At 09:25 PM 7/9/2005, Mathew Quaife wrote:
>First I would like to thank Dave from Muncie, IN and his father for
>coming out and assisting with the beast of a nuisance.
Glad to do it, and glad to meet you!
It was an interesting situation. After all the theories, the facts
turned out to be thankfully simple.
I've just got home, after being on the road for the last 18 hours, so
I hope this is reasonably coherent :)
The maggiore exciter was not outputting a spur. It was more like a
comb generator. Bunches of them all over the place.
Sort of a "spread spectrum" approach, say from DC to about 350 MHz.
Matt didn't have any easy way to couple his test gear into the
system, so he couldn't see this, but I've left a line sampler with
him to make that relatively easy.
This behavior can be tuned out in the exciter, but it varies with
supply voltage, and we were able to find some tuning points that were
bad all the time, and some that were bad for only a few seconds after
keyup. This appears to be a situation where the change in loading on
the astron supply caused just enough voltage drift to take it in and
out of the problem area.
Matt's system is set up with diode transfer between the Astron and
the batteries, rather than having the battries float charged by the
power supply and sitting in parallel with the load. This probably
caused some of the comings and goings of the problem.
The exciter was also sensitive to the load impedance changes, but
tuning the final a bit seems to have minimized that.
We replaced a long coax attenuator with a short N connector 3dB pad,
which I just happened to find at the Indy Hamfest this AM.
That should provide a more flat resistive load than the coax would,
plus it's less bulky.
Thinking on this on the way home, I think between cans 3,4, and 5 in
the exciter, there is a problem.
It may be a bad solder joint, and it may or may not be visibly bad.
I'd be very tempted to replace everything in that area with new
parts, except probably the inductors.
While it would be interesting to know the exact fault, it's not
really necessary, and the parts are likely only a couple dollars.
I'd put my money on a failing transistor first, but then it becomes a
random guess really, at shifted resistance values, cracked ceramic
caps, or who knows what.
> It seems there is some very critical coils inside the exciter,
> that has very little
>tolerance. A slight movement and the thing goes whacko.
About 1/16th of a turn, from a couple of spurs at <-60dBc to
WAHoooOO! and back to quiet, and that point seems to move around as
the loading on the power supply changes.
Check the exciter schematic, does it internally regulate the
voltages? If not, that might be a very useful mod.
The Daniels modules use a regulated 9.5V to feed all the modules, and
they internally regulate to 6V in each module, so that there's no
passing of noise between modules on the 9.5V bus.
> I'm glad it was the exciter and not a mix, a much easier fix.
Indeed, sometimes these things are pretty nasty.
It's not dead yet, but Matt knows where it lives now, and I think
he's got a good handle on getting rid of it permanently.
FWIW, the system was very nicely done. Well organized, and good
craftsmanship all around.
I'll cut out a piece of that foam for you tomorrow, and mail monday.
I'm sending a chunk of MosFoam, a very effective RF absorber that I
use in boxes that might have unwanted RF flying around.
Enclosed in a ziplock bag, it's completely safe for the system, and
it can help a great deal in cutting down RF in tight enclosures.
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