I find it hard to imagine that the final could exhibit symptoms of being
out of neutralization on 80 meters. The grid / plate capacitance is
simply too low to cause feedback on this band.

The Heath SB series used an unusual pre-selector / driver tuning
circuit. On 80 meters a single coil with a coupling link is used in the
tank circuit on both the input and output of the driver. It is tuned by
variable capacitor, and tracking is adjusted with a variable powered
iron slug. On the other bands additional slug tuned inductors are
switched in parallel with the 80 meter inductor. These inductors are
connected in series and are shorted out one by one between 40 to 10
meters by the band switch. On 80 they are disconnected from the
circuit. IF the 80 meter inductor were open the circuit would not tune
correctly on that band, but the series connected inductors might still
resonate close enough on the the other bands, on 10 meters the open 80
meter inductor would not even be noticed. Since this circuit is part of
the neutralization bridge it would be worth checking.



On 07/16/2011 04:55 AM, Antonio Vernucci wrote:
"Kenneth Scharf" ha scritto nel messaggio
...
If the problem is ONLY on 80 meters then the issue may be with the
output loading capacitor. On 80 meters the three gang variable loading
capacitor isn't big enough so the band switch adds an additional fixed
cap in parallel. Maybe that cap has gone bad or the switch is not
making contact on that band. Try putting a 500pf 500v cap in parallel
with the output loading capacitor and try it on 80 meters.

Should the output Pi network capacitance be too low (bad additional
fixed capacitor or faulty switch), one would just see a dull plate
current dip and get low RF output power, due to the Pi network
overcoupling to the antenna. I see no reason why it should also cause
the final stage to become non properly neutralized (as evident from the
fact that the plate curent dip does not occur as the same plate
capacitor setting as for maximum output power)

73

Tony I0JX
Rome, Italy