[Amps] Refurbishing CB amp

Dr. William J. Schmidt, II bill at wjschmidt.com
Tue Oct 4 00:14:48 EDT 2005


Well, the obvious question to be asked is what kind of transformers did you 
use to replace the existing transformers?  To make sure you are using the 
right core material and design, you can take the transformer out of the 
circuit... put a load resistor on the secondary (input to the transistor of 
about 5 to 10 ohms) to mimic the impedance of the transistor circuit... and 
then sweep the transformer (vary the frequency and watch the SWR).  If this 
is ok.....

Scope the base of one of the transistors to see if you are, indeed, 
saturating and over-driving the transistors.  Characteristic curves for the 
455 will tell you the whole story.  It could be that the RF is getting into 
your new bias circuit...changing the bias voltage when RF is present.

I see you've change the frequency-gain compensation network.  I've done lots 
of these... on the first one I attempted, the capacitance of the resistor I 
was using created problems for me... had to change resistor types to 
strictly carbon composition (be careful you don't use glass film or wire 
resistors in this network!!!).

Hope this helps.

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

"If computers get too powerful, we can organize them into a committee; that 
will do them in."  -- Bradley's Bromide


----- Original Message ----- 
From: "Stefan" <ea5fy at ure.es>
To: <Amps at contesting.com>
Sent: Monday, October 03, 2005 10:49 AM
Subject: [Amps] Refurbishing CB amp


> Hi friends
>
> I am trying to refurbish an CB amp. I am converting from class C to class 
> AB
> (adding Vbe polarisation) and intend to change the output trafo to avoid
> saturation. But I have noticed a bad bhaviour at lower frequencies (below
> 14MHz). I have alreday added an network to lower gain (lower input signal)
> at loer frequencies to compensate the gain-frequency behaviour of the 
> MRF455
> transistors (as stated in the MOT application notes). I have also lowered
> the input (driving power) to avoid saturation. But I still find a high SWR
> at the rig (input impedance change at the amp?) which lowers even more the
> rig's output power.
>
> So,
> why does the signal distort that much (parasitic oscillations?)? and
> when lowering input power and polarising properly the transistors, the 
> gain
> should be maintained, or no?
> saturation to means, increase of current consumption and reduction of
> impedance of the trafo (drastical reduction of Lmag); is this what is
> happening? Where, at the input trafo? or the output trafo?, if I lower 
> input
> power I should be prevent saturation to some extend, but I see the same
> behaviour.
>
> I would like some comments. In the menatime I will continuo to play around
> with the amp (change trafos) to see if soemthing becomes better.
>
> Waveformas are at http://www.uv.es/~esanchis/radio/
>
> Thanks for your help.
>
> Stefan
> EA5FY
>
>
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