About a month ago, JE1BMJ reported a similar problem with his SB1000. In
his case, the problem was found to be inadequate bypassing of the plate rf
choke. Apparently enough rf was reaching the bypass capacitor to heat it up
and cause it to change value and that was enough to detune the plate circuit.
73,
Gerald K5GW
In a message dated 1/25/2012 7:20:22 P.M. Central Standard Time,
at6c@bellsouth.net writes:
Carl/Paul,
There isn't a fix loading padder (Carl, I think you are referring to a
capacitor pad for 160M correct me if I am wrong) in the HF 1250. There
is a tapped ferrite transformer/coil that is switched in for 40-80-160.
That being the case the problem could be something shorting out part of
that transformer in the 160M position. Also the band switch could be
bad. Have you checked all the components in the RF compartment for
visual damage/overheating/value? The only difference in the
tuning/loading circuit between 80M and 160M is the position of the band
switch and which part of the above coil is in the circuit.
Another point to consider is that the LP100A has three different
selections for measuring output power(average/peak/?pulse?). I find
that if you are tuning a twitchy amplifier a needle type watt meter is
easier to use than the LP100 (maybe it's just me). I think if you are
not using a pecker tuner, peak is recommended, check the manual. Also
carefully follow the instructions in the HF 1250 manual as to the way
you tune the amp. I found those instructions will prevent surprise grid
current fluctuations. Good luck.
73
George
W2GS
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