As Stuart writes, many used JVM matchboxes have coils with melted air core
coils.
Matchbox buyers beware.
I am unaware of any good modification papers written in ham journals,
however I am aware of where you can find EXACT instructions for improving
the JVM.... on my web site:
See:
http://www.dj0ip.de/antenna-matchboxes/symmetrical-matchboxes/j-viking-upgra
de/
73 - Rick, DJ0IP
(Nr. Frankfurt, Germany)
-----Original Message-----
From: TenTec [mailto:tentec-bounces@contesting.com] On Behalf Of Stuart
Rohre
Sent: Tuesday, July 19, 2016 9:07 PM
To: tentec@contesting.com
Subject: Re: [TenTec] OT: Question to the group- re Match Boxes (Johnson)
For those newer hams, a word about power rating of the Johnson Match
Boxes: These were designed in the days of power input ratings, and AM phone
operation where you had carrier power plus modulation power.
Thus the "275 watts" box probably would on low duty cycle, easily handle
"kilowatt" SSB rigs today with a reasonable duty cycle to keep heat from
building up. One could also install a fan to move air over the coils.
The main concern is the plastic used for coil insulators, which may also
decay, just sitting around for decades. In shopping for Match boxes, take
close looks at the coil mountings, and be prepared to eventually replace the
plastic insulating strips with new ones.
The trouble with the Johnson design is some coil taps are fixed, not
adjustable, and thus some of the modern bands may not tune within efficient
range of the fixed taps.
The fixed loop coupling is another issue, and adding a variable cap in
series with the coupling loop if often the route to handle that. Good
modification papers have been written in the ham journals and noted earlier.
-Stuart Rohre
K5KVH
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