The AM Forum

Where have folks tapped off audio output in a Viking II so as to make a trapazoid measurement?  Any precautions?

thanks

Gary - W5GW

Gary,

I would think in order to get close to a true representation of "actual audio"  you would want to take it off the top side of the modulation transformer.  Taking it off earlier would not be a true representation.  I would use a suitable size DC blocking cap which would need to withstand the HV DC plus the peak audio voltage.  That would then go into a rather high impedance resistor divider network.  Size the two resistors such that you get the correct amplitude to the scope.   

Joe,
W3GMS

Joe is correct

Audio is taken from the class C side of the mod xfmr, between the xfmr and the plate RFC.  The cap has to be at least 3-4KV and as large a value you can get.  I use two .1ufd 2KV caps in series.  Then you add resistance, usually a string of resistors 250K-500K.  The last one can even be a pot for adjustment.  This set-up is in any ARRL handbook.

The RF is taken from the tank output with a pick-up coil.  DO NOT make any direct connection to the RF tank circuit for the RF pick-up.  You can also use the RF pick-up for the usual wave envelope pattern.

Fred

Look at any of the 50's or early 60's ARRL handbooks. The equations for C, R1,& R2 are in there. Previous posts already give the location for C. A small project box will hold the pot and the string of resistors.

Never played with a Viking II, but getting the audio sample out of the transmiter is an issue. If an unused solder post & rear connection are there, use it with consideration to voltage going to the project box & insulation of the wire.

Craig,

Gary,

Put the grounded series divider network in the transmitter that way your bringing a low voltage audio sample out of the rig.  I would not bring it through a FT cap since your source will be very high impedance.

Joe, W3GMS     

Correct again,

I put the whole network INSIDE my HB xmtr.  Therefore you can't get sapped by the high voltage.  The last resistor is grounded and you can bring the output out to a BNC connector.  The output is taken from the junction of the last two resistors,  or the wiper if you use a pot as the last resistor.

Fred

Some BC rigs take the RF sample off just before the output coax receptacle.  That is, if you are running coax output as well.

They use a tappable choke that represents high impedance to 50Z connected directly, the other end grounded, the sliding tap goes to the sample out jack.

I've seen others that used a few turns in a link, coupled to the last series inductor in the output network.

73DG

I have never been able to get a trapezoid pattern to work right with a simple resistive dividing network without adding some capacitors to compensate for phase shift; otherwise the pattern is all muddled up. One capacitor like the circuit they show in the handbook never worked for me.  I eventually came up with a circuit that would produce a clean pattern, using trial and error.

To K4KYV:
Could you please share the information on the resistor capacitor circuit?
I have a Valiant II that I would like to do a trapezoid monitor on.
Thanks,
Don W9BHI