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" <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
>>>
>>>
>>> _______________________________________________
>>> Amps mailing list
>>> Amps@contesting.com
>>> http://lists.contesting.com/mailman/listinfo/amps
>>
>>
>>
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>
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