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.

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...

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....

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...

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".