[Amps] GS35b FWB issue

[Mike Tubby]

On 11/07/2011 05:10, 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
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I run the "ultra fast" UF5408 diodes in all my EHT packs and I use 12 
diodes in each leg rather than 8 ... just a healthy bit of over 
engineering ;-)  I don't use any equalising resistors or capacitors and 
in over 15 years have never had a problem with any of my three amps at 
home or in the field contesting...

I buy UF5408s from a quality supplier like Premier Farnell in batches of 
100 which is enough to make two power supplies and have a few spare.  
This way all the diodes are from the same batch and likely to be good.

Mike G8TIC