>R Measures wrote:
>On Nov 9, 2005, at 6:09 PM, <johna8119-amps@yahoo.com> wrote:
>
>> I should probably be ashamed to ask this. I have this SB-220 that I
>> have been unmodifying, and I
>> have run into a situation that I am wondering about.
>>
>> When I tune this rig up, with excitor power anywhere from 5-100 w, I
>> have to turn the load
>> capacitor fully clockwise (unmeshed) to maximize power output. I'm
>> getting decent power out with
>> it. Could this be due to the 50 ohm dummy load I'm working into? Is
>> it a good thing to have
>> "full" loading?
>
>Two possibilities: The load R is not 50-ohms -- or the bandswitch has
>some vapourized contacts due to previous arcing. This is more likely
>to occur to the 15m and 10m contacts.
>The load R's ohms can be measured with a DMM.
I had previously checked the dummy load with DMM (47.8 ohms) and with a MFJ 239
SWR meter (50 ohms
and 1:1 swr). There is no change with switching the bandswitch. I have
polished the contacts
with a bit of grocery bag paper. No change.
>Dr. William J. Schmidt, II K9HZ wrote:
>Sounds like you are using the same piece of bad coax between the amp(s) and
>the dummy load. A fully un-meshed loading cap indicates that the output
>load is not 50 ohms!
I thought so, too, about a month ago, so I made up new (different) cables with
RG-8U, and the same
thing happens.
>Robert Dorchuck W6VY wrote:
>Hi John,
>I have four 220's here and none of them load like that. At low
>drive the load setting is quite low. My load settings are
>roughly - 75 meters -- 6 - 7
> 40 " 4 - 5
> 20 " 5 - 6
> 10 " 6 - 7
>
>
>There are very small differences between the amps.
>
>Would seem like something is amiss. Are the caps fully meshed
>when the knob is set to zero? Do you have normal grid and
>plate current?
The caps are fully meshed when the knob is turned to zero. Plate is normal.
Grid current is a
little high at 100 w drive. I've checked this on all 5 bands, and the load cap
setting is the
same (fully unmeshed) for each.
On the amp I am "unmodifying," I have replaced the PC board with the Harbach
board. I have also
replaced the anode wiring circuit with the low-Q circuit recommended by Rich
Measures. The other
220 is pretty stock, though I have replaced the zener with a 7 diode string. I
don't see how
either of these mods could do this.
Thanks
John
w4kv
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