Yesterday, I cleaned up the input relay wiring on my SB-220. It has been
modified to use a small Hi-G d'Italia vacuum relay for the input, and a
Kilovac HC-1 for the output. Initially, when I did this modification, I
wasn't too careful with the cabling -- two of the three cables connecting
the input relay to the original RF input cables had their shields grounded
only through the relay case, for example. I left the original RF cabling
exactly as originally built, and connected it to the input relay with short
miniature teflon coax (no more than 1" long in any case). The input
pi-networks are stock, and have not been readjusted or modified at all.
The modification worked fine through several years' contest operating. I
did notice, however, that with certain lengths of cable between the
transceiver and the amplifier, the amplifier seemed to deliver MORE power
than it should, and that the input SWR on that band (40m) was relatively
high (just under 2:1). With another length of input cable (and another
transceiver), the problem shifted to 20 meters, where it showed almost 1500
watts output into a dummy load.
The "cleanup" consisted of making sure the shields of the miniature teflon
coax leads to the input relay were grounded at both ends and, in the case
of the connection bypassing the amplifier, replacing a length of hookup
wire (no shield at all) with a length of RG-174 with the shield grounded at
both ends.
To my surprise, though, there was no improvement. These are the results of
some tests I just ran into a dummy load. In all cases, the transceiver was
delivering no more than 110 watts drive (as measured on its own internal
metering). Output power measured by an RF Applications VFD.
3501 -- 1192 watts, 700ma IP, 150 ma IG, input SWR 1.25:1
7001 w/o xcvr tuner -- 780 watts, 530 ma IP, 100 ma IG, input SWR 2.2:1
(transceiver folding back drive to ~ 50w)
7001 w/tuner -- 1150 watts, 650ma IP, 185 ma IG, input SWR 1:1
14001 -- 1505 watts, 1 amp (!) IP, 200 ma IG, input SWR 1.9:1
21001 -- 1009 watts, 720 ma IP, 135 ma IG, input SWR 1.4:1
28001 -- 665 watts, 620 ma IP, 170 ma IG, input SWR 1.35:1
It seems to me that I have three options:
-- I can redo the input relay interconnection cabling yet again, on the
theory that I must not have done something right when I "cleaned it up."
-- I can redo all the input RF cabling to make the interconnection as clean
as possible. This would probably involve using Teflon 50-ohm miniature
cable to replace the original RG-58, routing it as directly as possible to
and from the relay contacts.
-- I can live with it, using the transceiver tuner on 40 meters and backing
off the drive on 20m to avoid excessive dissipation.
I'm reluctant, though, to take any of these courses of action without some
idea what is going on. I'd greatly appreciate any advice anyone might
have. If it would be helpful, I can post a close-up digital photo of the
relay arrangement on my web site.
73, Pete N4ZR
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