>I thought I had read about a way to adjust the power out on the 38s
>last week. After looking through 1.5 weeks of digests I don't see
>it.:( Am I dreaming or did someone actually write something on this
>and I can't find it?
>72 de Ron, N8VAR
Ron:
I am working on adapting the 38 special to 40m. Mine is now variable
power from about 1.5 to 5w.
I did this by going back to the original amplifier configuration and
using the coupling cap to feed the MOSFET amp. I think this is a
better configuration anyway because it appears that some people have
smoked their finals by getting the drive stuck to +8v when tuning the
drive cap, turning on the very MOSFET hard. AC coupling takes care of
this problem.
There are three resistors in the gate circuit (I do not have the
schematics in front of me). I left in only the 220 ohm. To the
capacitor side of this resistor, I applied a variable voltage to set
the power level.
I did this by first using a 50 K pot and the R- voltage. The R-
voltage is just about 8v during transmit. Using this voltage to bias
the MOSFET makes sure the bias is off during receive, and is a
regulated voltage, as opposed to the problems people were having with
the 12v being raised to 13.8v.
I varied the pot to find what voltage my MOSFET needed to get to 5W.
Mine required about 5.25v. So I placed a 47K resistor in series with
the 50K pot. From the 7.75 R- voltage, I now have a transmit only,
0-5.25v voltage variable source. I connect this to the bypass cap
side of the 220 ohm MOSFET gate resistor.
Like I said, I now get about 1.5w - 5w of output range.
- Dan Tayloe, N7VE (ex-KK7BD), Phoenix, Az; QRPL # 696, Az ScQRPions
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