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<div dir="auto">I would be interested in tune up procedure for the SB 200 in AM mode.</div>
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<div dir="auto">------ Original message------</div>
<div dir="auto"><b>From: </b>Kenneth G. Gordon</div>
<div dir="auto"><b>Date: </b>Tue, May 15, 2018 3:02 PM</div>
<div dir="auto"><b>To: </b>Bill;</div>
<div dir="auto"><b>Cc: </b>Heath@puck.nether.net;</div>
<div dir="auto"><b>Subject:</b>Re: [Heath] DX40</div>
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<div class="PlainText">On 15 May 2018 at 1:18, Bill wrote:<br>
<br>
> Hi Mike and Ken.<br>
> <br>
> Yes, I will be replacing the 6146, but should I get a 6146 OR a 6146A?<br>
<br>
Doesn't matter. Any of the 6146 family will work fine in the DX-40. Even the 8298, and 6293
<br>
(if you can find one) are quite suitable. The DX-40 is not picky as far as final amp tubes are
<br>
concerned as some of the later rigs are. You should avoid the 12.6 V filamented versions,
<br>
though, unless you change the filament voltage (!)<br>
<br>
> I have a copy of the original schematic and the build manual. They both show a 0.1uf cap.
<br>
> Physically it is across from the 2 power supply electrolytic. However, on the DX 40 I acquired that
<br>
> capacitor has been replaced with a 30uf cap.<br>
<br>
Hmmmm....I am still at a loss as to where this is. Your description doesn't help me much.<br>
<br>
On my schematics, I find only one 0.1 uFd 600 V capacitor, labeled C-3, which is in the
<br>
modulator circuit. One end is connected to the junction of three 470 K resistors, and the
<br>
other end goes to ground. If that one was replaced by a 30 uFd capacitor, I would think that
<br>
it would, essentially, badly effect the audio coming from the 12AX7 speech amp by severely
<br>
reducing "lows". I suspect that who ever changed it, had some hum in the transmitted audio
<br>
and thought that by increasing that capacitor to 30 uFd, the hum would be reduced.<br>
<br>
If that was my rig, I'd change that back to 0.1 uFd 600 V.<br>
<br>
Usually, hum in the audio is caused by poor shielding of the mic cables.<br>
<br>
In any case, that capacitor is not going to effect the transmitter when in CW so unless you
<br>
intend to use the DX-40 on AM, you can safely ignore it.<br>
<br>
BTW, such controlled-carrier AM rigs as the DX-40 make excellent drivers for a <br>
grounded-grid linear amp for AM, such as the SB-200 or SB-220.<br>
<br>
Ken W7EKB<br>
<br>
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