[Amps] GS35b FWB issue

[David H Craig]

    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