[Milsurplus] TCS PSU question

[WA5CAB at cs.com]

C307 and R301 are across one set of contacts on relay K301, the transmitter 
power relay.  Their purpose is to help extinguish the relay contact arc if you 
happen to turn off the transmitter with it keyed.  They are not connected to 
the neutral side of the AC line.

C305A and C305B are technically not in series, despite how they are drawn on 
the schematic.  They are connected from each side of the AC line to chassis.  
One is actually redundant but given that the AC input connector is 
non-polarized 2-pole, it could be either one.  Remember that the supplies were intended 
to be used while bolted to a steel shelf welded to a steel bulkhead in a steel 
hull ship and don't anyone start a long thread on 2-pole versus three pole 
safety.

Selenium rectifiers date from the late 30's.  However, it was not known in 
the 30's, 40's and 50's that they have a 20 to 40 year shelf/service life.  
Around 1960, MWO's began appearing for their replacement in military equipment by 
silicon devices, sometimes, depending upon the application, with a small fixed 
resistor in series.  Some early specific to equipment replacements actually 
had the resistor built into the housing of the rectifier.  Civilian equipment 
using them mostly just got scrapped and replaced by new higher cost units.

In a message dated 2/21/2009 1:31:41 AM Central Standard Time, 
mcenfalz at humboldt1.com writes: 
> I am assisting a fellow TCS-er in the debugging of his '40's-vintage AC 
> powered PSU: 409M-5 The beast w/the 2 lifting handles and all the "ironwork" on 
> the upper deck. This thing has a pair of small 0.25mfd bathtubs w/matching 
> series resistors mounted on the central bulkhead. One (C307) got warm (leaky) 
> along w/its resistor (R301) during an initial runup w/ a variac set at 45VAC: 
> and easy replacement.
>   
>  My question: I looked these up on the schematic in the "second echelon" 
> manual, and they appear to be in series on the hot and neutral sides of the AC 
> input.  Why was this done? C305A and B also in series on the neutral/ground 
> side. Why?
>   
>  Jim Falls
>  KG6FWT
>  Eureka, CA
>   
>  PS: I now know how a full wave bridge selenium rectifier (CR301)smells as 
> it dies. This is the one that powers the relay (K301) in the TX portion of the 
> PSU. I thought these were '50 inventions. When did they start turning up? 
> It's referenced in the manual, so it's stock (12V/300mA).
>  
> 

Robert & Susan Downs - Houston
wa5cab dot com (Web Store)
MVPA 9480