National HRO: A Comprehensive Guide to the Classic Radio

The National HRO, a classic radio that's still loved by many today. This radio was designed for high fidelity sound and was a favorite among audiophiles.

It was introduced in the 1930s and quickly gained popularity due to its exceptional sound quality. The National HRO was known for its high sensitivity and low noise floor, making it perfect for long-distance reception.

The radio's design was sleek and modern for its time, featuring a compact chassis and a beautifully crafted cabinet. Its durability and reliability made it a staple in many homes and workplaces.

The National HRO was also known for its versatility, with the ability to receive a wide range of frequencies, including shortwave, medium wave, and long wave.

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The National HRO receiver has a fascinating history that dates back to the 1930s. It was designed by James Millen, an amateur radio enthusiast with the call sign W1HRX, who was in charge of the mechanical design.

The electronics were designed by Herbert Hoover, Jr. and Howard Morgan in Hoover's garage in Pasadena, California. They worked closely with Dana Bacon, who would later write about the receiver as a co-author with Millen.

The HRO was initially called "Hell Of a Rush" by National Radio's tool makers, but management decided to change it to HRO, a name that would become synonymous with high-quality radio receivers. The HRO was announced in QST magazine in October 1934 and shipped in March 1935.

The price of the HRO receiver was $233, which included less tubes, and an external power supply was available for an additional $26.50. A 7000 ohm speaker in a rack panel cost $30.00 extra.

The HRO found widespread use during World War II as the preferred receiver of various Allied monitoring services, including Y-Service stations associated with the code-breaking group at Bletchley Park in England.

The National HRO radio boasted some impressive features, especially when it came to its tuning capabilities. The micrometer-type dial was designed to allow for continuous analog tuning, while also digitally indicating incremental progress over a range of ten full turns of the large tuning knob.

This dial was incredibly precise, with ten times its circumference allowing for great frequency resolution - equivalent to 12 feet or nearly 4 meters. The four standard sets of coils, A, B, C, and D, covered frequencies ranging from 14-30 MHz to 1.7-4 MHz, respectively.

Each radio was custom calibrated at the factory, a process that took nearly 4 hours, and also featured bandspread modes set by moving screws for amateur radio use.

The National HRO radio receiver's coil system is a remarkable feature that allows for easy switching between different frequency ranges.

Each coil pack is specifically designed for its band and can be optimized for improved performance and reliability.

The different coil packs enable operation from frequencies as low as 100 kHz right up to 54 MHz.

Here are the coil pack types and their corresponding frequency ranges:

Coil packs A to D have a band-spread capability on the amateur radio bands mentioned, allowing for a narrower frequency range to be selected.

The HRO-60 was replaced by the solid-state HRO-500 in 1964 as the top receiver of the National line.

The HRO-500 was available in a portable version with a case, internal speaker, and rechargeable Ni-Cad batteries, designated HRO-500P.

It was also available as a table model with a rack mount adapter. This new receiver covered from 0 to 30 MHz without plug-in coils.

The HRO-500 used a phase locked loop, which was locked to the harmonics of a crystal oscillator and a linear VFO that covered 500 kHz.

The receiver had an internal reduction system allowing either fast tuning or using an added vernier knob on the main tuning knob.

Selectivity for the HRO-500 was provided by cup core coils operating at the new 262 KHz IF frequency.

These coils were variable from 500 Hz to 6 kHz in bandwidth, making it a popular choice for communications companies like Tropical Radio and Telegraph.

The HRO-500 was almost as large as the original HRO, weighing 26 pounds, and used a combination of germanium and silicon transistors.

Performance-wise, the HRO-500 was fairly poor in strong signal conditions compared to modern solid-state receivers.

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The National HRO is a fascinating piece of radio history, and its various models are a testament to its versatility and widespread adoption. The first model, the HRO-Sr, was introduced in 1935 and remained in production until 1943.

One notable variation of the HRO-Sr was the HRO-Jr, a more affordable version that lacked some of the advanced features of its predecessor, such as the crystal filter and signal strength meter. The HRO-Jr was marketed as a more budget-friendly option, priced at US$100.

The HRO-Jr was also used in conjunction with the U.S. Navy's RAS model, which featured general coverage coils and a 175 kHz IF to receive the 500 kHz distress frequency. This model was likely used for emergency communications and navigation.

Here's a list of some of the notable models of the National HRO:

The HRO-M was a particularly popular model, widely used for British Y Stations that intercepted Nazi communications.

The National HRO receivers were produced in several models between 1947 and 1972. The HRO-7, released in 1947, was the first model and used 12 tubes, including 2 miniature tubes.

It was followed by the HRO-50, which featured built-in tuning dials and power supply, push-pull audio amplifier, improved styling and performance. The HRO-50-1 was released in 1951, with increased IF selectivity.

The HRO-60, produced from 1952 to 1964, was a significant upgrade with dual conversion for coils B and A, and heater current regulation for the HF oscillator and mixer tubes. It was a popular model among amateur radio enthusiasts.

The HRO receivers were known for their outstanding performance and continued to be popular even after more expensive and advanced receivers became available.

The wide availability of the National HRO as surplus led to many modifications, making it worth twenty times its original price if done correctly.

The most common modification is replacing the High Frequency Oscillator (6C6 or 6J7) with a miniature 6C4 triode oscillator and adding a voltage regulator like the 0A2.

Adding a noise blanker, as adopted by National on the HRO-5A1, is also a popular modification, which typically consists of a 6H6 and a 6J5.

Replacing earlier vacuum tubes with later versions, such as octal or miniature tubes, was another popular change, often affecting the RF amplifiers, mixer, and oscillator.

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A study found that it takes 66 days to form a habit, but only 18 days to break one. This means that catching a problem early on can make a huge difference.

Ignoring early signs can lead to more severe consequences, such as increased stress and anxiety. In fact, a survey revealed that 75% of people experience stress due to unaddressed problems.

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Early signs can also be physical, such as changes in appetite or sleep patterns. A study found that 80% of people experience changes in their physical health due to stress.

Ignoring these signs can lead to a downward spiral, making it harder to modify behavior in the long run.

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The HRO receiver has been modified in various ways over the years. One notable modification is the addition of a voltage regulator, which was done by some enthusiasts to improve the receiver's performance.

The Signal Corp produced the R-140 by modifying the HRO-M, MX, and 5 for use in a frequency measuring system. This involved removing the B+ from the speaker terminals and allowing an audio signal to be introduced to the second detector plate.

Some modifications can actually decrease the receiver's value, such as replacing earlier vacuum tubes with later versions. Depending on when the modification was performed, this might be octal vacuum tubes in place of the 6 pin glass tubes, or miniature vacuum tubes in place of either the octal or 6 pin glass vacuum tubes.

The British modified the HRO-M and HRO-MX to better suit the requirement for a direction finding receiver. These units had a panel over the normal antenna inputs to accept inputs from a direction finding antenna (usually Babcock), and a sense antenna.

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In some cases, modifications were done at the factory, such as the addition of a noise limiter and new small knobs on a 1st model HRO. This is why it's essential to carefully examine the receiver's circuitry and components to determine whether the modifications were done by the factory or an enthusiast.

The R-140 modification also involved tying together the leads to the B+ terminals on the rear of the receiver and connecting one of them to ground and the other to the plate of the detector.

Reverse engineered HROs were made by the Germans and the Japanese during and after World War II. They even copied the iconic emblem on top.

The Germans made two copies of the HRO, one during the war and one after the war in East Germany. The latter was supplied to the Chinese.

The Japanese made a copy of the dial and drive mechanism, and even supplied receivers with a different dial mechanism but the same layout and plug-in coils as the HRO.

The Australians made three versions of the HRO, with the AWA AMR-100 and AMR-101 being particularly well-made. The Kingsley AR-7, also from Australia, used a single tube for the HF converter and would have suffered from pulling of the HFO.

The Collier and Beale 941 SWB built in New Zealand is a very close copy of the HRO, using imported dial mechanisms manufactured by National.

The National HRO receiver was developed by James Millen and his company, The National Company. James Millen wrote about the HRO receiver in his 1935 book, "Radio Design Practice", which included a 4-page excerpt about the receiver.

The National HRO receiver was formally advertised in QST magazine for the first time in October 1934. This marked the beginning of the receiver's popularity.

Here are some key documents related to the National HRO receiver:

These documents provide valuable information for anyone looking to restore or learn more about the National HRO receiver.

The National HRO is a well-regarded radio receiver, and its technical details are worth noting. It's a guest post by Dan Robinson, a contributor to the SWLing Post.

The National HRO-500 is a specific model mentioned in the article, and it's worth noting its unboxing and initial tests. This model is likely to have similar technical specifications.

The article doesn't delve into the technical specifications of the National HRO, but it's clear that Dan Robinson's guest post is a valuable resource for those interested in learning more about this receiver.

Power Supplies are the backbone of any electronic device, providing the necessary voltage and current to power the circuitry. They can be classified into two main types: Linear and Switch-Mode power supplies.

Linear power supplies use a transformer to step down the voltage, while Switch-Mode power supplies use a switching circuit to convert the voltage. The efficiency of a power supply is crucial, with Switch-Mode power supplies being more efficient than Linear power supplies.

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A well-designed power supply can help reduce energy consumption and heat generation in electronic devices. For example, a 5V power supply for a smartphone can be as small as 1 inch by 1 inch.

The output voltage of a power supply is typically regulated to ensure a stable voltage supply to the circuit. In a typical power supply, the output voltage is regulated to within 5% of the nominal voltage.

The power supply's efficiency is also affected by the type of components used, such as the transformer and the switching circuit. A high-quality transformer can help improve the efficiency of a Linear power supply.

In summary, a good power supply is essential for the proper functioning of electronic devices, and choosing the right type of power supply can make a big difference in terms of efficiency and reliability.

The National HRO radio receiver used pluggable coil packs to provide switching between different frequency ranges, significantly reducing the size of the radio receiver and improving reliability and performance.

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These coil packs enabled operation from frequencies as low as 100 kHz up to 54 MHz.

The different coil packs provided a range of frequency coverage, with some coil packs covering a wide frequency range for general coverage, while others had a narrower band-spread for amateur radio bands.

The coil packs were labeled A to D, and each had two frequency ranges, with the ability to add serial and parallel capacitors to reduce the coverage to approximately the width of an amateur band.

Here are the frequency ranges for each coil pack:

The National HRO-500 is a true time capsule of radio history. It's amazing to think that radios from the 1960s can still be found in their original boxes and condition.

In recent years, several rare radios have appeared on the used market, including a Panasonic RF-9000, which was sold on Ebay in new, opened box condition.

The HRO-500 was considered a high-level receiver when it was introduced in the mid-1960s. It's rare to find tube or solid-state communications receivers from that era.

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A seller in California advertised a HRO-500 in 2016, claiming it was purchased in 1967 and still in its original shipping crate. The asking price was $2500.

The radio was said to be in new, unopened condition, but concerns arose about its usability since it had never been used. Unfortunately, it's unclear if the radio sold.

Another HRO-500 was recently discovered, also in its original crate, with the original box and National Radio Co fabric cover still intact. The manual was even still in its plastic wrap.

This particular radio had some minor issues, including a missing metal cap for the MODE knob and a frozen dial calibration clutch knob. The good news is that the PLL lock circuit, which often fails in used HRO-500s, was still operational on every band.

It's worth noting that the PLL lock light on this radio did not light up. However, the PLL itself did work, which is a relief for anyone restoring or using this vintage radio.