[Amps] GS35b FWB issue

[Jim Thomson]

Date: Mon, 11 Jul 2011 00:10:05 -0400
From: "David H Craig" <davidhcraig at verizon.net>
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