[Henry] Classic 3K X Tuned input

[W5AMI]

I have the above amp with the single 8877 and have some issues with my
tuned input on 75 meters.

I've had high swr on the input for some time, so decided to try and change
the mica caps on the pi network.  Tuning the slug would not get me close at
all.  As it was, it seemed fine way down in the bottom portion of the
band.  After pulling the RF deck out, I saw that it had three 470 pf caps
in parallel on each side of the coil, input and output.  Looking at my
manual, it calls for three 470's on the input and on the output, two 820's
and one 620.  Since I used the book before looking in the amp, I ordered
what the book called for.  Either a previous owner changed these, or now I
know I can't trust the manual...

It would seem to me that if I need to raise the resonant point of the input
circuit, I would need less capacitance on each side of the choke than
either configuration would offer.

I decided to measure the coil and adjusting the slug will give me a minimum
of 2.23 uH and a maximum of 3.16.  Using a Q of about 3.1, 50 ohms input
(from rig), and 54 ohms (8877) load, I come up with 1180 pF on the input
and 1220 pF on the output of the Pi.  That's with the slug set about midway
between min and max inductance, about 2.7 uH, and a freq of 3.935Mhz.  I
order the proper combination of micas, install them, and still I'm way
off!  Drive power also varies the reflected power considerably too.  At 3.7
Mhz with about 10 watts of drive I see about 1.4:1, but when I increase to
about 40 watts, it goes well above 3.0:1.  Going higher in frequency makes
it even worse.

I have this project all put aside, but want to get it going again.  I did
not try other bands since I don't run much above 75 meters, but need to do
that to see if this is a common issue with all bands.  Also, I need to
inspect the bandswitches to make certain they are all in sync with the
chain drive.  I guess it's possible that the input switch may be off one
link or something?

I have used this amp on 75 meters with the 3 470's on each side of the
input coil for some time, but never saw anything below 2.0:1 around 3.935
mHz.

One last question; what is the safest way to adjust the slug on these coils
without getting killed??  Since this Henry does not have access holes on
the outside, you have to slide the rf deck out to gain access to them.  Can
I simply remove the HV, key the amp/relays and get an accurate indication
as I adjust the slugs?  I don't want to be anywhere, or poking around
anything with 4KV in the area!

I am at a loss where to go from here, and starting to feel pretty stupid!

Thanks in advance!
Brian / w5ami




-- 
Brian Sherrod - A.R.S. W5AMI
My website: "Arkansas Airwaves <http://arkansasairwaves.net>"
Committee - QWHA <http://menahamfest.org> (Queen Wilhelmina Hamfest
Assoctiation)
Arkansas Assistant Section Manager
ARRL Arkansas Official Observer Coordinator (OOC)
Section Website: ARRL Arkansas <http://ararrl.org>
Quarter Century Wireless Association (QCWA) #35737