This question relates to the Ameritron AL-800H, but the circuit in question
is identical to that in the AL-80B and perhaps other Ameritron amps. I had
to replace the toroid used to sense forward/reflected power, as well as the
diodes, a couple of resistors and a couple of disc caps in the circuit. The
secondary winding of the toroid is loaded with a 51 ohm resistor and the
forward and reflected voltages are taken off of the ends of that resistor.
The winding is center-tapped and goes to ground through a 1.8K resistor
bypassed with a 270 pf mica cap. The center tap is also connected via a 10
pf fixed cap and a series 3-12 pf ceramic trimmer to the RF output line
(which passes through the toroid).
The question is: what is the function of the caps connected between the
center tap and the primary conductor? I assume that the adjustment has
something to do with the calibration of the forward/reflected power meters,
but don't know how it comes into play. There's a pot to ground connection
in each of the legs to calibrate the meters with. Perhaps this variable cap
is used to correct the power reading at higher frequencies?
When ordering the parts for this unit, I asked the question of the
technician at Ameritron and he was reluctant to give me the adjustment
procedure. He said it wasn't written down and he wasn't sure about it.
K8AC
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