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Re: [Amps] GS35b FWB issue

To: "Jim Thomson" <>, <>
Subject: Re: [Amps] GS35b FWB issue
From: "David H Craig" <>
Reply-to: David H Craig <>
Date: Wed, 13 Jul 2011 08:06:46 -0400
List-post: <">>
    Thanks for the input all.  Still a bit baffled- as Ian wrote my issue 
HAS to be in the PS side only- it goes into failure mode with NOTHING at all 
connected to the tank (this is a single chassis unit BTW) & bypassing the 
glitch resistor, HV meter resistor string, and, just for kicks, once 
removing the B- lead to the plate current meter.  It is drawing current (the 
ampmeter shows it before the breaker trips if connected to the B- lead on 
the FWB on powerup) which means something, somewhere, shorted to ground in 
the DC supply.  Maybe this is wrong, but it appears that a short HAD to be 
in the capacitor bank somewhere- I can power up the transformer without any 
load, no problem.  Add the repaired FWB (unconnected to any load) and no 
problem.  Add the cap bank & viola, shorting noise (sometimes a rattling 
high current relay straining from the overdraw of current) followed by a 
blowing breaker (or me manually hitting it off to save the "next" component 
failue in the FW bridge.)  I have not powered it up again, and won't til I 
find the issue.
    I fixed the FWB, again, but with the assumption that a transformer 
insulation breakdown under load wouldn't hurt the diode string, just blow 
the breaker, I pulled the cap bank apart.  I hipotted each 800mfd Mallory 
450V nominal 550v surge CGS801T450VAL with my HB tester, and NONE read a 
short.  I have no scope here.  Granted, my test was a little on the low side 
of max voltage rating as I didn't want any explosions, but each cap showed a 
5 microamp leakage at ~400 volts DC.  Finally, with the thought that it 
still HAD to be in the board, I removed one side of the last string of 
bleeders (BTW, my bleeder system style was stolen shamelessly from an ETO 
MRI supply I have) and hipotted the trace on the board.  Placed on a 1/2 
thick piece of teflon, at 5,000 VDC I found no visible/ audible shorts & ~ 5 
microamps leakage.  At about 6500VDC, however, I could hear & see breakdown 
on the micrometer (slightly bouncing needle) and at around 7 kv I found one 
place on the trace (lights out in the room) with a pale blue arc appearing 
almost "inside" the Lexan, straight line between 2 traces.  Other places 
"appeared" by sound to be arcing but nothing was seen, and I wasn't game to 
take my noise-damaged ears close enough to the board to figure out which 
spot or spots the arc was coming from.  At this point leakage was reading at 
best, 10 microamps.  It sure wasn't pegging the meter...
    My next thought, as suggested in this thread, was to hipot the HT 
transformer, though why that shorting under load would kill a diode string 
escapes me.  Would it be logical to try this - removing mains leads & 
testing the HT betweeen primary & high secondary tap at, say, 3500V (500 
over output rating DC) & see what happens?  I am afraid to test the system 
as-is with my large oil-filled 5700kv nominal cap, though in fairness the 
other cap bank has not gone "boom".
    In sum, when the thing failed I was sitting here, the amp was running, 
but had not been used for a couple hours when whatever failed, failed & the 
20 amp magnetic breaker on the amp tripped (along with my 30 amp wall 
breaker) which is 8 ft away & feeds the amp w/ #10 wire.

    Thanks for all the input, it is an ongoing  learning experience.

    73 Dave N3DB

----- Original Message ----- 
From: "Jim Thomson" <>
To: <>
Sent: Wednesday, July 13, 2011 4:31 AM
Subject: [Amps] GS35b FWB issue

> 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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