[Amps] Alpha 87a High SWR between Exciter and Amp vs Bad Bandswitch

[JOS Earthlink]

Hi Mike,

When I said the 87a "autotunes" what I meant was I would set the band
setting on the 87a to lets say 10m or 15m and then I would xmit on 80m and
the 87a would "autotune" = auto-bandswitch back to 80m.  At this point, if I
do it a few times, there still is an intermittent high SWR > 5 and the
TS-950 folds back power to the 87a.  I did this to prove to myself that the
intermittent problem was in the 87a and not the TS-950 bandswitching, etc.
The latest commenta from Tom says RF on the shield should not be a problem
unless there is a break in the shielding somewhre but I don't buy this as
the drop in my SWR was :

1) dramatic across all bands -  now 1.0-1.1
2) independent of the coax jumpers I used
3) same with a different dummy load (both relatively new items)
4) verified by interposing my Powermaster meter between the TS-950 and 87a
to verify the improvement in SWR

I don't have an RF current meter but the line isolator seemed to make a big
difference.  I even tried it with with and wtihout a different T4 isolator
and same results.

Any thoughts on the 87a intermittent bandswitch problem?


73 Jerry
K1JOS
CCA #11906


-----Original Message-----
From: Mike Schatzberg [mailto:cherokeehillfarm at earthlink.net]
Sent: Tuesday, January 02, 2007 11:21 PM
To: JOS Earthlink
Subject: Re: Alpha 87a High SWR between Exciter and Amp vs Bad
Bandswitch


Hello Jerry:

Interesting results.  Yes, if there is RF floating on the shield, you can
get faulty readings.  Since you have the line isolator already, you might
want to take your solution length coax, approx. 20 feet, and place the line
isolator at the input to the amp with a short jumper, or right angle with a
double male adapter, and see what happens also.  You could also try the line
isolator at the other end of the solution length coax at the transceiver.  I
think it should be better to trap off the RF right at the linear, but see
what your experiment determines.

I assume that you are under no circumstances using an internal tuner on the
transceiver while operating on 80.  I don't see how it could be the
autotuner circuit because the autotuner does not adjust the input circuit.
You might start by merely using an appropriate spray cleaner on the contacts
and rotate the wafer switch thru its rotation to wipe them clean.  Also, you
could reheat the connections associated with the switching to make sure that
there is no cold solder joints in the 80 meter input.  I assume the input
wafer is ganged along with the rest of the tank switching, and if the tank
has switched bands, then the input has switched also.

Whatever solution you finally choose, you have certainly learned quite a bit
about driving an amp with an exciter, and it was worth the time and energy
to do the experiment.  I would be interested to learn about what happens
when you insert that line isolator.  Certain antenna installations are very
prone to RF pickup on the shield, which carry the RF back into the shack.
It often manifests itself as distortion on the audio, or RF burns in severe
cases.

You have now a way to avoid power foldback, and you have eliminated the need
for the internal antenna tuner, and the additional problems that it can
introduce.

I'll look forward to hear how the line isolator works with your solution
length as compared to being in the middle of the 6 foot lengths of coax.
Also, you might want to just insert the line isolator into your original set
up, to see what happens with the original jumper length.

Nice going.

73,

Mike
W2AJI