On Wed, 2007-01-10 at 08:41 -0800, Denton wrote:
> Hi Jerry, thanks for the most informative reply
> ----- Original Message -----
> From: "Dr. Gerald N. Johnson" <geraldj@storm.weather.net>
> To: <tentec@contesting.com>
> Sent: Tuesday, January 09, 2007 6:15 PM
> Subject: Re: [TenTec] TT Omni VI final overcurrent protection?Parasitic
> Oscillation Protection?
>
>
> > 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.
>
> The batteries are actually AGM units.....mine are Powersafe Hawker Series
> SBS 40's and particulars are below:
> http://www.enersysreservepower.com/documents/US-SBS-RS-003_1106.pdf
> If these are not applicable, then what is?
So they are NOT gel cells. Sealed, with electrolyte in fiberglass mats,
are not gel cells. Their electrolyte is more mobile than in a gel, and
so they appear to be able to handle larger currents. At least that
catalog page shows no limits other than the inherent resistance when it
shows a short circuit current rating of 2 KA. But that catalog sheet
doesn't say they are suitable for starting service and rapid charge
either. Look for applications notes that in the fine print limit load
and charging currents.
>
> I fail to comprehend how the blocking diode will knock the power supply out
> of regulation.
The regulation after the diode will be poorer because the diode drop
depends on the load current. Diode drop is only 0.600 volts at low
current. Most power diodes drop a volt or more at rated current. You
could reduce that drop with power schottky diodes but you still won't
keep it from varying with the current.
> >>
> >> 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.
>
> I agree on the filters
>
> >
> > 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?
>
> Coax is not the problem nor are the antennas....I use two ladderline fed
> antennas with true link coupled balanced transmatches in the shack and
> extensive grounding. Feeding each hf rig with Ten Tec 961 power supplies
> currently allows me to have both rigs going at the same time, each rig
> transparent to the other, as long as each hf rig is on different bands (i.
> e. Omni VI on 20 meter psk and Paragon II on 80 meter ssb).
> >>
> >> 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.
>
> The Omni is my only hf rig having problems with common power supply....mine
> is a Omni VI Plus upgraded to option 3....one of the earlier ones....and my
> online research indicates that they are prone to final overload...unless
> they are used with a 961 power supply.
So maybe you want the current limit circuitry from the 961 between your
batteries and the picky radio.
>
> >
> > 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
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