PDP 11 gear finally moved

tony duell ard at p850ug1.demon.co.uk
Wed Jul 22 06:36:31 CDT 2015

> > that are running perfectly "just in case"...
> How do you -know- they are "running perfectly"? Just because a widget
> itself is functioning, you have no way of knowing if that capacitor is
> working 100% properly /unless/ you actually remove it from circuit and run
> a full battery of tests on it. Simply measuring the capacitance with a DMM
> while a capacitor is in circuit isn't good enough.

I am reminded of something that was written either in the manual for my 'scope
or in Tekscope (I forget which). Namely that 'The best tube tester is the circuit
that uses the tube'. Tektronix deprecated the use of tube/valve testers for finding
faults in their instruments

It applies to capacitors too.

If the _circuit_ (power supply or whatever) works correcty the the capacitor is good 
enough for that circuit. Whether or not it meets some published specs or not.

> Given that a typical aluminum electrolytic capacitor costs anywhere from
> $0.12-$0.15 (4mm or 5mm diameter radials) to about $1.00 (12mm or 16mm
> diameter radial), it also doesn't make much sense to desolder a 20 year
> old part, spend at a minimum 5 or more minutes testing it, and then solder
> it back in. It it much more economical to pull the old part and install a
> new one and be done with it. (You also don't have to worry if the
> desoldering and resoldering process might have damaged the original parts
> end-seals.) That said, I personally pre-test new parts, in bulk, before I

I don't remove parts unless they have something to do with the problem I am
solving. If the power rails are the right voltage with sufficiently low ripple then
I look elsewhere for probkems.

> put them into my stock, so I know ahead of time that I'm installing
> known-good parts.

You claim that electrolytics deteriorate with time whether used or not. How 
do you know the ones you install haven't deteriorated since you tested them?

> On many occasions I've cut open old aluminum electrolytics, and the guts
> very much do deteriorate with age. In addition to corrosion of the foil
> (black spots and pitting) and foil to terminal junctions (corrosion), one
> thing I particularly noticed was the more operating hours an aluminum
> electrolytic capacitor had on it, the more its electrolyte and paper
> insulator tended to smell bad compared to an otherwise identical (same
> brand and series) part that had very low hours. These are all clear signs
> of deterioration.

Firstly the paper is not the insulator. After all, it is soaked in electrolyte. And
'smell bad'??? OK, so some chemical change has taken place, but how do you
know it is detrimental to the performance of the capacitor?


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