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Re: [Amps] 30S-1 screen-boost question

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Subject: Re: [Amps] 30S-1 screen-boost question
From: <>
Date: Wed, 27 May 2020 22:15:56 +0200
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I don´t understand the "boost" question on the screen voltage.
The 4CX1500B should be run at 225V screen voltage as AB1 RF linear.
The voltage for the 4CX1000A is higher already.


-----Original Message-----
From: Amps [] On Behalf Of Joe Giacobello,
K2XX via Amps

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 
expert information.

73, Joe
> <>
> 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
> CW.
> 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
> achieve?
> Thanks in advance!
> -Gord Pritchard (VA7GP)

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