Scott Dorsey wrote:

In article ,
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?


Bad caps.
--scott

I assumed the caps had been replaced. But, replacing them is a GREAT idea for
several reasons if you have not yet done so.

There are four electrolytics. The two 40 uF in the high voltage supply. Mine
warmed up immediately on power application and started to leak. They were
replaced with aluminum electrolytics for about $9.

There is a 20 uF 450 vdc cap on the audio, 12AX7, plate line with a one second
time constant. Check that capacitor. The time constant matches your symptons.
That makes it a good place to start.

The final electrolytic is a 2 uF connected between the 6DE7 output and the 6146
screen. I replaced mine on principle ... I had no problems in this part of the
circuit.