[TenTec] TT Omni VI final overcurrent protection? Parasitic Occilation Protection?

[Dr. Gerald N. Johnson]

On Tue, 2007-01-09 at 09:08 -0800, Denton wrote:
> I have posted this thread before...but thought would try again and see if I 
> can come up with any new ideas or info..
> Currently, I have a Paragon II and an Omni VI Plus upgraded to option 3 
> running off separate TT 961 power supplies and all is working very well.
> 
> I want to revamp the power supply setup to a home brew system consisting of 
> 35 amp power supply, blocking diode and battery pack consisting of 8, fused, 
> 38 amp hour 12 vdc gel cell batteries. This will feed, via a MFJ power 
> strip, the two hf rigs, two vhf/uhf rigs and assorted items in the shack 
> that use 12 vdc. I will be using clamp on chokes on all dc leads.

Your choice of battery is odd, strange, and poorly selected. Gel cell
batteries are limited in charging and discharging current and I suspect
your power supply will charge them too fast for their own safety. That
and the blocking diode will tend to wreck the voltage regulation the
power supply had.
> 
> I will also be using Hopkins 48-440 Hz 20 amp rfi filters in-line with the 
> dc input to the hf rigs. Although these are designed for tesla coil use, I 
> think they have sufficient filtering for dc use. I will also have the 
> recommended Airpax circuit breakers to each TT hf rig in the dc plus leads.

They would only be completely effective if in the back wall of the radio
or the cable between the filter is thoroughly shielded between the
filter and the radio. But these filters will introduce a bit more
voltage drop than you will like at 12 volts. And not be all that
effective at keeping out RF.

If you really want to keep RF out of the radio from the power wires you
need a feed through capacitor made for the purpose. Sprague used to make
such capacitors with 20 amp current ratings.

And if you are getting that much RF to get feedback at a VHF parasitic
frequency through the radio's internal low pass filter and from coupling
from the antenna, it may be time to move the ham shack or antenna to
increase their separation.
> 
> I have had problems in the past using a common power supply battery setup to 
> feed the Omni...apparently parasitic oscillations cause the drivers and 
> finals to draw overcurrent faster than the Airpax can react, and I have had 
> to send the Omni back to TT 3 times for repair to have the finals and/or 
> driver transistors replaced...not much fun!

Are you absolutely certain its not a coax problem, an intermittently
shorted coax connector from a stray braid strand or an open from not
being properly soldered?
> 
> I have had numerous rigs utilizing a common power supply and battery 
> setup....only the Omni has given me problems.
> 
> TT says it due to lack of electronic current limiting and parasitic 
> oscillation taking out the finals, despite the omni feeding a flat load. I 
> think it may be due to RF getting into the finals via the dc lead...

A tuned load is only FLAT at the frequency its tuned to. Its not flat at
harmonics or parasitic frequencies. Only a dummy load is flat over most
frequencies but it tends to be a poor antenna for either receiving or
transmitting.

If its truly a PA or driver parasitic oscillation there may be a need to
move DC wires away from those boards inside the radio, and to be sure
those boards are properly grounded at all points and that the RC
parasitic suppressors that are surely part of their circuits ( often
series RC from collector to base on each RF power transistor) are all
connected properly and that the resistors haven't drifted high in value.

If the parasitics are at Low frequency, it might be that the added
filter raises the impedance of the in line filter and the longer wires.
Perhaps a large tantalum at the radio power connector in parallel with a
0.1 disc ceramic is in order, though that Sprague power feed through
would supply a large chunk of capacitance with less inductance than the
disc ceramic.

How come we don't hear of other hams using whole shack power supplies
moaning about loss of PA transistors?
> 
> I could just leave the Omni on it's own power supply and feed the rest of 
> the station with the home brew power supply and batteries, but that kinda 
> defeats what I want to accomplish.

And are you sure the home brew power supply isn't sensitive to RF and
raising its output voltage to fry the RF power transistors?
> 
> Any ideas or comments?
> 

-- 
73, Jerry, K0CQ,
All content copyright Dr. Gerald N. Johnson, electrical engineer