If you check the screen voltage of the 6146 I expect you will see it pulsing
from 70 +/-10 volts to over 100 volts, possibly to 160 volts. Seventy volts is
normal screen bias in the AM mode. With modulation it follows voice peaks to
~150 volts.

The time constant of the 6DE7 to 6146 RC circuit is about 0.025 seconds. So, I
would exclude the RC between the 6DE7 and the 6146 screen.

I would check pin 6 of the 6DE7. It should be stable in the AM mode. The input
circuit to the 6DE7 has a 0.1 second RC time constant. If the input is stable
[pin 7] and the output is pulsing the replace the 6DE7. If the input [pin 7] is
pulsing then check the RC from the 12AX7.

/s/ DD

PS: let us know what you find out.
+ + +

Roger D Johnson wrote:
While we are on the subject of DX-40s, I have one with a problem
that's driving me nuts. In AM mode, just sitting at idle, the plate
current pulses up about every second or so. The scope shows a pulse
on the plate of the second 12AX7 audio stage. It's acting like some
sort of relaxation oscillator. I've rebuilt the audio section to no
avail. Anyone have an idea what the heck is going on?

73, Roger





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