[AMPS] RF Concepts Amplifier Question

[Ian Roberts]

I had this exact problem with a Motorola design for 70 cm using MRF309's
and offered as a kit by Communication Concepts Inc. Initial power was 20
W instead of the target 100 W!
Remedy: Solder trimmer caps of a few pF across the collector circuit (to
ground) just before the combining transformer, position is critical.
The trimmers must handle the RF voltage, if they go short circuit the
collector current will destroy the associated RF power transisitor
(don't use plastic foil trimmers!). Therefore, use low drive to find the
spot, then try increasing drive.

Playing with the bias will only increase (or decrease) the splatter in
SSB, while changing the quiescent current.

Hope this helps,
Ian ZS6BTE

Jon Ogden wrote:
> 
> Hi all,
> 
> I have an RF Concepts 4-310 UHF brick.  It's spec'd to put out 100 Watts
> with 30 Watts in.  However, I get power different from that!
> 
> I finally have a power meter for UHF so I decided tonight to check out my
> 432 setup and see how much power I was really getting out.  On 432.200, I
> found out that I was getting only about 60 Watts out of the amp with
> about 30 Watts of drive.  I checked the voltages and opened the amp up.
> Voltage right at the collector choke is around 13 Volts.
> 
> The I decided to tune up and down the band.  My output power gradually
> increased to around 90 Watts right around 439 MHz.  Then in the FM
> portion of the band it dropped down to around 80 Watts.
> 
> The amp consists of two MRF648s driven in parallel.  

Are you sure that are not in push-pull?

> At the point where the input microstrip splits to go to each transistor's base, there is an
> adjustable cap.  

This improves the match, monitor the exciter's SWR as well.

> Adjusting that tunes the amp so that I can change where
> the power peaks.  If I peak the power down in the small signal portion of
> the band, I can get a max of about 80 Watts.  This is then the maximum I
> can get out and everywhere else on the band is either the same or lower.
> There are no other adjustments except for a couple of bias pots and the
> pot that controls the release timing of the relays.  Adjusting the bias
> pots doesn't seem to do much.
> 
> So I am at a loss here.  Is this typical behavior for this sort of amp?
> Operating 40 Watts below its specified power in some parts of the band
> and 10 Watts below rated power in others?  Or do I have a bad device??
> 
> Any help would be appreciated.
> 
> 73,
> 
> Jon
> KE9NA
> 
> -------------------------------------
> Jon Ogden
> KE9NA
> 
> http://www.qsl.net/ke9na    <--- CHECK IT OUT!  It's been updated!!!!!
> 
> "A life lived in fear is a life half lived."
> 
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