At 3:25 AM +0200 6/16/04, Listar wrote:
>Date: Mon, 14 Jun 2004 22:52:26 -0700 (PDT)
>From: Tim Hughes <timhughes@xxxxxxxx>
>Subject: [OM] Re: T20 charges but will not fire..
>
>
>Clint Wrote:
>
> >>First, discharge the main and trigger cap (300v is gonna bite you
>otherwise!). Then check from the x-pin to the white wire going to
>the middle of the top circuit board. It's not likely the problem,
>but...
><<
>
>Whatever you do when you discharge the capacitor don't short it out
>directly, as you may sputter copper vapour in your eyes from the
>immense spark, if it is still fully charged! You should do it with
>a resistor but not too low a value as it could also shatter. I did
>this once with a high wattage resistor (20W,20 ohm) and it shot a
>piece of epoxy across the room. Ideally you need to use something
>like a 1Watt 5k Ohm resistor and you need to leave it on a while,
>say 30sec to be sure it is fully discharged. Better yet, make sure
>the cap has ~0V on it, at the end by using a voltmeter. A general
>good idea when working on flashes, is to put a cliplead across the
>capacitor after you have discharged it, so you can't inadvertently
>charge it up.
I bet that 20-ohm resistor was quite inductive, and not rated for 300
volts, and it sparked over. Right through the epoxy conformal coat.
>If you don't have a suitable resistor hook up a voltmeter across the
>capacitor and wait until it has self discharged to less than say
>15V. It may take a while but it will drop eventually. The T20 self
>discharges reasonably quickly compared to some other flashes.
I would add one thing: Leave the shorting cliplead in place, as such
capacitors are subject to dielectric absorption. It's a long story,
but the effect is that you discharge a capacitor down to zero, remove
the short, and soon thereafter a substantial charge has reappeared as
if by magic. It isn't magic, and at a 300 volt charge, the rebound
voltage (at most 50 volts I would think) probably won't hurt anyone,
but it could be quite the surprise. (I have not measured the rebound
on such a capacitor, so the 50 volts is a guess based on other types.
It should be easy to measure, once one has the flash apart.)
Many flashes have a drain resistor across the flash capacitor,
precisely to dissipate such absorbed charges.
Joe Gwinn
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