[Amps] GS35b FWB issue

[Ian White GM3SEK]

David H Craig wrote:
>    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


What transformer voltage?

Also, have you checked the mains voltage?

You imply that there have been some lightning strikes recently. Might 
the electricity company have been messing about with transformer taps?


-- 

73 from Ian GM3SEK
http://www.ifwtech.co.uk/g3sek