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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....

Friday, June 8, 2018

Stancor 10P Transmitter - Part 1

On May 18 I was fortunate to find a Stancor 10P transmitter at the Pavek Museum annual "garage sale". The ads claim this little rig will run 12 watts
AM or 20 watts CW on 160-10 mtrs. It should be a fun rig to have on the air. Now's the time to assess what I have and decide what needs to be done.

Both the front and top views show this rig to be unmodified. Excellent. I'm always sorry to see an other wise neat radio suffering from Black and Decker Syndrome.  But what's been done under the chassis? Connections hanging in mid-air? 6.3 VAC filament voltage run to the crystal socket (maybe for a VFO connection)? But everything important seems to be there so it looks like the first step will be to strip the chassis down to stock parts/layout, next, test the transformers and chokes then replace the electrolytic and paper capacitors. After that I'll see what else needs to be replaced or rewired.


To be continued...

Wednesday, June 6, 2018

NC-57 Bandspread

One receiver that I like to use as an example of early novice class gear is my National NC-57. It is a good vintage and performance match for transmitters like the Johnson Adventurer and Heathkit AT-1.
Unfortunately, like most two dial sets, the bandspread is designed to cover the entire 500KHz 80 mtr band. The 25 KHz of 80 that I'm most interested cover only 4 divisions (out of 100) on the dial. Bandspread on 40 is little better. I needed to bandspread the bandspread. This modification is reversible and changes those 25 KHz of 80 to cover 45 divisions on the dial instead of 4. Now I have a chance of grabbing the right crystal when answering a CQ and easily tuning in a station with the Q-Multiplier turned on.

Basically this change greatly reduces the effective value of the bandspread capacitor. This can be done by removing rotor plates from the bandspread capacitor but a simpler change is to put a low value capacitor in series with each section of the bandspread capacitor.

You'll need three 10 pf capacitors.

Using a fine tooth hack saw blade carefully cut the three wires between the main and bandspread capacitors. After cutting these three wires your NC-57 should behave normally except it will have no bandspread.

Solder three 10 pf capacitors between the main and bandspread capacitors electrically replacing the three wires just cut.
That's it electrically but you'll probably also want to make up calibration curves for you favorite parts of the bands.

**** Warning   Warning **** 

When cutting the three connections between the main tuning and bandspread capacitors there is a chance something will go wrong. You may be able to fix it by running an interconnect jumper across the top (insead of capacitors) or you may have a boat anchor in the nautical sense...sorry.

Sunday, May 20, 2018

It Was A Very Good Weekend

This past weekend was the annual Northland Antique Radio Club (NARC) "Radio Daze" about 90 miles north of me in Plymouth, MN. I try to make this one every year. While the club is more oriented towards broadcast sets, obviously there is a lot of overlap with ham radio boat anchors. I always see several calls in the parking lot and some ham gear shows up in both the Friday night auction and the Saturday parking lot swapmeet. It's been a while so Beth said she'd come along and we would make an over night outing out of it.

NARC has close ties with the Pavek Museum of Broadcasting so Radio Daze starts out with a Friday Pavek garage sale. I've picked up a few things at the Pavek before so I joined the crowd queued up at the door waiting for 10AM. This time I was not disappointed. Out for sale was a Millen 90800 transmitter, a Central Electronics 10A SSB transmitter, a McMurdo Silver 801 receiver and a Stancor 10P AM/CW transmitter.

I was already familiar with the 10P. Several years ago I had built my own 6J5/6L6 transmitter based on the CW portion of that rig. This one was a no brainer. This nice condition 10P was my Pavek purchase for the day. I'll probably pair it with my NC-81X as an example of a nice entry level 1940 station.

Last Friday happened to be a fairly nice day here in Minnesota. We've been waiting for spring here. Beth and I headed over to French Regional Park for a picnic lunch. Beth had reading material and I had my KX3 along. Despite poor conditions I thought I'd try "QRP in the Park". What I found was that I got a lot of  20 mtr hits on the Reverse Beacon Network but most of the hits were pretty weak...no responses to my CQs. I decided to move to 30 mtrs and 10 watts. This, at least, netted a QSO, one with KC5K in SE Texas. The 2018 QRP season is open for me. Given conditions I'll probably rethink my antenna situation, though. Right now it is optimized for 20 and 30. I need to put it on 40 and 30.

Friday night was the NARC Radio auction. This one had several Hallicrafters general coverage receivers, a BC348, a Mon-Key code monitor/keyer and a Knightkit T60. While not a lot of ham gear it was still at great place to meet others and talk about radios.

Saturday morning's  swapmeet was scheduled to start at dawn. I took this to mean start when you don't need a flashlight. At 5:45 two of us, one seller and one buyer were on time. The rest caught up by about 7.
Again, mostly broadcast stuff but also good parts and a few pieces of ham gear. I did talk to Roger, KA9BKK. He had a nice looking Ranger I and a  Heathkit AT-1 to sell. I have a Ranger already but the AT-1 is a good example of early novice gear. 15 mtrs isn't even on the bandswitch. We worked out a deal and I now have a transmitter that is a good match for my NC-57. I'll have to decide how many of McCoy's Oct 1955 modifications I want to add.

Like I said, "It was a very good weekend".