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