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[Amps] GS35b FWB issue

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Subject: [Amps] GS35b FWB issue
From: "Jim Thomson" <>
Date: Wed, 13 Jul 2011 01:31:54 -0700
List-post: <">>
Date: Mon, 11 Jul 2011 00:10:05 -0400
From: "David H Craig" <>
Subject: [Amps] GS35b  FWB issue

    I lost a diode string on an 8x4 string 1N5408 FWB on my single GS35b 6m HB 
amp.  To whit, 8 5408s per leg of a FWB for a total of 32 diodes.  Presumably a 
lightening strike / blown lightening arrestor, but now I wonder.  
    I replaced the diodes on the blown string & tested again- worked fine with 
just HV trans running to the bridge.  

##  OK, maybe use a small variac, [fused of course...]  and  fed directly to 
the pri of the plate xfmr [ all step start bypassed] .  

My bet is now on  a short /arc  from one end of xfmr to casing or ct to casing 
or part way along the sec winding to casing. 
The thing is.. he  has a 250 ohm-50 w  resistor between B- and chassis. 

##  Plan B.    temp replace  xfmr with  something else... heck, and old tv xfmr 
will do.... something with say 100-400 vac sec. 

##  remember, this hv supply worked for a long time. 

## This is prob not recomended... but he could insulate the entire xfmr casing 
from the chassis, via  a sheet of plywood,
and a thin layer of some type of insulator/mylar, just abt anything else..  

##  On the broadcast TX forums, this has been done b4... with plate xfmr 
sitting on plywood.   Won't arc to casing if casing is no longer
at chassis  potential.   On any supply that uses oil caps,   I will always 
insulate the oil caps from chassis  [ and each other] with a sheet
of micarta .   Then u take all the stress off of them. 

Later...... Jim  VE7RF
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