[Amps] GS35b FWB issue
Jeff Goldman
k3dua at erols.com
Mon Jul 11 11:29:21 PDT 2011
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 at jeremy.mv.com>
> To: "David H Craig" <davidhcraig at verizon.net>; <amps at 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 at verizon.net>
>> To: "Carl" <km1h at jeremy.mv.com>; "Ian White GM3SEK"
>> <gm3sek at ifwtech.co.uk>; <amps at 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 at jeremy.mv.com>
>>> To: "Ian White GM3SEK" <gm3sek at ifwtech.co.uk>; <amps at contesting.com>
>>> Sent: Monday, July 11, 2011 9:19 AM
>>> Subject: Re: [Amps] GS35b FWB issue
>>>
>>>
>>>>
>>>> ----- Original Message -----
>>>> From: "Ian White GM3SEK" <gm3sek at ifwtech.co.uk>
>>>> To: <amps at 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
>>>>
>>>>
>>>> _______________________________________________
>>>> Amps mailing list
>>>> Amps at contesting.com
>>>> http://lists.contesting.com/mailman/listinfo/amps
>>>
>>>
>>>
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>>
>
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