Gary, can you terminate the power amplifier module with a 50R load and
measure RF/DC power there? Just thinking about some bad LPF.
Carlos VK1EA
On 3/14/19, MadScientist <dukeshifi@comcast.net> wrote:
> I have more time in this than I ever wanted, but I know the Scout pretty
> well now…
>
> It works fine in every way EXCEPT that the output power goes down from 50
> watts on 20 to 20 watts on 40.
>
> When I measure the current draw but the Power amp, it draws 12 amps on 40
> and 8.4 amps on 20, just the opposite of what one would have expected..
>
> Distortion of the waveform at the output TMP compactor is quite a lot worse
> on 40 than on 20, and its amplitude is actually higher at this point than on
> 20.
>
> I assume that the added current draw in the final is due to the distortion
> products, which get filtered out by the low pass filter in the module(s) and
> that it why I see more input power on 40 than 20 but less output power on 40
> relative to 20.
>
> I calculate that the final is pulling 166 watts on 40 and producing only 20
> watts output at the antenna terminal (14% efficient). On 20, the final is
> pulling 118 watts and producing 55watts, still only 46% efficient.
>
> In the middle is the 30 meter band, which produces 26 watts. Thus I am
> safely ruling out band modules as an issue.
>
> Has anyone else experienced this sort of thing?
>
> I did find that the schematic is wrong regarding C7 and C8 on the final
> board. The schematic calls out 0.1 uF whereas the radio has 0.01 uF in these
> positions. That calculates out to about 10 ohms reactance (total for the two
> capacitors, which are effectively in series in this circuit) at 80 meters so
> I wonder why that value was chosen. In fact, why are they there at all?
> There’s no DC to block so what purpose do they serve?
>
> PS: The schematic is also wrong in showing the order of the Jones filter and
> the fixed filter in the signal path on the receive board!
>
> One thought that comes to mind is the condition of the broadband transformer
> cores. Heathkit had a similar problem with the HW7 and HW*, wherein the
> power output on lower bands would falter due to aging of the ferrite used in
> the tuned circuits for the lower bands. Typically 15 meters would be fine on
> those radios because hey used a different coil core material.
>
> Ferrite DOES age, losing permeability.
>
>
> Gary
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