The AM Forum

I have an old HRO needing restoration, and I haven't yet been able to accurately identify it's model number or find a schematic.  The tubes in the receiver are different from the chassis markings, so I'm seeking advice from all of the HRO mavens lurking about.

Tube complement:   

          Function         Chassis Marking          In the Receiver
          1st RF             6K7                           6SG7
          2nd RF            6K7                           6SG7
          1st Det.           6J7                            6K7    ?
          Osc.                6J7                            6C4
          1st IF              6K7                            6K7
          2nd IF             6K7                            6K7
          2nd Det/Audio  6SQ7                          6SQ7
          Audio Out        6V6                            6V6     
          Noise Lim.        6J5                            6J5
          Noise Lim.        6H6                           6H6

 If you can tell me with certainty what model HRO this is likely to be, and where I can find a proper schematic for it, I would certainly appreciate your efforts.

Tnx es 73.

-al hart, w8vr

           
           

http://www.radioblvd.com/National%20HRO.htm

Henry has posted extensive information and links about HRO models, and production.  Rather than duplicate, please start here.

The best indicator for you to begin with is the serial number which is probably stamped into the chassis next to the antenna push terminals.  You will need to lift the lid and use a flashlight.   Your radio may, like mine, have coil packs with matching serial numbers.

Lots of good reading. Enjoy.  Bill

[edit] http://bama.edebris.com/manuals/national/

National manuals and schematics here.

The tube line up seems closest to some flavor of an HRO 5a or 5a1.

Is there a BFO?

Al,

Clearly it was modified since the 6C4 wasn't used until the HRO-7 and the earlier "double ended" RF tubes were changed to the 6SG7 single ended type.  One of your first decisions will be whether to return it to stock which is always my bias, it depends upon how hacked up it is from tube type changes.

I agree with Roger.  BTW, if a pre-WWII HRO uses the 6C4 in the HFO, it was probably modified after Loren Windom, W8GZ, published his article titled "Modernizing the Prewar HRO," and appeared in the June, 1942 issue of QST.  In addition to the 6C4 mod, he discusses comparisons with the HRO-7 and shows how to add an OA2 VT tube -- just like the HRO-7.  The 6C4 and OA2 were added together on a small chassis plate.  Link below if you're a League member:

http://p1k.arrl.org/pubs_archive/29312

Depending on the similarity between the mods, you may be able to follow Windom's diagram and reverse-engineer the HFO back to the HRO's original condition -- or at least understand what's happening in the event you decide to keep it as-is.

Paul, W9AC

Thanks to all respondents es 73!


al hart, w8vr

Al,

If you really get into HRO's, you will learn a lot from reading many of the yearly published AWA Review publications that dealt with the subject.  Here is a link to the master index: 

http://www.antiquewireless.org/awa-review.htm

Subjects in the yearly released AWA Review are dealt with in more detail beyond what can be published in their quarterly Journals. 

Many of the issues are still available and I believe all are available on a DVD. 

73,
Joe, GMS