On 26/01/2012 02:40, TexasRF@aol.com wrote:
> 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
>
This appears to be not uncommon across several amp designs - even the
venerable TL-922 shows lower output on 160m than it does on 80m/40m and
no doubt if the input power is there then the output is going
somewhere... okay in the case of the '922 it might not be causing
de-tuning and grid current but I bet soemthing's getting warm ;-)
Mikie G8TIC
>
>
>
>
> 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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