Reforming caps and CRTs

Mr Ian Primus ian_primus at yahoo.com
Tue Apr 21 08:00:44 CDT 2009


--- On Mon, 4/20/09, Eric J Korpela <korpela at ssl.berkeley.edu> wrote:
> 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,
> 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.

Does the Osborne have a tantalum capacitor in it? Those tend to blow without warning, for no good reason. When they go, you get lots of magic smoke and burning - but your device tends to stay working. Filter caps can go with smoke too - and depending on the deivce, it might keep going, or fail. 

I think people get really way over-concerned about electrolytics. These devices are old... but not that old. Slow powerup would be a good idea for seriously old stuff (pre-50's, mainly), primarily just to check for shorts. I use a series light bulb for that. But if the cap is bad enough that it's going to exlplode on powerup, it isn't going to reform to a useable state.

This is one of those often discussed "holy war" topics, and comes up once a year or so. My mindset has always been to cross my fingers and plug it in, especially for common, documented things like an Osborne or a TRS-80. If a capacitor blows up, then you know that it's bad, and should be replaced - and it's marked clearly with a smoking hole. For rarer things, I try to go in stages, testing the power supply first with a dummy load to ensure that it's good before going on.

But the fact of the matter is, when you're working with old stuff, you are going to be replacing a fair number of electrolytic capacitors. They just don't last. And the typical failure mode has them simply operating poorly - not exploding.

A tradition at VCF East is the releasing of the magic smoke (Andy is usually on top of this one), and while spactacular, all affected devices have since been repaired. (Canon Cat, Apple IIe, Kaypro, Apple ProFile...)

-Ian



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