Dave, you might consider adding some current limiting in the transformer
primary while testing.
A couple of 100 watt incandescent light bulbs in series will work great for
that. If all is well, the bulbs will glow very dimly. If there is a fault,
the bulbs will glow at near full brilliance and the current will be low
enough to protect the diodes and everything else in the circuit.
73,
Gerald K5GW
In a message dated 7/10/2011 11:10:12 P.M. Central Daylight Time,
davidhcraig@verizon.net writes:
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
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