Testing a modulator without an RF section

[W4RFM]

This may have been covered but I could not find anything on the subject:

Is it possible to test a newly built or restored self powered modulator by finding a correct resistance and wattage resistor and placing it across the secondary of the mod transformer, and then clipping on a .5 cap and listening to it on an amp?

[Opcom]

Like these? Its more or less what I do but all disclaimers apply. There is a topic somewhere.

A cap directly to the audio amp input is not good because the aduio amp wants 100mV to 1V for drive, but the modulator makes hundreds or thousands of volts.

Also a reasonably low resistance to GND is needed across the audio amp input or it can blow the amp input when your dc blocking cap charges. The bigger the blocking cap the worse it is, so a small one even 0.01uF, with a high resistance divider is better.

The second diagram is better if you are measuring freq response because it will change with the DC current flowing through the mod transformer secondary.

[W4RFM]

Thank you Sir, I will act accordingly. I thought that should work, but I never trust my simplified judgement. 

[WD5JKO]

One additional comment. You might be tempted to keep the modulation transformer secondary at DC ground, and just lay a load resistor down on something that can take some heat. For safety reasons, this idea has some merit. But consider that there will be high voltage between the primary and secondary windings. This could lead to a breakdown, and transformer failure. I like Patrick's second circuit.

Jim
Wd5JKO

[Opcom]

R1 should say: string of reistors  enough to withstand peak modulating voltage

R3 should say: load resistance, enough resistors to withstand peak modulating voltage and power, plus usual RF stage plate current.

sorry bout that, it was late probably.

[W4RFM]

A question: But...there is no B+ for the RF, I have not built that part yet, I just want to test the modulator. The amp "should" deliver a peak of 65 or so watts. (2x 6DQ6's with 550 volts on the plate) I planned to place the 3K  100 watt resistor across the secondary and tap off it with a cap and series of resistors and feed the amp.

[GW0FZY]

Ive used a load of light bulbs in series to test a 300w modulator...the 240v100w filament type; seemed to work OK

[Opcom]

A question: But...there is no B+ for the RF, I have not built that part yet, I just want to test the modulator. The amp "should" deliver a peak of 65 or so watts. (2x 6DQ6's with 550 volts on the plate) I planned to place the 3K  100 watt resistor across the secondary and tap off it with a cap and series of resistors and feed the amp.

That low a voltage there should not be an issue with the mod secondary or inter-winding insulation if the transfomrer is new or in good shape.

But you don't need a RF B+ supply, just borrow from the modulator plate supply since you are not drawing any DC current for the non existent RF. It's just to 'charge' or 'elevate' the secondary. So either way, withoor without the high voltage on the secondary your choice it should not be a problem.

Yes place the 3K across, and tap off it with a cap, and a voltage divider like shown.