On Thu, 2006-12-07 at 14:01 -0500, Craig Roberts wrote:
> Dr.Johnson makes a good point about the possibility of stray RF tripping
> the magnetic circuit breaker in the power supply. I'll try a couple of
> shielding and bypass techniques.
Look where the power wires run inside the radio. If Tentec paid the same
attention to that as in my 2m to 6m transverter (no care at all) the
wires could be close to the PA output transformer or a low pass filter
coil and RF could be induced there.
>
> In answer to Don Kemp's question: the problem occurs even when the SWR
> is flat (on my lab grade dummy load) with a short coax run straight to
> the load. At any rate, my antennas are resonant to within a 1.7:1 SWR
> at worst (double bazookas). Since no changes were made in the station
> setup, the sudden appearance of the problem leads me to suspect a
> component failure upon powering up. I'm hoping it's in the power supply
> -- a Model 225. Electrolytics are always a reasonable suspect.
Electrolytics shouldn't be involved in RF filtering. Aluminum
electrolytics tend to be rolled up and to have lots of inductance. Solid
tantalums are much better but not commonly available in the values
needed for a 10 or 20 amp power supply filter.
>
> Since I'm primarily a tube guy, I have a question about solid state
> finals. Do they ever "go soft" or invariably fail catastrophically?
> Ten Tec suspects the finals may be bad, I can get full output from them
> if I increase the circuit breaker trip point (but, of course, I don't
> wish to operate the rig in this manner). The circuit breaker is
> triggered by excessive current draw (10A), but I don't understand how
> the finals themselves could play a role in that. Would someone please
> explain?
Lots of power RF transistors are an integrated circuits made up of many
small transistors hooked in parallel. Often with many emitter and base
connections (because its the periphery that does the control) and one
common collector diffusion underneath. So it is possible to have a
gradual degradation as base emitter junctions go shorted and then blow
open. To force current sharing there are individual emitter resistors
for each emitter and the shorted junction blows away the associated
emitter resistor. Then to make power the remaining junctions run at
higher currents so distort more. Greater intermod or harmonic distortion
can be a hint of PA transistor degradation.
>
> 73,
>
> Craig
> W3CRR
>
--
73, Jerry, K0CQ,
All content copyright Dr. Gerald N. Johnson, electrical engineer
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