The National Radio Company

HRO-600 Receiver

THE NATIONAL RADIO COMPANY launched its legendary "HRO" series of communications receivers in 1935 with the 9 tube single-conversion HRO. After 40 years, the HRO dynasty finally ended with the solid state, synthesized HRO-600, pictured at the right. Intended primarily for government and commercial users, and priced at $5600, the HRO-600 is today rarely seen. (Reportedly, fewer than than two hundred were manufactured.)

The HRO-600 was introduced in 1970, shortly after National declared bankruptcy. The reorganized company struggled for several years, and during this period both the HRO-500 and HRO-600 were sold, primarily to government and marine customers.

The HRO-600 shown here is serial number 75, which I acquired in Spring of 2005 in non-working condition. As described below, its repair and restoration posed quite a challenge and the investment of many hours of workbench time.

The HRO-600 looks positively diminutive, next to the earlier HRO-500 receiver (shown mounted atop an LF-10 low frequency preselector). Performance-wise, however, the HRO-600 runs circles around its older brother. Of course, National afficionados believe there can never be an adequate substitute for the legendary "PW" dial, a fixture of all HRO receivers, until it was at last supplanted by the HRO-600's digital frequency display.

Most of the HRO-600's "mainframe" wiring consists of interconnects for the 36-pin circuit board edge connectors. Teflon-insulated wire is used for all point-to-point wiring, and all shielded cables are marked with identifying numbers. The rear panel folds out (right photo) to provide access to the rear connector wiring.

Diodes, diodes, everywhere. This diode matrix is used to decode the MHz band selector.

A unique feature of the HRO-600 is its plug-in frequency control options. Shown here is the Model 601 tunable VFO module, which features a four-digit Nixie tube display. The VFO slides into an opening in the radio's front panel and seats into a 50-pin Centronics type connector. The two upper knobs lock the unit into place. Color-coded shielded cables carry the VFO's 4-5 MHz output, a 5 MHz fixed BFO output, and also a 5 MHz negative logic clock used by the synthesizer. The 5 MHz oscillators are oven-stabilized. For specialized uses, a Model 602 thumbwheel-operated frequency synthesizer (with no frequency display) was also available, as was a Model 603 crystal oscillator (users could select 12 crystals in the 4-5 MHz range). Obviously, most users preferred the versatility of the Model 601 VFO.

CIRCUIT DESCRIPTION: (Please refer to the simplified block diagram, which you can open in a new window by clicking HERE.) When tuning signals from 16 kHz to 1 MHz, the HRO-600 acts as a single conversion receiver. In this range, the received signal is added to the 4-5 MHz VFO output, resulting in a fixed 5 MHz IF. After amplification, the IF output passes through a wideband 8 kHz crystal filter, and then, if desired, through addtional narrower bandwidth crystal filters. The amplified and filtered 5 MHz signal is then detected by the product detector/BFO, and the detected audio is then amplified by both a line audio amplifier and a speaker amplifier. AGC is IF-derived and is applied to several of the amplifier stages.

When tuning signals above 1 MHz, the HRO-600 uses a double conversion scheme. The 1 -30 MHz signal frequency is mixed with the 56-84 MHz output of the first injection synthesizer, with resulting difference signals falling in the range 55 - 54 MHz. This difference signal is then mixed again with a variable 50-49 MHz local oscillator to produce a 5 MHz fixed IF. Subsequent processing is identical to that described in the preceding paragraph.

The most complicated part of the HRO-600 is the frequency synthesizer, which produces 56-84 MHz sine waves in 1 MHz steps. This circuit is a phase-locked-loop design and uses a voltage controlled oscillator in combination with a programmable frequency divider. When this circuit was designed in the mid-1960s, integrated circuits had limited functionality, and the designers had to engineer the digital logic from the ground up using flip-flops, and gates. To further complicate matters, the circuit logic used a very early family of now-obsolete Motorala emitter-coupled-logic (ECL) ICs. These ICs use negative voltage logic, requiring the HRO-600's power supply to produce several negative as well as positive voltages.

RESTORATION AND A TALE OF WOE: I knew when I bought it that the radio was dead on all HF bands, so I expected to face some repair problems. Fortunately, an original manual had come with the radio, so I figured troubleshooting would be straightforward. I figured wrong.

There were two big complicating factors. First, the manual only included circuit diagrams of the mainframe interconnections. National had expected users to send defective circuit board modules back to the factory for repair, and try as I might, I couldn't find any board-level schematics. In short, I hadn't a clue what was inside all those circuit board modules!

Second, the dead bands turned out to be caused by a defective 1st injection phase-lock-loop synthesizer. Aside from the fact that I knew nothing about synthesizer design, and had no schematic diagram to guide me, the synthesizer module was filled with long-obsolete Motorola integrated circuits. Fortunately, the story does have a happy ending, which you can read about by clicking HERE on the banner at the bottom of the page.

OPERATING IMPRESSIONS: The HRO-600 is a surprisingly good performer. The audio is crisp and clear, and the selectivity is excellent, as one would expect from the 8-pole crystal filters. SSB and CW signals have a very clear tone, quite unlike signals on the HRO-500, which made CW notes sound like they were being played on a harmonica. The stability is excellent, with no discernable drift. The sensitivity is also excellent, even on the higher bands.

The receiver really shines on the VLF bands, thanks to the built-in preselector. The bandswitch has six bands below 1 MHz, each tuning an octave (16-32 kHz, 32-64 kHz, etc.), and the preselector peaks up sharply on signals within each band. Changing the bandswitch engages the motor, and there is some whirring for a second or so, until the preselector settles into its designated range. Quite a unique feature.

The AGC time constants seem well-chosen, and the attack time is sufficiently short that I couldn't hear thumping on CW signals. The gain distribution among the receiver stages seems well thought out, because the receiver seems lively but not noisy.

My only quibbles with the HRO-600 are that the 0.1 kHz Nixie tube flickers between digits, which I find distracting, and secondly, that the feel of the main tuning knob has just a tinge of sponginess, a result no doubt of the simple dial cord linkage to the VFO capacitor. Maybe I'm just spoiled from the solid heft of the PW dial.

All-in-all, the HRO-600 was a pleasant surprise, performance-wise. I'd rate it roughly on a par with the Drake DSR-1 or DSR-2, which were a later design. The tuning is superior to that of the Drake receivers, which had a clunky way of changing bands, but then the Drake radios had an effective noise blanker, which the HRO-600 lacks. The radio's performance is definitely better than that of the analog Collins 51S1, which was a contemporary of the HRO-600 but was based on an older technology.