The AM Forum

DX-100 causes a couple seconds worth of pops in receiver as the high voltage decays after letting up on the PTT button.  Installed bypass caps around clamper that Johnson Viking Valiant uses; better, but still there.  Any ideas?  Ed.

I've had that happen in amplifiers when the cathode standby resistor is either too high in value or open.

Might help ya out,  dunno.   

--Shane
KD6VXI

A snubber diode across the relay coil?  Providing it uses one, I am not familiar with small ham TX.

73DG

Hi Ed,

I've experienced this problem many times, especially with voltages above 2KV. It is usually a bleed off path that bypasses the filter cap bleeder path. It can be so many things, like leaky filter caps, corona off sharp points, a terminal or lead that is too close to ground, etc.

It may even be related to parasitics or other instabilities that act up only when the HV is reduced. Try loading the grid of suspected tubes to be sure. Also, unplug tubes to divide and conquer from the rest of the circuit.

As a test, make the room totally black dark and look closely at the various parts as it pops. A big mirror behind the rig can help to see more angles at once. You may see a faint spark. Also, use an insulated pick up loop that goes to an AM receiver via a coax to sniff around for the loudest pop. This will often expose the culprit. Be careful with HV.

Also, as it pops, use an insulated stick (like a fiberglass rod) to push and pull parts. Often the popping will change character as the guilty part is moved.  

Tighten down HV connections, look for corroded connections and re-solder connections in a shotgun approach.

It could also be a transformer winding that is breaking down, so prod that area and sniff around.

It's a matter of detective work. You will find it.

Tom, K1JJ

Try turning the audio gain down all the way down and then re-running the test. If you don't get any pops then the HV is decaying too slowly and the audio section is providing a feedback path through the receivers speaker. So if it's unmodified, suspect open or higher value bleeder resistors. If you have larger electrolytics or solid stated Hv and Lv, you may want to provide for some form of audio cutoff bias or muting when you switch over to receive. Had the same issue in a highly modified Apache.

  Steve
KB1VWC