..
In a message dated 10/17/01 6:48:59 AM Pacific Daylight Time,
b_bradfield@yahoo.com writes:<<
Hi Tom - -
Kilowatt Matchboxes show up at hamfests around here (Dallas/Ft
Worth/North Texas) in the $200-300 range typically, with the low power
tuner in the $90-125 range. As someone else said, the meter doesn't
seem to add an awful lot to the price. If you want to remove all those
darn screws to look inside it, check to see if all of the stator plates
of the variable capacitors are still there. Some were occasionally
removed to make for easier tuning on the higher bands. This is ok, but
may leave you a bit shy of capacitance on the lower bands.
Also, L.B. Cebik has a lot of excellent information on link coupled
tuners, including the Johnson Matchboxes on his web page at
http://www.cebik.com/link.html and http://www.cebik.com/link0.html.
73,
Brad, W5CGH
>>
Brad: I would take any MB apart totally and check each contact as some have
run big power into them and formed arc paths across the switch contacts. It
make it possible to start fresh on all the contacts. It's been over 50 years
since they were made. I take the front case off and install a clip or swtich
arrangement so I can change the link coax tap from 2 to 3, 4 or 5 turns or an
occasional odd load. This really extends the MB's range and allows
maximizations of output. There is not one load I have not been able to match
with it so far. In one case I had to reverse the connections of the
differential variable. The MB and a One Knob Balanced L circuit I have are
my most used and most efficient (no heating) balanced tuners I have used for
over 50 years on just about every antenna there is. I still try other
circuits but have not found a better one as yet. The MB is a "Best Buy." I
do use resonant lengths of open wire line to reduced additional reactances
being created by the feedline. QST insists on recommending "Open Wire of ANY
length." That literally asks for a load hard to match. I told them many
times but they keep recommending random lengths in the last 30-40 years where
before that they they and other Handbooks like Frank Jones Radio Handbook of
the 30's had charts of "Specific Lengths" for particular antennas and gave
the recommended tuner configuration.
I also add another SO-239 connector for balanced 100 ohm coax (150 ohm
balanced also) and it matches long wires. I add RF ammeters or shunted light
bulbs in the LW or in open wire lines to check for balance and maximize the
output. When I use balanced coax I add 2 matched SWR bridges for the initial
tune up of the 100 ohm load.
I added a "plug--in inductances" on the ends of the tank coil for 160M at
right angle to the main tank circuit to eliminate any "shorted turn coupling"
to it when I short it out when not in use. Fuse clips are used to secure a
50 or 100 uufd vacuum for 160M.
With an open box I don't have any RF in the shack problem although it's nice
to have it closed. On my 6M MB with a 100 ohm balanced coax load I have no
TVI on 6m with 100W. I use a balanced 200 ohm link input on 6m. And yes I
have a 2m MB also.
The MB doesn't need a series Xc in the link for conventional loads as do the
conventional Parallel & Series configurations. QST screwed up on that. I
have home made universal versions of the MB with plug-in 1-KW, 500, 150 & 75W
coils or a back to back rotary coils with a mechanical differential inbetween
them so they turn in opposite directions from the center. I added adjustable
dual links for coupling to the rotarys. I have on occasion used the series
Xc in the link in this version. Calibrated tuning knobs make it easy to
quickly set for all the bands. I added another tank variable of about 20
uufd each side for easier tuning on 15&10M.
The MB will work on 30,17&12M but may not be the optimum setting for max
output. In the 275&1KW version I have installed band switches with 5 more
sets of contacts on each side and added taps for 160,80,30,18&12m.
The good news is a Mfg is interested in making it again as soon as I can
send him my prototype. It's a great buy now and will be when available
again. I will provide typical antennas and feedlines in the instructions
manual for those who will need the help. There are very simple ground rules
of Basic Fundamentals for use..
W6AM used a 1-KW MB just in the receive circuit. I would think that in high
RF levels like in LA, the additional selectivity of the MB would reduce
fundamental overload to our so called "untuned mixer front end receivers."
At the next Flee Market make arrangements to get in early and look for the
275 or 1KW MB. The 275W AM rating MB will handle a balanced 100 ohm coaxial
load when driven by a Collins 30L1 600W linear on SSB. It will take more SSB
on the lower frequencies.
The MB is one of the most useful tuners there is despite its Critics and
only has 2 knobs to turn. "A Critic is one who thinks he knows the way but
can't even drive the car." (Old K7GCO Axiom)
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