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RE: Re: DEM Pre-Amp



I purchased the kit version of the 13ULNA from DEM.  I'm assuming that you
have the pre-assembled version.  In my instructions after the unit was
built, I was to turn that potentiometer until the unit was drawing between
20 - 30 mA.  I set it to 25 mA.  I am using the 10V supply from my Drake fed
into the power feed thru capacitor on the case.  My unit is working GREAT
with no problems! You may want to check the total current draw of the preamp
and the output of the 5V regulator inside the unit.  7.5V does seem very
close to the minimum overhead of the 78L05 regulator.

Regards,
Tim
K3TZ

> -----Original Message-----
> From: owner-AMSAT-BB@AMSAT.Org [mailto:owner-AMSAT-BB@AMSAT.Org]On
> Behalf Of ken durham
> Sent: Friday, July 27, 2001 11:06 PM
> To: amsat-bb@AMSAT.Org
> Subject: [amsat-bb] Re: DEM Pre-Amp
>
>
> My .7db NF preamp arrived several days ago and just got put in the
> system with the converted Drake (spec 3 db NF.) It seems to have a
> problem or is causing a problem. It increases the S meter reading
> from S1.5 to about S3, but the s/n ratio seems worse. It seems to
> work only at about 7.5 V for a decent gain and NF.  S meter increases
> to  >S4 at this supply voltage and the NF improvement is obvious
> listening to the AO-40 beacon. My 12V supply isn't useable
> for this preamp. The spec sheet says that the supply range is
> 7 to 16 V. The spec doesn't have a curve for NF or gain vs supply
> voltage.
> Does anyone know if this is typical of this preamp (model 13ULNA)?
> I sure hate to send it back since it took so long for them to ship
> and it's up on my tower now.  I'm tempted to just use a 7.5V supply
> and keep the preamp. The schematic shows a variable resistor from the
> IC that generates the internal negative voltage to ground and the wiper
> to the gate of the FET.  It may be that this pot just didn't get
> adjusted
> at the factory to a proper voltage for the gate. I don't want to take
> a chance on blowing the FET by trying to adjust  this for optimum
> gain and s/n ratio. Any suggestions?
>
>
> Ken   K5MBV
>
> ----
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