AWA 6AQ5 Transmitter (continued)

 

At https://youtu.be/nJgJenvi2x4 I've posted a demonstration video of an entry/novice station as it might have existed in the mid 1950s running on 40 mtrs. It uses a National NC-57, a receiver that was introduced in 1948. The transmitter is my single 6AQ5 running 6-8 watts out. For more information see the video description.

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.

More NC-57

I added a Heathkit QF-1 Q-Multiplier ( http://www.heathkit-museum.com/ham/hvmqf-1.shtml ) to my NC-57. The QF-1 provides additional selectivity or signal rejection as desired. It was marketed by Heathkit starting around 1956 and was a fairly common accessory. It requires a connection to the IF stage plus B+ and filament voltages available at the NC-57 accessory socket.

I found this to be a big help. The QF-1 effectively eliminated the CW audio image (ie. a readable signal each side of zero beat) common in receivers that don't have a crystal filter. Eliminatling the audio image eliminated half the QRM on the band.

NC-57 Project with comments about the NC-125

For the past few days I've been going through my NC-57. This radio was marketed by National in the late 40s and early 50s. Features include a built-in speaker, electrical bandspread (uncalibrated), regulated B+ for the HF oscillator and a tuned RF stage. Advertised frequency coverage was .54-55MHz. See http://www.io.com/~nielw/nat_list/nc57.htm for more information.

My set had been already gone through by a previous owner, recapped and aligned. This set includes an extra dial string in the bandspread tuning assembly to overcome slipping. While this radio generally worked it also would go dead once and a while for no apparent reason. Close inspection revealed a cracked solder connection on a filament pin of the 6H6 detector. Resoldering this connection fixed the problem.

Properly aligned, the calibration was excellent and sensitivity good but this radio is still only one step removed from a simple SWL receiver. Mechanical and electrical stability is marginal, especially above 40 mtrs. It was no surprise that SSB and CW signals easily overloaded the receiver and BFO. Operation with the BFO requires running the AF gain wide open and controlling volume with the RF gain control. I did like the positive feel of the main tuning. While National used a failure prone dial string for bandspread tuning, main tuning uses a gear/pinch wheel mechanism.

Three years after introducing the NC-57 National came out with the NC-125. Under the covers this "new" receiver was simply a NC-57 with the internal speaker replaced by an S-meter and Select-O-Ject audio filter. Both of these features were available as options on the NC-57. National also replaced the round dials with slide rule dials. Unfortunately National took a step backwards by using slip prone dial strings on both the bandspread and main tuning. The Select-O-Ject feature, while unique, does nothing to remove the audio image. I believe a Q-multiplier similar to the Heathkit QF-1 would have been more useful.

I acquired a tilt base with my NC-57. Besides tilting the radio front for easier reading, this also allows a ham to add extra station controls and some accessories without cluttering up the shack. High on my list is a 1MHz crystal calibrator to help set the main tuning for repeatable bandspread calibration over each ham band.

I plan to use my NC-57 on 80 and 40 with one of my early novice transmitters.