Reforming caps and CRTs
ard at p850ug1.demon.co.uk
Tue Apr 21 13:21:53 CDT 2009
> Some of the smoke came out of my Osborne 1 this weekend. It doesn't
> appear to have been fatal, as it was still functioning when I cut the
> power. I'm giving the CRT some time to fully discharge before I open
> it up, probably next weekend. It was probably a failing capacitor,
I don't know why people are so worried about the residual charge on a
CRT. Unless you're working on the CRT or flyback transformer, there's no
way you're going to come into contact with it.
And I am not sure what good waiting a week does. If there's a bleeder
resistor, it'll discharge in minutes. Similarly on a _working_ device,
the beam current will often discharge the CRT at power-down (I find it
rare to find much residual voltage on a CRT final anode connector). If
neitehr of these is the case, then a week may well not be long enough.
> although it didn't explode, it merely got really warm over a period of
> hours. But since there are some similar machines that I haven't
> recently used, this question came to mind.
> I've used the variac technique to reform capacitors, but thus far
> haven't done this on a machine that contains a CRT because I don't
> know what the reduced voltage is going to do to the CRT. The next
The CRT is the least of your worries.
I am not sure what good applying a Variac to the mains input of a machine
using regulated supplies actually does. It's noy going to do much
reforming of capacitors deep in the circuitry. And if the PSU is a
switcher (and a lot are), it can do damage (switchers are constant
_power_ loads to a very good approximation, they draw more current at
if you want to reform the capacitors, IMHO you need to remove them and
run them up on a current-limited DC supply.
Even then I am not sure what the point is. I've found capacitors,
particularly tantalum bead types, just fail at random. I don;t think
reforming them would help.
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