[Swan] 500-CX Tech Assist Needed

[Dan Hatcher]

Folks:
Hopefully, someone can provide help to solve a most unusual problem I'm
having w/ a really nice 500-CX.  Radio had been working fine and then
.........
In receive, on 80 & 40 M, it breaks into an oscillation when I adjust the
P.A. GRID control.  On 80 M, any position of the PA GRID will cause a raspy
oscillation but it will peak at a certain setting; on 40M, the oscillations
occur over a narrow range of PA GRID control movement.    20/15/10 M seem
normal.

Problem seems independent of dial frequency or any other control - loading
(coarse/fine) has no effect on the oscillation.

I've not read any problem like this in the Swan Compendium or Newsletters.

I've checked the receive RF amp (6BZ6, in this variation of 500-CX) & mixer
(12BE6) tubes  & the xmit driver (and swapped them out w/ known good ones
also).  I've cleaned the 500-CX power connector pins,  tried 2 other power
supplies, measured the various voltages (seems within tolerance) & even
resoldered all tube socket pins on the rec amp/mixer - all w/o success.

It appears as though some feedback path exists in the mixer area - I can
duplicate the effect w/ the rec amp tube removed, which doesn't make much
sense at all, since the PA GRID control effects the receive signal path as
applied to the rec RF amp.

I would appreciate any insight you Swan lovers might have - maybe I'm to
close to the problem & am overlooking the "obvious".

73 & thanks in advance
Dan KD3CQ

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