. The muting circuit is the known weak point, often traced to capacitor C714, a 4.7 µF/50 V electrolytic. Power supply filter caps, output coupling caps, and muting circuit caps are all worth replacing as a matter of course, and calibration is a must after any work.
Asking prices on eBay for untested or as-is units run roughly $60 to $300. The observed listings spread wider than that: a working unit at $300, a "Pro Series Rack" example at $340, a restoration recap kit at $125, and one optimistic seller asking $799. The $799 listing is an outlier, not a market signal. The realistic entry point for a project unit is the low end of that range. A restored, aligned example is worth more, but only if you can verify the work was done. Schematics and service manuals are available from several sources.
When it's working, the sound is the reason people bother. One reviewer describes it as "a very good sounding tuner" with "an open, airy FM sound" that "can hold its own against tuners built decades later," and calls the AM section "surprisingly good." Another owner, listing a unit on Reverb, says it "pulls in even the hardest to get stations" and calls it "perhaps the best tuner I have ever used" with the proper antenna. A retrospective in Vacuum-Tube-Valley (p.12) was less kind. One reviewer's notes on an unaligned unit read "weak bottom, narrow sounding, needs work, boomy." That unit hadn't been serviced.
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