Viking II Trapazoid Measurement

[w5gw]

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

thanks

Gary - W5GW

[W3GMS]

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

[KA2DZT]

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

[WD8KDG]

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,

[W3GMS]

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     

[KA2DZT]

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

[W7TFO]

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

[k4kyv]

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.

[W9BHI]

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