[Amps] SB-220 load capacitor question

[Dr. William J. Schmidt, II]

You might also clean the contacts on the TR relay... that could do it too...

Sincerely,

Dr. William J. Schmidt, II  K9HZ
Trustee of the North American QRO - Central Division Club - K9ZC

Email: bill at wjschmidt.com
WebPage: www.wjschmidt.com

"It's not what you take with you... but what you leave behind that counts. 
Live each day as if it were your last."


----- Original Message ----- 
From: <johna8119-amps at yahoo.com>
To: <amps at contesting.com>
Sent: Thursday, November 10, 2005 12:15 PM
Subject: Re: [Amps] SB-220 load capacitor question


> >R Measures wrote:
>>On Nov 9, 2005, at 6:09 PM, <johna8119-amps at 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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