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Re: [Amps] GS35b FWB issue

To: "Ian White GM3SEK" <>, <>
Subject: Re: [Amps] GS35b FWB issue
From: "David H Craig" <>
Reply-to: David H Craig <>
Date: Mon, 11 Jul 2011 03:02:10 -0400
List-post: <">>
    Hi Ian,

    Transfomer voltage is 240- 3000VAC.  Line voltage 245 AC (tongiht 
anyway).  No line work that I know of ( & I would know it where I am).  My 
pole pig is 50ft from my home.  Mains, with caps in line when testing, 
is/was 245VAC.  No blown diodes with just no-load (no cap bank in the 
circuit) at that voltage. Just hums along.  Under the cap load, it blows. 
That is why I wonder if the loose screws were the issue, but I don't want to 
keep blowing the diodes to find out.

    That said, I have a lot of "brown-outs" here, and some may come at night 
when I am not aware of them.  I often let it run 24/7 during Es season. 
Bear in mind though that this occurred when I was up, no brown-outs.

    Thanks for the reply,

    73 Dave

----- 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
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