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Re: [Amps] GS35b FWB issue

To: amps@contesting.com
Subject: Re: [Amps] GS35b FWB issue
From: Jeff Goldman <k3dua@erols.com>
Date: Mon, 11 Jul 2011 14:29:21 -0400
List-post: <amps@contesting.com">mailto:amps@contesting.com>
Do you have a soft-start circuit in the primary of the transformer?   
The capacitor bank (12-800 uf caps = 66 uf) looks like a dead short  
the instant the power is turned on, and there will be a huge current  
flow.

73,
Jeff, K3DUA




> ----- Original Message -----
> From: "Carl" <km1h@jeremy.mv.com>
> To: "David H Craig" <davidhcraig@verizon.net>; <amps@contesting.com>
> 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" <davidhcraig@verizon.net>
>> To: "Carl" <km1h@jeremy.mv.com>; "Ian White GM3SEK"
>> <gm3sek@ifwtech.co.uk>; <amps@contesting.com>
>> 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" <km1h@jeremy.mv.com>
>>> To: "Ian White GM3SEK" <gm3sek@ifwtech.co.uk>; <amps@contesting.com>
>>> Sent: Monday, July 11, 2011 9:19 AM
>>> Subject: Re: [Amps] GS35b FWB issue
>>>
>>>
>>>>
>>>> ----- Original Message -----
>>>> From: "Ian White GM3SEK" <gm3sek@ifwtech.co.uk>
>>>> To: <amps@contesting.com>
>>>> 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
>>>>>> 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
>>>>>
>>>>
>>>>
>>>> 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
>>>>
>>>>
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>>>
>>>
>>>
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>>
>
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