[Amps] 30S-1 screen-boost question
Joe Giacobello, K2XX
k2xx at swva.net
Wed May 27 14:17:37 EDT 2020
Gord, you're really testing my alleged memory. I did, in fact, make
this mod, but I sold my 30S-1 with the mod intact about 15 years ago.
It's very easy to do and there should be plenty of references to it on
the web and among the Collins aficionados.
I believe the mod involved connecting a 1:1 (115VAC in to 115 VAC out)
isolation transformer across the primary and secondary windings of the
main power transformer that were used to provide the screen voltage to
the corresponding rectifiers and filters. These added windings have to
be done in such a manner that the added windings "boost" and don't
"buck." One also had to replace the screen supply filter capacitor to
accommodate the increased voltage. I used the original 4CX-1000A and
was able to get 1.5 KW out, although the power transfomer routinely
groaned a lot.
That's my recollection of the process, but it's all very vague in my
mind, and I would verify all the details.
I'm surprised that there are no Collins forums on the web that you can
consult. Accordingly, I've copied the Amps mailing list on this reply,
and I'll wager that someone there can unhesitatingly provide you with
> gordon.pritchard at gmail.com <mailto:gordon.pritchard at gmail.com>
> Tuesday, May 26, 2020 6:16 PM
> Hi, gentlemen:
> I Googled and found some email-archives from a decade
> ago, where you both were emailing about 30S-1 screen boost mods.
> I hope you're both (still) doing well...
> I overhauled a new-to-me 30S-1 which was far-from-
> working. I changed the tube to a 4CX1500B, and I added a boost-
> transformer. But I'm not really happy with how it works, so I'd
> like to ask what you did, how you connected this screen-boost,
> and how it works for you?
> I opted to use a 100VA transformer, with 120/240 dual
> primary windings, and a 120V secondary winding. I connected the
> dual-primaries exactly to the main transformer T201 on the 'SSB'
> connections. The idea was:
> -I wouldn't get any screen-boost until after warm-up and then
> only when HV enabled.
> -by using the SSB primary-winding taps on T201, I anticipated
> that my boost would be ~120VDC on SSB, then reduced to ~65VDC for
> During no-load DC start-up testing, my anticipated CW
> boost was only ~40VDC; lower than expected. I did not worry
> about this.
> During full-load operation now, I see significant sag to
> my boost :-( With a common 500mA plate-meter reading for both
> CW, and single-tone SSB (the only changes are plate-voltage and
> bias-voltage), my total screen (T201 + boost):
> SSB total screen = 320VDC, down from idle-only (250mA) 370VDC
> CW total screen = 220VDC, down from no-load/no-Ip 271VDC
> My questions for both of you:
> Where did you wire-in your boost-transformers? Is it the
> same - and how much is it - between SSB and CW?
> What sort of Power In / Power Out are you able to
> Thanks in advance!
> -Gord Pritchard (VA7GP)
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