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Re: [Amps] GS35b FWB issue

To: "Carl" <>, <>
Subject: Re: [Amps] GS35b FWB issue
From: "David H Craig" <>
Reply-to: David H Craig <>
Date: Mon, 11 Jul 2011 14:03:53 -0400
List-post: <">>
 Hi Carl,

    Yes, I checked for shorts & nada.  The transformer, sans FWB & etc 
performs flawlessly.  The repaired FWB also is fine fed by the transformer 
without the cap bank load...   Unless the transformer (or cap[s]?) is/are 
shorting only under load...  Problem seems to be (IMHO) now in the cap bank, 
though it doesn't show up that I can see with the Fluke.  The diodes go only 
once the cap bank (isolated from the RF deck) is attached to the bridge. 
BTW, if I leave the B- lead from the ampmeter to the bridge installed, it 
reads current before failure with the amp out of line, and bypass switch in 
the "off" position.

    73 Dave

----- Original Message ----- 
From: "Carl" <>
To: "David H Craig" <>; <>
Sent: Monday, July 11, 2011 1:06 PM
Subject: Re: [Amps] GS35b FWB issue

>I was considering mentioning the 6A10 (aka 6A100 by a few mfg) but that 
>wont solve the problem. It could be caps arcing (an open equalizer resistor 
>can create havoc). An arcing transformer is before the diodes. I assume the 
>wiring and connectors has all been checked for arc paths.
> Carl
> KM1H
> ----- Original Message ----- 
> From: "David H Craig" <>
> To: "Carl" <>; "Ian White GM3SEK" 
> <>; <>
> Sent: Monday, July 11, 2011 12:22 PM
> Subject: Re: [Amps] GS35b FWB issue
>>    Hi Carl,
>>    AC secondary is 3kV where I have tapped it (has 3 more lower V taps). 
>> AC line is around 245VAC.  Diodes came from Mouser a few years back.  The 
>> ones I used were from a loose batch, not a reel, though I have some reel 
>> (a couple of those in the reel "blew" me away, less than 1 MEG inverse!). 
>> No caps were warm that I could tell but they are in 4 x 2 configuration 
>> on the board so I cant handle each one quickly, and the "blow-time" of 
>> the breaker is so short I doubt I'd be able to tell anyway.  I agree, I 
>> have never experienced anything like this before with 5408s @ almost 
>> double the voltage configuration need built in.  I'll try the caps next & 
>> see what happens- I appreciate the suggestion.  To me it HAS to be an 
>> amperage issue...  It ran for 2-3 years without a single problem, so I 
>> was taken aback to have this happen twice (2nd time without the tank in 
>> line)....  I suspect something else is causing the issue, caps or diodes. 
>> I doubt the transformer, but who knows (it was poorly built ab initio & I 
>> did have to resolve a huge current issue with it before the amp was 
>> complete -the all-thread on the E-I wasn't properly insulated & it got up 
>> to almost 300deg F w/out load before I fixed the bolts & insulated the 
>> thread)  FWIW I checked my 5408s as I put them aside to use & found quite 
>> a few that measured less than 1.5 MEG on the inverse side (rather than 
>> the normal 1.8- 2.2MEG I usually saw).  Maybe they are all indeed junk 
>> for FWB purposes, as the board was made from a different batch I no 
>> longer have? Should I just go to the 6 amp 2kw version (forget the 
>> designation?) & forget the 5408s?
>>    73 Dave N3DB
>> ----- Original Message ----- 
>> From: "Carl" <>
>> To: "Ian White GM3SEK" <>; <>
>> Sent: Monday, July 11, 2011 9:19 AM
>> Subject: Re: [Amps] GS35b FWB issue
>>> ----- Original Message ----- 
>>> From: "Ian White GM3SEK" <>
>>> To: <>
>>> Sent: Monday, July 11, 2011 2:23 AM
>>> Subject: Re: [Amps] GS35b FWB issue
>>>> 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 
>>>>>    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
>>> Where did the diodes come from? What is the AC voltage of the 
>>> transformer
>>> secondary?
>>> After the failure and a full discharge are any of the caps warm?
>>> With 8000V of diodes and 5400V of caps I would certainly not expect a
>>> problem.
>>> Place a scope on the primary and check for excessive spikes. A .01 AC 
>>> rated
>>> cap from each 240V leg to ground will get rid of some of it. What gets 
>>> thru
>>> the transformer can be removed with a .0047 6 or 10KV cap across each 
>>> leg of
>>> the bridge and another to ground at the DC output.
>>> Im somewhat suspect of the diodes as being relabled or just plain junk 
>>> to
>>> start with. Ive run 1N5408's since they come out and havent lost one 
>>> yet.
>>> Carl
>>> KM1H
>>> _______________________________________________
>>> Amps mailing list
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