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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.