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