I've been experiencing a rapid failure rate on the Mallory 450VDC 400mfd
filter caps in my Titan 425 AC supply.
Symptoms:
1) RST 591 in contests
2) Reducing drive level on Omni-V.9 to 5 watts still causes Grid overload
LED to flicker
3) HV Meter reads about 1800VDC (normally nearer to 2400 no-load).
I had one fail last month. Put a new one in. A week later #2 went.
Three days later #3 went. I finally shotgunned the remaining 5 caps with
new ones from Newark Electronics (Mallory and Newark Part #CGS401T450R5L).
The ones that failed all came from the same batch dated the 43rd week of
1988 (362 8843 56699). If your caps have that date and you lose one...order
a full set of replacements. Ten-Tec gets about $30 each. Newark about $22
(cheaper through corporate accounts).
I cut a couple open (as did Bob K2TK) and one lead from the screw terminal
is a nice wide aluminum strip. In mine the other lead was....gone. Just a
pile of brown slime. Bob found some of the lead material in place in the
one he cut open.
His theory is that the leads fold back and forth accordian style as the cap
is assembled. Possibly thermal expansion between full power on and stone
cold off flexed the lead back and forth (up and down actually) until it
cracked where it meets the screw terminal crimp. But the aluminum strip is
like a spring and pressed against the rivet at the bottom of the screw
terminal allowing a resistive electrical connection to form. What other
chemical process takes place if it arcs over or cooks is unknown but by the
time the pile of brown slime is created the cap has opened up.
The new batch of caps are dated Jan 2001 and Oct 2002 so they are still
being manufactured. I bought the last 5 that Newark had in stock but they
show 70 on backorder so there should be some in shortly.
Has anyone else experienced this? I'm just glad they fail-safe.... Hate to
think about replacing them all plus the diode strings if one shorted!
Tom K2TA
tom@k2ta.com
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