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Monday, December 10, 2018

Further 6J5/6L6 Transmitter Developments

My shack continues to be in a state of flux, Nothing is built just once. As I rethink things, stuff is added or changed.

In this case I decided to pair my 6J5/6L6 transmitter up with my HRO Sr rather than my Simple Superhet. I wanted something of the right design vintage and looks to sit next to my HRO while running either barefoot or driving my TZ-20 amp. The core design of my 6J5/6L6 transmitter is right out of a 1940 Stancor manual so the design was fine. It just didn't look like it belonged with the HRO Sr. What I did was a cosmetic change replacing the front panel and then repackaging the transmitter in a metal cabinet.


If I ever need a more upscale transmitter in that spot I have a Meissner Signal Shifter waiting in the wings.

Sunday, December 9, 2018

Novice Bookends

Many identify the Novice Class between 1951 (when it was introduced) and the early 70s (when VFOs, 250 watts and renewable licenses became part of the novice license) as the golden or classic era for novices. During the 50s and 60s the novice class was clearly a learner class. During this time amateur radio manufacturers marketed rigs specifically designed for novices.

I have two that I would call bookends to this classic era. One is the Hallicrafters SR75 introduced in 1951 just as the Novice Class was introduced and the other is the Heathkit HW16 (1967-1976). Both of these are transmitter/receivers in the same case with features and capabilities that match the restrictions of the
Novice Class. Hallicrafters chose the add a crystal oscillator to a S38B along with circuitry using the 50L6 audio amp as the transmitter final. The resulting rig looks, from the front, almost exactly like a S38B. Performance and usability would have been disappointing.The transmitter was only rated for 10 watts and changing bands or the crystal required removing the back of this AC/DC set. Also the SR75 receiver section was basically a S38B, an entry level receiver that many beginners would already have had. In addition the price delta between the S38B and SR75 was $40. Several crystal controlled transmitters were on the market in the early fifties in this price range that would have looked better to the novice and he could build his own transmitter even cheaper. By the time Heathkit tackled this particular niche in the mid 60s they designed a rig from the ground up that had features better tuned for the novice.

Two bookends on an era but Heathkit did a better job. I'm using my HW16 while the SR75 remains a shelf queen good for discussion.

I've uploaded more SR75 information to  www.prismnet.com/~nielw/SR75/

Saturday, December 8, 2018

Winter 2018/19

After missing the AWA Bruce Kelley 1929 CW Party I'm looking forward to the vintage ham radio opportunities still coming up over the next few months:

ARRL Straight Key "Night" (Any equipment, Any band CW) Jan 1
http://www.arrl.org/straight-key-night

AWA Linc Cundall (pre 1951, 160/80/40 CW) Jan 2-6
http://www.antiquewireless.org/event-details-for-lc-cw-contest.html

Novice Rig Roundup (xtal control and less than 75W, 80/40/15/10 CW, 2 AM) Mar 2-10
http://novicerigroundup.com/

AWA John Rollins (pre 1960, 40/20 CW) Mar 13-17
http://www.antiquewireless.org/event-details-for-john-rollins-dx-contest.html

 Novice Rig "Night" (80/40/15/10 CW) Every Monday
http://novicerigroundup.com/novicerignight.html

I've six stations that I should have on the air for these events.

HRO Sr / 6J5-6L6 + TZ20 Amp
Simple Super / Utah Jr
FB7 / CW25Jr
R4B / TX4B Drake Twins
NC-303 / HW16
Drake 2B / Eico 720
















These will all be a great activities for those long cold Minnesota nights I'm expecting.

Wednesday, September 19, 2018

Winter 2018-19 Boat Anchor Season

As summer wraps up here and turns into fall I start shifting from QRP-In-The-Park to Boat-Anchors-In-The-Basement and the winter contest/operating event season. One station in the works here is a moderate capability mid 30s CW setup using a couple of TZ-20s (at about 50 watts) in the final and a National FBX-A receiver. It qualifies nicely for the AWA Linc Cundall Memorial CW Contest.

I've now finished up the transmitter power supply so I'll soon be testing it out on the air. The transformer is rated for 560V at 270mA so should certainly easily handle my 50-70W transmitter. At 31 pounds, mostly iron, I don't plan to move this one around much.

Next on the to-do list is to go through the FBX-A getting it on the air.

Friday, September 7, 2018

EF Johnson 50th Speech

Here is an audio file/recording of Edgar F. Johnson speaking on the 1973 50th anniversary of his company. He covers the early history of radio and then (starting at 8 minutes into the recording) how EF Johnson has grown:

Wednesday, September 5, 2018

The Western Historic Radio Museum

As I was looking for more information about my NC-81X I found the Western Historic Radio Museum. This is a great resource providing all sorts of interesting information and restoration tips.

Take a look at  https://www.radioblvd.com/ 

National pages are at 
https://www.radioblvd.com/National%20HRO.htm 
and
https://www.radioblvd.com/NC100.htm

National NC-81X

I've gotten my NC-81X back on the air! It is paired with my Stancor 10P for a mid-class 1939 AM/CW station. I'm using it on 40 and 80 meter CW.

The NC-81X sat in the National receiver price lineup below the NC-101XA. Comparing the two, the NC-81X had transformerless AC/DC power, no S-Meter and no RF stage but I found it really performs pretty good. Shifting the IF to 1560KC helped eliminate any image problem.

Along the way to adding it to my station lineup I did change a few things. First the obvious one, I changed out all of the paper and electrolytic caps. I left the original electrolytics in place but replaced them with modern electrolytics mounted under the chassis. I'm not a fan of transformerless AC/DC receivers and this one had a short between the chassis and the metal case. Unless run with an isolation transformer it was a shock hazard waiting to happen. I had a small isolation transformer big enough to handle the NC-81X B+ requirements so I wired it into the receiver to supply B+ and I left the filament string connected across 120 VAC. I addition I fused each side of the AC line and added a three wire/grounded cord. Now this radio is safe to use but repair work under the chassis still requires a "real" isolation transformer. The NC-81X is not an HRO though. While very usable it still tends to FM/wobble on strong CW signals. I suspect the B+ needs to be regulated.

The 1938 list price for the  NC-81X was $165 while the NC-101X listed for $215. This was back when the US average wages per year was $1,730 compared to about $60,000 today. I bet 10% of my wages towards a new radio would lead to some interesting dinner time conversation. No wonder National introduced the NC-44 at an even lower price point ($82.50) and by 1939 dealers sold these radios at a very steep discount (NC-101X for $129 and NC-81X for $99).



Sunday, July 22, 2018

Stancor 10P Transmitter - Part 4

This entry might better be called "The devil's in the details".

By the time I set up my Stancor 10P to operate with a NC-57 I had added all sorts of accessories, many of which I've covered before in my blog.....

Load control - The 10P as shipped has fixed link coupling to the antenna. This is fine if your antenna happens to match to load needed by the 10P 6L6. Even the 10P manual mentions adjusting the load between CW and AM. The solution is to add a variable cap between one side of the output link and ground connecting your feedline between the other side of the output link and ground. I've done this before. See my TZ-20 Amp Output Circuit entry dated Feb 4, 2012.

Transmitting Crystal Adapter - In 1940 crystals were still expensive. Hams typically didn't have several to choose from. They didn't move around the bands. Putting a crystals socket on the front panel was not a priority. Burying the crystal socket inside the transmitter, accessible only from the back, was good enough. I, on the other hand, have several crystals on both 40 and 80. I want to easily change frequency. I also have a two holder types, each with different pin sizes and spacing. The solution is an adapter that plugs into the 10P crystal socket and sits on top of the 10P. See Function Again Wins dated May 28, 2014. The cute solution works fine in the 10P and includes a bulb to give an indication of crystal current.

Key Isolation -  Cathode keying was common the simple rigs. With cathode keying comes high voltage across the key terminals. The 10P is no exception. 100V appears across the key terminals when the key is open. Definitely a case of keep your fingers out of where they don't belong. See Cathode Keying Safety dated April 23, 2018. I use a Keyall HV interface to keep high voltage off of the key terminals.

I've already mentioned the need to step down modern house current to match 1940 designs. See information about this at Stancor 10P Transmitter - Part 3 dated July 20, 2018. 

The Heathkit SWR meter helps with tune up. Both the 10P tank tuning and external loading need to be adjusted to maximize power out and efficiency as indicated by the SWR meter forward power and plate current meter.

Finally, the operating position includes a TR switch. In my case this is a simple SPDT toggle switch switching the antenna between the 10P and my NC-57.




Friday, July 20, 2018

Stancor 10P Transmitter - Part 3

I've continued to make progress on my 10P. All of my initial "to dos" are done and my new transmitter is coming to life. It puts out a good 10 watts on both 80 and 40 with just enough chirp to give it personality .... but (always watch out for the buts) ... my 10P is running a little too hot. Voltages are all significantly high, even filament and B+ straight out of the power supply.

The easiest issue to address is the high filament voltage. I measured it to be about 10% high. This makes sense. In 1940 house current was lower than today, about 110 V vs 120-125 V today. A power transformer designed to generate 6.3 V in 1940 wold now show closer to 7 V, about what I measured. To solve this I have step down transformers at both of my prewar operating positions. I decided to add a third.

I started with a 12V at 8A filament transformer that I  had on the shelf and rewired it with the primary and secondary windings in series. With the windings properly polarized 120V across both windings generates 108 V at the junction of the two windings. It becomes an auto-transformer with one tap. Since only the old 12 V winding sees full current, the 8 A rating now is the auto-transformer rating, about 900 watts. Electrically this auto-transformer sits between the 120 V wall outlet and the 10P. The 900 W power rating will allow me to plug my receiver, probably a NC-81X, into it also.

 As expected the 10P filament voltage dropped to 6.3 V but B+ still remains high. This may be because the replacement electrolytics I used in the power supply are larger than the originals. I'll need to work on this.

Addendum - The high B+ was a simple fix (but I'm not a fan of the engineering trade off behind it). I had removed the 6L6 modulator while testing CW. It turns out that this tube loads down the power supply, even in CW mode. I plugged the 6L6 modulator in and B+ now looks about right.

To Be Continued...

Monday, June 11, 2018

Stancor 10P Transmitter - Part 2

I've cleared out most of the obviously questionable stuff from under the chassis and tested what is left...no surprises and, most importantly, the iron tested good. At this point I have a to-do list:
  • Replace the electrolytics
  • Replace the carbon resistors (they had all drifted out of spec high)
  • Change power cord to a 3 wire cord with ground
  • Add fuse
  • Add a ground stud to the back (this will allow an external loading control and an easy connection to station ground)
  •  Rewire as needed

To be continued....