On 11/07/2011 05:10, 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 tu
rn
> 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
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I run the "ultra fast" UF5408 diodes in all my EHT packs and I use 12
diodes in each leg rather than 8 ... just a healthy bit of over
engineering ;-) I don't use any equalising resistors or capacitors and
in over 15 years have never had a problem with any of my three amps at
home or in the field contesting...
I buy UF5408s from a quality supplier like Premier Farnell in batches of
100 which is enough to make two power supplies and have a few spare.
This way all the diodes are from the same batch and likely to be good.
Mike G8TIC
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