The National HRO Receiver - A Historical Perspective from QST, January, 2010 (Volume 95, Number 1).
War Department Technical Manual TM11-885, Radio Receiver R-140/FSM-1 National HRO Series of Receivers is absolutely the best manual I've found for the care & feeding of HRO receivers. Publication date: August 1946, 88 pages, 8.5 MB PDF.
A close second in usefulness is this British 1954 publication: RECEPTION SETS R106, MKS 1 AND 2. The original was is fairly poor condition so I wrote a macro using Paint Shop Pro to process the scanned pages. That helped. Be aware that the originals were in the European A4 paper size format. 50 pages, 9 MB PDF.
This is the manual that came with my 1942 vintage U. S. Coast Guard HRO receiver. There are many copies and versions of this manual available on the World Wide Web but here's mine just in case there are differences amongst those available. The National HRO- 1942, 20 pages, 1.479 MB PDF.
Millen's 1935 Radio Design Practice carried a section devoted to the HRO receiver. Here is that excerpt: 4 pages, 410 KB PDF.
Millen described the National HRO receiver in two of his monthly advertisement letters in QST:
The National HRO was formally advertised in QST, I believe for the first time, in this advertisement from October, 1934.
Tube data sheets:
6B7 RCA Data Sheet, 1937
6C6 RCA Data Sheet, 1937
6C6 Tung-Sol Data Sheet, 1940
6D6 RCA Data Sheet, 1937
6D6 RCA Data Sheet, 1941
42 RCA Data Sheet, 1937
Pre-restoration photographs of my 1942 National HRO receiver
And now here's the best part of this web page for anyone restoring a National HRO receiver: fresh & clean coil charts. These started as scans from original equipment but were transformed into Adobe vector graphic files, clean and scalable. I have included two versions of each chart. One is white type on black background and the other is reversed, black type on white background. When you print these from Adobe Acrobat, be sure to select "Actual Size" and not the "Fit" option which is likely your default setting.
And it gets even better. Here are large copies of each chart! My HRO receiver sits on a desk and I got tired of laying my head on the table and removing my eye glasses just to read the charts. So now I have 8.5 x 11-inch copies of the charts!
Band spread 1.7 to 4.0 Megacycle coils
Band spread 3.5 to 7.3 Megacycle coils
Band spread 7.0 to 14.4 Megacycle coils
Band spread 14 to 30 Megacycle coils
General Coverage 1.7 to 4.0 Megacycle coils
General Coverage 3.5 to 7.3 Megacycle coils
General Coverage 7.0 to 14.4 Megacycle coils
General Coverage 14 to 30 Megacycle coils
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