On Thu, 2006-07-13 at 11:57 -0400, ROBERT CARROLL wrote:
> I had recently questioned whether the Omni or similar rigs had a design
> problem if they needed to be protected by an Airpax breaker. One of the
> Airpax advocates responded that the issue was not oscillation but the
> ability of the rigs to put out full power into a mismatch causing a quick
> thermal failure due to inadequate heat-sinking. To me that sounded like a
> questionable explanation, but again a misdesign problem if true. But these
> failures do indeed sound like oscillation problems. Strange indeed. My
> condolences. It is a painful experience to have to ship a rig out for
> repair.
> 73
> Bob W2WG
Its not oscillation, its merely the effects of off impedance loads on
the PA. And the PA is protected by the power supply, either current
limit or circuit breaker.Its a design CHOICE, not a design fault. I've
been protecting my Corsair II with fuses and I have blown fuses without
destroying the PA transistors. My secret is to use 15 amp fast blow
(ATC) fuses so that they are running 125% of rated current at the 20
amps the PA draws when matched. Since the ordinary fuse is rated to
carry 125% indefinitely this intentional overload makes the fuse
sensitive to a bit higher current, significantly more sensitive than a
20 amp fuse would be. And the 15 amp fuse blows rapidly enough to
protect the Corsair II PA transistors. A 20 amp fuse would likely not be
fast or sensitive enough.
Fact is the PA output Z is much lower than 50 ohms and so acts more like
a voltage source with the output power depending on the load impedance
presented to the PA transistor. And because the output LP filter can act
as an impedance transformer, its not sure that a low impedance on the
connector is a low impedance at the transistor collectors.
I've had two TS-120 and a TS-130 where the RF protection was very
effective. Many times, I've laid a brick on the key with full drive and
tuned the antenna tuner manually for maximum power from the radio. Over
a decade of doing that with three radios, I didn't loose a PA transistor
so that design technique was also successful. The Corsair will blow the
protective fuse if I do that to it. But it hasn't lost any transistors.
--
73, Jerry, K0CQ,
All content copyright Dr. Gerald N. Johnson, electrical engineer
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