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Re: [Amps] GS35b FWB issue

To: "David H Craig" <>, <>
Subject: Re: [Amps] GS35b FWB issue
From: "Carl" <>
Date: Mon, 11 Jul 2011 13:06:46 -0400
List-post: <">>
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.


----- 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 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
>> 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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