'orrible 'unts a first??

[Chris_C]

While going through the usual steps on an Ekco Mariner, I found that all of the Hunts caps were on spec and not leaking and no, it's nowhere near April 1st.

Is this a first? This included the mains filter capacitor. Which was rated at only 300 volts AC (this will be replaced)

The cap to the output valve grid was leaking but this was one of the unbranded fawn coloured ones like the Mullard type.
The set is from around 1966 and the only visible repair was a replacement o/p valve cathode resistor.

I know these old caps get bad press but I've just finished working on a Philips Romance and all the Hunts were within tolerance on that set too.... Maybe the produced a " good " batch at some point
Rich

[wireless_paul]

I have just replaced 3 Hunts capacitors in a power supply. A 2mf measures 6mf, another 2mf measures 2.5mf and a 1.0mf measures 0.99mf. Apart from the obvious duff one I would have thought the other two were well within tolerance. I still believe that far too many capacitors are replaced for no other reason that they might be something wrong with them. I read an interesting article recently regarding this topic, pointing out that if you were repairing something like an oscilloscope, where the number of capacitors might be going into the 100s, that replacing them just for the sake of it would not be feasible.
Paul E
Paul E

[Alan_Douglas]

If you're using a digital capacitance meter as opposed to a bridge, the 2 is reading 6 because of leakage fooling the meter, and probably the 2.5 is also. You can try the experiment of measuring a known good cap, then adding a high resistance in parallel to see if the meter reading goes up.

[Sideband]

Hi All.

Replacing capacitors just 'for the sake of it' may sometimes seem reckless.....probably many of those changed would carry on working for ages. However there is the question of long-term reliability. Any capacitor that is used as a mains filter or an HT decoupler or tone correction across an output transformer I believe is best replaced as a matter of routine irrespective of how it measures on test. Capacitors in low voltage areas could possibly be left in place if they measure OK. My experience has been that odd 'rustling' noises can be caused by some of these however. It's probably a question of personal preference and the number of capacitors that you have in the spares box.

Electrolytic types should always be suspect. Strangely enough I find smoothing capacitors are more reliable than the lower voltage ones. I find I nearly always change the cathode decoupling capacitor on an output valve but rarely have to change a smoothing capacitor. These normaly reform very well.

Rich.

[JHGibson]

While certain makes of capacitors are more prone to failure than others, the caps operational history seems more important. For example, the caps operating in a hot environment like a compact table radio turned-on 10 hours or more every day, seem more likely to fail than those in say in an oscilloscope, such as the Tektronix 500 series, which have a giant cooling fan sending a hurricane of room temp air around all the components, even though the running hours could be similar.
It could very well be that the caps in an old radio with only a few hours on it, will check OK. However if the radio has been stored in a damp area, that will be as bad for the caps as running them at a elevated temperature. John.

[Paul Stenning]

There's also the attitude that while you have got the set apart and are working on it, you might as well change the lot. Waxies and Hunts caps are notoriously unreliable (although to be fair they were never intended to last 50 years or more and spend part of that time unused in less-than-ideal storage), and are the cause of many problems both obvious and obscure with sets. Modern caps are better quality and should be good for at least another 50 years.

[paulsherwin]

It's also difficult to test old caps. You really need to take them out of circuit to test the value, and even then leaky ones often only leak under high voltage and/or when they're starting to get warm. If a radio only has, say, 6 high value wax caps and a couple of low value electrolytics (not unusual) it's easier to just change them, even if you have all the right test gear (which I don't).

[jim_beacon]

I tend to change all the caps in domestic sets in one go, past experience says that this is the easiest way (especially if the radio belongs to someone else).Considering the cost of new caps, against the disapointment of repeated failures, and the time wasted in re-visiting sets, it's just not worth doing anything else.

Test equipment is a different matter, the caps tend to be higher qaulity, and far more conservativly rated, hence their reliability is much greater, though I'm still suspicious of any electrolytics.

TV seems to be the break even point - they tend to use the lower qaulity components, and run them near the top end of their ratings, but then there are an awful lot of them. In older sets, I just change the lot, but when you get towards the dual standard sets, it becomes impractical, and I tend to change only the faulty ones - it's a good couple of evenings work to change all of the wax caps out of a TV24!

Jim.

[Alan_Douglas]

I've restored eight or ten Tektronix scopes, never replaced any of the twist-lock cans, and never had one fail.

Now the molded Bakelite paper caps with the color stripes that Tek used, well, that's a totally different story. I've never found a good one of those.