Hi Folks,
consider this situation:-

I have a problem with 2 HF transmitters, both misbehaving in the same
manner.

On all HF bands (from 3.5 to 28Mhz) bands, the alc meter shows drive
variable by mic gain control - this is set up by watching meter
deflection on voice peaks as per instruction manual. In CW mode, power
output is max 100 watts each band.

The EXCEPTION is 80m, where
1.ALC does not register on meter irrespective of voice drive/mic gain
setting.
2.Power output is 180 watts in CW mode.

Something is wrong - I have checked drive level to RF power amp
section with my CRO, same level all bands. I have checked the LPF on
the PA output, the switching relays work OK and every component has
been removed from the board and checked - the caps have, the toroidal
coils were examined for burning/shorted turns/continuity, but not
inductance measured.

Can you suggest what has gone wrong - I am running out of ideas and
knowledge!
My limited theory states that a portion of the RF output is sampled
and fed back to the exciter stage to reduce gain and thus prevent
overdriving and splatter. This ALC detector circuit is common to all
bands, so its unlikely to be faulty. All test results were on a 50R
dummy load, so antenna variations have been eliminated.

Thanking you,

Andrew Blight VK3BFA