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Re: [Amps] GS35b FWB issue

To: David H Craig <>
Subject: Re: [Amps] GS35b FWB issue
From: Mike Tubby <>
Date: Mon, 11 Jul 2011 18:02:55 +0100
List-post: <">>
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 tu
>   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
> _______________________________________________
> Amps mailing list

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

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