[VHFcontesting] The 432 Report

[John Santillo]

Well I bet all of you are just sitting at the edge of your seat waiting for
another long drawn out email from me, especially Howie.  I know he reads
everyone one of them.

 

Abstract:   I have made good progress on the 432 station.  Now you can
either stop here or continue reading.

 

The FT-847:  The Yaesu FT-847 has been modified and transverter ports
installed.  I disabled the adjustable RF power out feature so no mistakes
can happen.  The Tx transverter port is fixed at a constant +6dBm.  This is
more than adequate to drive the transverter to full output power if needed.
I also had to disable the HF Power Amplifier.  It was generating too much
28MHz RF noise when the radio was keyed.  For FD I'm going to have to enable
everything I disabled.  

 

The 432 Transverter:  This transverter is a beautiful piece of hardware.
The oscillator accuracy is within 10Hz and the noise figure is less than
0.5dB.  It's sensitivity is better than 2M at -144dBm which is great since
the 432 background noise is exceptionally low.  The transverter has a
built-in 4-way sequencer too.  I had to modify the sequencer to put out
12Vdc to enable the 432 SSPA.  I set the drive level of the transverter to
5W max.  This is the power needed to drive the SSPA.  The drive is easily
adjustable from 0 to 25W to drive the old 220W 432 box in the event of a 432
SSPA failure.

 

Now the part you've all been waiting for.

 

The 432 SSPA:  First off this amp has been a royal PIA.  I am not able to
get the full 1000 watts I expected but I am getting closer and I believe I
understand why I'm not getting a KW.   All along the power was low by ~400W.
I suspected that one of the MOSFET's was defective (which still might be the
case) but now I'm thinking after experimentation, that the two MOSFET's are
not closely matched.  The Gate bias (Vg) to the MOSFET's are tied together
via a common bias circuit so there in no independent adjustment.  But once I
modified the circuit to increase Vg to one and not the other I saw an
improvement.  If the MOSFET's aren't balanced then the 50 ohm termination on
the Branchline Hybrid coupler burns up (which it did on me twice).  The
other issue I found is that the input chip caps fracture due to heat and I
had to replace them with disc ceramic.  Anyway, right now I'm getting 800W.
For me to achieve the 1KW I'm going to have to redesign the bias circuit for
independent adjustment.  At this time, I'm going to wait until after the
June VHF contest to do this.  This way we can "burn-in" the SSPA.  If the
one MOSFET fails then I know it was defective.  If not then I know to
redesign the bias circuit.  In the meantime I will make every attempt to get
the heat away from the 50 ohm termination.

 

Now the integration starts.  Nick what's the status of the remote box?  Does
anyone have 30 to 40 feet of four conductor shielded wire?

 

 

73,

 

John

N2HMM