On Thu, 19 Apr 2007, Roger D Johnson wrote:

Chuck Harris wrote:
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?

Have you tried replacing the 12AX7?

-Chuck

Yes....several times! I don't have a spare 6DE7 however.

You ought to get one. Tubes can sometimes have weird "heat-up-short"
cycles and if everything else is _new_, look at what you have yet to
not yet replaced.

You might try "bridging" in -- with aligator clips--external electrolytics
at those locations in the circuit where an on-board electrolytic might be
going through some kind of "charge-up-discharge" cycle to cause the spike.
One of the guys who answered this had a good idea about that.

Did your rebuild the audio section with all new capacitors? Sometimes even
new components can include a run of defects in a given lot.

What else is the second plate of the 12ax7 connected to?

Are the working voltages of the new capacitors you put in all higher than
any voltage you expect on those circuits?

Roger

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