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[Amps] GS35b FWB issue

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Subject: [Amps] GS35b FWB issue
From: "David H Craig" <>
Reply-to: David H Craig <>
Date: Mon, 11 Jul 2011 00:10:05 -0400
List-post: <">>
    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.  Then, put back together w/ cap bank- 
blown fuse again & no solid HV.  Checked again, another leg of the FWB blown.  
Replaced those 8 5408s & tried again with the cap bank inline. Ergo, another 
string was blown.  Replaced them, and then tested with just HV & the FWB.  No 
problem.  Then connected the cap bacn (overkill, 12 x 450V in series, nominal 
800 MFD Mallories w/ 6 x 27k ohm Rs across each cap) WITHOUT the RF deck 
connected, and another audible glitch.  Tested all the 5408s in the bank, and 3 
of the (just) replaced diodes show x Meg ohm in the WRONG direction (yes, I 
tested all before & after installation before poweup).  All the rest were fine. 
 Then, tested each cap in the 12 x 800 MFD bank, and discovered nothing (none 
shorted), but on a hunch, tried to tighten each screw, and low and behold, 
after 2+ years in service, EVERY screw needed 1/8 to 1/4 turn 
 to get tight.  Question:  Could the combined losening of the screws create 
enough resistance to draw enough current to blow a diode string in the FWB?  I 
have checked everything else in the PS, and the common denominator seems to be 
the cap bank.  Am I missing something?

    Without load at least, the high current relay works 100% & the HV 
soft-start relay is working 100% as is the 24 volt ps that works the relay.  
The 2 second TDR that controls the soft-start is similarly fine.

    The 250ohm 50w B- float resistor is also fine (tested).

    Having isolated the problem to the HV supply, I am baffled.  I plan to 
replace the now-bad diodes, again, but am not sanguine it will work, and I am 
running out of 5408s...  I don't want to waste bigger diode blocks on this if 
they'll short too.

    Any advice or suggestions would be appreciated.

    73 Dave N3DB
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