The linearity of a modulated RF stage can be monitored with a scope by applying the RF to the vertical plates and the audio to the horizontal plates. The resulting waveform is a trapezoid. This implementation is specific to my Valiant, but it can be adapted to similar rigs like the DX100, Apache, Viking One and 2, and so on.

I added a sample point for the modulated B+ to feed the horizontal deflection of my monitor scope. This allows the RF to be on the vertical deflection for a trapezoid pattern as shown in some of the 50s handbooks. At the completion of these modifications, I got a perfect textbook pattern. It is a lot easier to judge a straight line on the side of a triangle than guessing at a peak of a speech pattern for linearity. You can strive for linearity in the audio system and fail to get the RF biasing correct and wind up with distortion that is hard for a receiving station to characterize. Circuit constants shown are good to below 100 Hz for the sample circuit. The modulated high voltage sample comes out to an RCA jack just below the SSB jack. Its hard to find a spot to mount anything on the back apron without ruining a harness. All my instrumentation is located here.

To alter the circuit for other radios, be sure you have capacitors of at least 3 times the DC plate voltage of the modulated stage. You can use series capacitors as I did of 2 KV each. These were salvaged years ago from a TV set.

Capacitor value should be more than 0.004 divided by the total resistance in the divider string in Megohms. This is necessary to avoid loss of low frequency response and associated phase shift which will distort the trapezoid pattern. Refer to handbooks of that era to learn what conditions are indicated by bad patterns. Frequency response to ten cycles per second can imply unacceptable phase shift up to 100 cycles. Linearity tests performed mid band at 1KHz should be valid for most all frequencies. The exception is that if the screen bypass resistors of the RF stage are too big, the high frequency response may be changed by the resultant phase shift in the screen circuit.

Also do NOT use just one resistor for the high voltage sample. Many do not know that resistors have a voltage coefficient that affects the accuracy. More significantly, they are not rated for use above a certain voltage across the resistor and may cause a safety problem by exhibiting much lower effective resistance than the value marked. I bought a box of resistors of the value shown, so that is what I used to get the total resistance required to attenuate the signal to the desired level. The Radio Amateurs Handbook of 1954 suggests a value of 250K for each 100 Volts of DC plate voltage of the class C plate modulated RF stage. NEVER use higher than 470K resistors to make up the total resistance; with the value I used, 180K, 1 Watt resistors are OK, but verify it by your own calculations. I am just trying to get you in the ballpark here.

The bottom resistor in the string should be substantially small in resistance to provide the correct voltage to the horizontal amp in your scope. WARNING: modern solid state scopes do not tolerate high voltages well. Do not risk shock hazard or failure of the scope with inferior blocking capacitors or resistors of insufficient wattage. IF IN DOUBT, start with a smaller bottom string resistor than you think is appropriate. If the signal is too small to get a big enough pattern, increase it in small steps. READ THE SPECS of your scope to be sure what is expected.

BE ABSOLUTELY CERTAIN THAT A PROPER GROUND BETWEEN THE SCOPE AND TRANSMITTER AND EARTH GROUND IS IN PLACE. If you are not experienced with proper electrical safety procedures, get someone who is to help you or do not attempt this idea.

click on thumbnails to enlarge.