OM's 1946 AM Rig About To Be On Air

[Tom WA3KLR]

Guys, I would like to keep this thread on topic.  Please use the Private Message system for admonishments.  Please start another thread for computer topics.

Thanks for all of the nice comments about my father and his home-brew equipment.

Joe W3GMS,

I do intend to put the 1946 812H rig on the air someday.

W4AAB,

the 10 meter repeater’s output was on 29.493 MHz and the input was 29.626 MHz., for a frequency difference of 133 kHz.  The spacing was 1 mile between sites and a phone line was rented for the link.  This was 1969 and then the club repeater system evolved to 2 meters FM a few years later when that became the big thing.

The 29493 kHz operating frequency was established in 1949 based on WWII surplus crystals on 7373.333 kHz.

Carl WA1KPD,

I sent you a PM concerning the photos.

Fred KA2DZT,

I have one HRO from my father’s estate – a 1935 HRO Senior he inherited from a legendary “Elmer” of my father’s area.  It has been out of service since 1967 when the original owner passed away.  It is on the workbench now to get all of the capacitors changed.  They appear to have been changed once before, a long time ago.  The receiver filaments were never turned off during WWII by decree of the owner, a Naval officer, away for the war.  It probably ran 24 hours a day after the war too.  Many of the resistors will need to be changed also.

[KM1H]

Tom, if you replace any of those white National resistors I'll take them please.

Carl

[Tom WA3KLR]

Carl,

I'm going to try to put new 1/4 watt film resistors in those ceramic tubes when the time comes.  I have read that this can be accomplished.

[Tom WA3KLR]

Walt W2DU,

The photo of your father's shack shows an extremely impressive assembly of gear, equal (or better) to commercial communications stations of the era.

I recognize the "danger high voltage" sign at the lower right of the photo.  It looks identical to one in some of my father's other early shack photos.  He gave me the sign card when I was a kid and was looking for it today but couldn't find it.  I probably threw it out many years ago.

[AMroo]

Thanks OM really great pics, the kinda pics a guy could use in private.

 And sure better than pics of kitty cats, doggy woggies and flowers.
 

[KA2DZT]

Tom,

The caps in my HRO-M seemed to have been changed years ago.  They are all the green ones, forgot what brand.  They seemed to be all good as the receiver works fine.  I don't think any of the resistors were changed in mine.  I would have to pull the receiver out of the rack to see if they look original.  You may check them first before replacing them as they may be within tolerance.

Not sure 1/4 watt resistors will be a high enough wattage, especially if you put them inside an old resistor body.

I added an output xfmr in mine, as many HROs didn't have one.  My output tube is a type 42, 7K load to spk.  When I run out of type 42s I'll have to switch to a 6F6 and make a socket adaptor.

I have all nine plug-in coils that were made for the HRO along with two rack mounted coil housings.

Whats really rare is finding an original HRO rack with the speaker.  It's worth more than the receiver.

I don't have the dog house PS that went with the receiver so I built my own PS.  250V at about 80ma.  My receiver had a filament center tap resistor in it.  I think it was 100ohms CT to ground.  I did away with that resistor as the filament winding has a CT.

Someone added a tone control in mine, looks like it may have been something National did.  I redesigned the tone control circuit to make it work correctly.

The last thing, I redesigned the S-meter circuit.  I'm not sure what National was thinking with the way it was designed.  The S-meter works correctly now as it closely follows the S-meter readings on my NC-183D.  The S-meter in your older model I thing differs from mine.  Mine has S9 plus dBs above S9

My HRO had a switch to shut off the S-meter, not sure why I would want to do that.  So, I rewired that switch to turn the receiver PS on and off.

I have an original manual if you need help with anything.  Although, my HRO may differ from yours.

Fred

[Tom WA3KLR]

Fred,

I won’t replace resistors unless they measure out of tolerance.  I already know that many measure way out though. I won’t put ¼ Watt resistors back in the tubes if that value is going to dissipate more than ¼ Watt.  I will use appropriate wattage carbon comps in that case.  I saved my father’s resistor drawers that have old carbon composition resistors.  Although they may not be real reliable, they would be older than new stock

My HRO has 6 volt tubes.  There is an audio output transformer installed under the chassis - a Merit A-3020 7k to 3.5 Ohms.  The output tube is a 42.  The speaker secondary comes out a phone jack/hole that was added.  The two terminal strip on the back for the transformer-less audio output was re-wired to be a Switched B+ terminal.  I do have an original matching National speaker cabinet with output transformer.  When the receiver is restored, I will see what gives with that National speaker assembly (bad speaker or transformer?) and go from there as to whether I run the additional internal transformer or not.  It is convenient though to not require an additional speaker to sit at the operating position.

The doghouse supply I had was the lower B+ voltage (170) output.  You want the higher 240 Volt supply to run an HRO anyway.  When I checked it out, I found one of the output chokes open.  That National doghouse is the cheapest p.s. I have ever seen (Depression market design).  So I drilled out the spot welds on the bottom of the supply chassis and stripped out the potted transformer/choke assembly.  I have a power transformer that is perfect for the job.   And I will use silicon diodes.  Sacrilegious yes, but practical.

Thanks for the offer on manuals, but my father had put together quite a collection of HRO civilian and military manuals.  I have the original serial numbered manual that came with the receiver chassis – matching S/N’s with the entire original set of 4 coils.  I have 3 additional low frequency coils also which cover 2 MHz down to 230 kHz.

[KA2DZT]

Tom,

National must have made a number of different coil sets.  I have nine matching coils, 50-100KC, 100-200KC, 180-430KC, 480-960KC (notice the missing 50KC), 900KC-2.05MC, 1.7-4.0MC, 3.5-7.3MC, 7.0-14.0MC, 14.0-30.0MC.  The last four are the band spread type that can be switched to just cover the 80/75M, 40M, 20M, 10M ham bands.  The coils are switched by moving small flat head brass screws to the adjacent holes on the contact strip.  I'm sure you're probably familiar with this.

My receiver has an original phone jack and the two-terminal strip on the back was wired to cut the B+.  It also has a B+ on/off switch on the front panel.  Use it when changing coils while the set is on.

I mentioned earliar about the on/off S-meter switch.  You may notice that the S-meter will go through a soft pin when the set is first turned on.  Could be the original reason for the S-meter on/off switch.  The S-meter works on a balance of screen currents being drawn by the 6C6s and/or 6D6s and the B+ itself (IIRC).  The point,  I used a 5Y3 rectifier for the B+ supply to retard the rise in B+ which reduced the pinning of the S-meter.

Using SS rectifiers may make the pinning worse.

Fred

[KM1H]

National marketed a few 160 and 15M bandspread coils in the 50-60's for use in all the older HRO's. I had a 160 in a 5TA1 many years ago. The HRO-7 had an AC coil available.

There were also speciality coils made for commercial customers.

Ive also made my own replacements for those white resistors, pretty time consuming.

Carl