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EB5AGV's National HRO-500 pages

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HRO-500 front panel HRO-500

The National HRO-500 was the first complete solid state receiver manufactured by National. It is a real transition unit as it is built just like a vacuum tube radio, except that transistor sockets replace the tube sockets. It is all point-to-point wiring and thus straightforward to get at the components, and uses also the famous National PW dial, found in older HROs. It weighs 32 lbs, not bad for a solid state device!. So I classify it as a 'semi-BA'.

With respect to serial numbers, the first two digits specify the series. Here are the series and (approximate) dates of manufacture:

HRO-500 Series
SeriesYear (approximate)

The largest circuit changes occured beween series 75 and 88, and there were some factory bulletins specifying most of the upgrades.

My unit is serial number 75 1257, so it is from the first series. It was made in 1965, according to the dates found in some of the components.

HRO-500 Specifications
Frequency range5kc to 30Mc in sixty 500kc bands, continuous coverage
Calibration accuracyWithin 1kc over the entire 500kc selected tuning range; within 250cps when zeroed at nearest 50kc calibration point
Synthesizer accuracyWithin 250cps over entire tuning range when zeroed at 10Mc
Dial backslashLess than 50cps
Frequency stabilityOver-all stability fron turn-on NMT 100cps in any 10 min period
RF input impedance50 Ohms unbalanced, separate high impedance unbalanced
AM sensitivity5kc to 30mc (with LF-10 preselector) better than 2.0uV for 10dB S/N
5kc to 500kc (without LF-10), 25-50uV for 10dB S/N
Preselector not required for 500kc to 30Mc operation.
SSB/CW sensitivity5kc to 30mc (with LF-10 preselector) better than 1uV for 10dB S/N
Preselector not required for 500kc to 30Mc operation.
Selectivity at -6dB points500cps, 2.5kc, 5.0kc, 8.0kc
Shape factor of filter2.5:1 at 2.5kc bandwidth
Passband tuning range6kc
Rejection tuning notch50dB
Image rejectionmin 60dB between 500kc and 30Mc
AGC meritLess than 10dB change in output for input change between 5 and 50000 uV
Tuning ratio10kc per revolution
Audio response200cps to 7000cps +/- 6dB
Speaker impedance3.2 Ohm
Size7 5/8" H, 16 1/2" W, 12 3/4" D
Weight32 lbs
Semiconductor complement37 transistors, 20 diodes

There are a couple of common troubles in the HRO-500. One of them is the crystal calibrator, which tends to have difficulty starting. One of the recommended mods is to use a larger emitter bypass cap in the oscillator in order to raise the gain. The capacitor is C127, and the original 0.1uF should be replaced with a 1-10uF. But the biggest problem with the HRO-500 pertains to all series, and that is audio distortion in SSB and CW. There are some very nice articles dealing with these and other HRO-500 troubles in Electric Radio issues 88 (August 1996) and 95 (March 1997).

In these pages, I will show the restoration process of my HRO-500 receiver. This is a 'live page', as I am going to update it regularly as I progress with the restoration. I would like to hear from anybody who has an HRO-500 and/or experience aligning them. Thanks!.

HRO-500 page INDEX

Any contribution or idea to enhance this page would be welcomed!.

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