HELP!! Valiant doing sneaky things!!

[Richard Carroll]

Brian Carling wrote:
>
> Hi guys! Any chance of your using a little expertise to help here?
>
> I am stuck with an annoying problem in my Valiant and had to shut
> down CX operation until I can figure it out!
>
> I hear a POP! sound about every 10-15 seconds when the rig is
> thrown into TRANSMIT, and the plate current will increase and then
> slowly climb back down to quiescent current of 10 mA unkeyed.
>
> Plat ecurrent will surge to about 400-500 mA and then quickly drop
> back to normal 10 mA!
>
> It will also happen when I am transmitting AM or CW. A POP occurs and
> the plate current (and power out) will kinda surge to a high level
> and then drop back. Happens more and more often now.
>
> It used to happen every 30-40 seconds. Now it's about every 10-20
> seconds. I am afriad that a transformer or choke might be arcing
> internally. I hav ereplaced all of the HV ELctrolytics recently and
> that did not help or change anything.
>
> I can't figure out WHAT is popping!
> It sounds like an ARC but I can't see where!
>
> The voltage on the hot end of  R16 remains pretty constant. 790V DC
> but on the other end and it is normally 140 VDC (clamping voltage,
> but will JUMP up to 280V DC when I hear the pop.
>
> Then it will slowly drain back down to 140V over about 1-2 seconds.
>
> Darn, I wish I could figure this out and get back on CX!!
>
> The RF Bias remains constant at -52V DC
>
> NOW, if I adjust the CLAMPER so that it is all the way clockwise,
> I STILL hear the POP, but there is no surge in plate current.
>

The Valiant has a weak point in the PI network  of the final, actually
*two* weak points--the fixed value TUNE and LOAD caps. The tune caps are
small low power fixed mica units in series-parallel to improve votage
handling vcapability, and on the one I had shoted. replace them with the
appropriate valaues in doorknobs that will handle the RF.  The fixed
LOAD cap is a stacked mica insulated unit that tends to short in one or
more sections, replace the individual sections with the proper size
doorknobs, or larger transmitting mica units. Note that if you undesize
them or use units that won't handle the RF current efficiently they wil
tend to drift, losing resonance. This goes fo trboth the TUNE and LOAD
units.

Luck and 73, Dick W0EX

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