[TenTec] Stubborn Scout
dukeshifi at comcast.net
Wed Mar 13 21:37:28 EDT 2019
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.
More information about the TenTec