[Elecraft] KPA500 - Rx Attenuation

[AG0N-3055]

On Sat, 09 Feb 2013 21:23:40 -0700, AG0N-3055 wrote:

> On Sat, 9 Feb 2013 19:51:09 -0800 (PST), John K3TN wrote:
> 
> > I thought 40M signals sounded kinda low, and the noise floor on the P3  was
> > down - but then noticed when I turned the KP!500 from OPER to STBY, signals
> > came about 15db. No fault indicated. TX power seems fine. Any thoughts?
> 
> I'm currently investigating a similar problem.  To speak to your issue, I have
> not seen it at all.  However, there is something similar happening on the
> frequencies below 160M.  

At this point, I cannot duplicate what you saw.  But...

I've just made a fairly extensive test run on all bands (except 60M)
because of my <160M problem mentioned above.  At first, I only checked
the AM broadcast band and lower, but then thought maybe I should check
all of the bands and did so, all the way to six meters.

Since I retired and lost access to the IFR to generate reliable signals
for testing, I used some local/regional AM broadcast signals for that
portion of the spectrum.  To check the frequencies under the BC band, I
used local/regional LF air-nav beacons.  The frequencies below the
broadcast band were where the problem was initially discovered,
specifically 60KHz WWVB.

To keep sensitivity of the meter fairly high, I kept the strength below
S-9 for all readings, except one or two more local broadcast stations. I
did it with either the attenuator, or tuning to the edge of a filter in
a couple of places.  I also have the XG3, so used it for all of the
later readings on all ham bands (again, I didn't check 60M).  When using
the XG3, I used 50uv signal with attenuator on for about an S-7 reading
on most bands.

Be aware that the S-meter calibration on the TS-590 is not 6db per
S-unit below S-9.  This is a personal aggravation, but most of the
manufacturers seem to have a similar problem.  

Generally, in all of the ham bands, I can see ONE BAR of S-meter drop on
the TS-590 when putting the KPA into OPERate VS STandBY.  That's not a
scientific measurement, just an observable change.  It is also pretty
insignificant, but it is very repeatable so does exist.  In some cases
there was no noticeable drop, so I slid off to the edge of the filter
and adjusted for one 10Hz step above the threshold of the S-meter bar
being lit and then put the amp in OPER position to see the change. There
was always a change, but sometimes I had to make that tweak to see it.

The spectrum below 160 meters is much more drastic.  Broadcast signals
suffer lots of signal loss, air-nav signals disappear except for locals,
and 60KHz WWVB is totally missing, along with all noise of any kind from
the antenna.  Switching the antenna in and out, had no effect on noise,
not even a pop is heard in the speaker when switching antennas or
disconnecting connectors when the amp is in OPER position.  In STBY, all
sounds normal.

Generally speaking, the broadcast band could be summed up by saying, the
lower in frequency you go, the more loss you are going to see when
switching the amp in and out.  It varies from an S-unit or two, to
several S-units at the bottom of the band.  Below that, it gets much
worse.  Only the strongest signals get through in the 250-350KHz range,
and just about nothing makes it below that.

One might ask why not just turn the amp off when not listening to the
ham bands.  Well, because I shouldn't have to, for one thing.  For
another, I don't always remember to.

These results are also being sent to support.  I don't know if there is
a pin-diode problem on mine, or if this is to be expected.  I don't like
it, but can obviously live with it if there's nothing to be fixed.

Gary
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