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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 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 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".

Monday, April 23, 2018

Cathode Keying Safety

As I participated in various radio events this past winter I thought about a safety issue common in budget and homebrew transmitters prior to about 1960. Many, if not most, of these transmitters used cathode keying with no regulation of the screen voltage. In these transmitters it is common to find over 100 volts across the key terminals. I measure 125 VDC at my Johnson Adventurer key terminals. Some of my friends checked their 1929 homebrew transmitters. They found as much as 191 VDC at the key terminals. All of these are basic one or two tube cathode keyed transmitters, the sort of transmitters used by many today to experience vintage tube gear.

What can be done about the high voltage at the key terminals?

- Keep your fingers off the key terminals: I've been doing this up to now. It usually works but my wife worries that it does not always work. It also follows the "If it hurts don't do it" rule.

- Covered Key Terminals: Several military keys and even some bugs are available that hide the terminals. This is a workable low tech solution but you have to find a suitable key or bug. One option today is the Bencher RJ-1 sold by Vibroplex ( ).

- Mechanical Relay: Use your key or bug to operate a relay that is actually keying the rig. I've never tried this but it appears that speed may be an issue. How well does the relay keep up?

- Electronic Isolator/relay: I used a "Keyall HV" from Jackson Harbor Press / WB9KZY. The Keyall HV includes an opto-isolator and MOSFETs rated for 1000V at 3 Amps. Besides low voltage at the key terminals it is also low current so it can be used to interface a modern keyer to the heaviest of our cathode keyed boat anchors. See for details.

The Keyall HV kit includes five parts and a small printed circuit card. This kit is just about as simple as one can get. Instructions are on the Jackson Harbor Press website.
I used a small aluminum utility box for my keyall housing. I could have used a smaller box but the only two battery battery holder I had was for type C flashlight batteries. The batteries needed all of the available width. I used a barrier strip for the transmitter connection because some transmitters do not have one side of the key connection grounded.

The Keyall HV interface sits in-line between my key and the transmitter. The key terminals now have less than 3 volts between them. The two C cells should be good for over 500 hours of key down time.

Friday, March 23, 2018

A New Rig

My new rig...

The Heathkit HW-16 was one of the last rigs designed specifically for the US Novice operator. It was marginally a transceiver. The receiver and transmitter sections share a common power supply and antenna coupling circuitry. The HW-16 covered only 80, 40 and 15 meters, the bands allowed for Novices in the late 60s. The CW only transmitter ran 75 watts and was crystal controlled, again novice requirements. The receiver section was double conversion with 500Hz selectivity. Good selectivity for the chaotic novice bands was a must. After my initial tuning around the 40 meter band I'm pretty happy.

Only 345 +- days to go to the NRR 2019. In the mean time look for me in the evenings around 7110-7120.

Tuesday, March 13, 2018

Novice Rig Roundup Recap

The 2018 Novice Rig Roundup is now history. During the nine day event I used 4 transmitters and 5 receivers to make 23 contacts in 15 states and British Columbia.

This was a great opportunity to try out several of my rigs. Which work best in a variety of situations? Which are easiest to operate? Which are most reliable? Which sound the best on the air? The list goes on....

( Warning: Personal Opinion, YMMV )

Drake 2B and Collins 75A4 with EFJ Adventurer My Adventurer position was set up so that I could easily switch between a Drake 2B and my Collins 75A4. The 75A4 is certainly a fun radio to use. I have a 500Hz crystal filter installed and, with the 75A4 bandpass tuning, I could easily sort signals out. KC frequency readout was great also. The 2B, though, could hold it's own against the 75A4. The only thing I missed on the 2B was KC frequency readout. For pairing with a simple crystal controlled transmitter like the Adventurer the 2B is my choice.

NC-57 with a 6AG7 Sucker Stick Xmtr This pair fits both as an NRR Novice 1 station and as an AWA Linc Cundall pre-1950 rig ( ) I've added a Heathkit QF-1 Q-Multiplier to the NC57 and spread out the bandspread so that 50KC covers most of the dial. It now does a pretty reasonable job on 80 and 40 CW. I just have to ride the RF gain to keep the set from overloading. 4-5 watts out for the 6AG7 transmitter, while expected and a cool design, makes for a tough time raising QSOs. My plan is never to run up the score but QSOs are still more fun that calling CQ CQ CQ. If I were only thinking of the NRR I'd probably replace the 6AG7 transmitter with my Adventurer, but the Adventurer does not qualify for the AWA Linc Cundall. I need a pre-1950 transmitter. Before next year I'll build up some sort of homebrew transmitter using pre-1950 tubes that runs more power than a single 6AG7.

NC-303 / Eico 720 The Eico 720 is my favorite "novice transmitter". I used one 50+ years ago when I was a novice and I like the looks. It has style. Before this year's NRR I replaced the power transformer and did a required tune-up. Now it works without a chirp on all bands. This one is a keeper. The NC303 is a fine late 50s receiver but it has to compete with my Drake 2B as I consider what to use. The 2B is the nicer receiver. Next year my Eico 720 will be paired with my 2B.

Drake R4B with matching T4XB Except for my Novice year I've always had a transceiver at the primary operating position.  This year for the NRR I ran my vintage Drake B Line separated. This worked great! I had frequency spotting, QSK keying, code monitor plus matching KC readout on both receiver and transmitter. The R4B receiver with variable/tunable bandpass is a great receiver. This is now my go-to rig when my KX3 won't quite do the job and it fits in fine as an NRR "Novice 2" station.

Saturday, February 24, 2018

2018 Novice Rig Roundup Update

It looks like I'll have three stations available for the 2018 Novice Rog Roundup: Drake 2B / EFJ Adventurer, NC-57 / 6AG7 Sucker Stick Xmtr and NC-303 / Eico 720.

Eico 720s seem to have power transformer problems. I have two, each with a bad power transformer. Lacking a drop in replacement I went to the 'net looking for a replacement.
What I found was a 120V to 480V @ 150VA industrial control transformer. Obviously it didn't have a filament winding but I decided I could live with a separate filament transformer. I changed the '720 rectifier to a silicon bridge rectifier, added a 10 mfd cap to the input of the power supply filter and installed
a "back porch" for the filament transformer. Now under load 530 VDC is delivered to the 6146 instead of the specified 560 VDC...close enough to go on the air at 75 watts input. It tests out just fine. Now I have a couple of minor cosmetic changes before I pair it with my NC-303.

I'm almost ready and really looking forward to this year's NRR.