Old electrolytic capacitors

[Andy Hall]

I didn't know that Jackson were still in business.

These were the typical ones

[Dave Plowman (News)]

Them were called condensers in them days, them were.

[Frank Erskine]

Be very careful there. Silicon diodes are much more efficient than selenium/copper oxide rectifiers and the resultant HT voltage may be quite a lot higher than original, causing other failures.

[The3rd Earl Of Derby]

Nope! the caps where called condesers. ;-) I'm older than you sonny :-P

-- Sir Benjamin Middlethwaite

[The3rd Earl Of Derby]

There used to be program on telly called "tricks of the trade" where some bloke did this to old radios, you been watching it? ;-)

-- Sir Benjamin Middlethwaite

[Dave]

I have always know them a tuning condensers.

Any old G3s reading this thread?

Dave

[Andy Hall]

Tony Hancock is long gone. It's 3G now, not G3.

[The Natural Philosopher]

Huge fun..thanks.

Imagine paying the equivalent of £400 quid for a radio, today...no wonder they still soldier on.

I ought to try and tune this one up...but te thought of having coils disintegrate on me if I do...no way.

It works.

[mike]

[e-mail] wrote in news: [e-mail]:

That's a nice site - well worth looking around the whole shop

mike

[Owain]

They pinched my idea :-)

Owain

[PeTe33]

Is a G0 close enough ?

[meow2222]

What changes is their V rating drops. The solution is to apply a variable supply to them, and raise the v very slowly, over hours. This reforms them, and they run fine again. Normally old 50s caps are fine, but there is a failure rate, so sometimes reforming is needed.

If a cap is beyond it, gut it and fit a modern one inside. But beware, old caps often used some very toxic chemicals like dioxin contaminated PCBs (?polychlorinated biphenyls iirc), and these chems should not be handled at all.

Do not succumb to the temptation to add more capacitance than was there originally, for a number of reasons. Valve rectifiers are rated with max capacitance loads, and those capacitanes are very small by todays standards. If you exceed these values, the valve sees a much higher peak current than it can handle, and you get valve failures. Similar prblems occur with selenium rectifiers, higher peak i means more I^2R and frying. Fried selenium is real fun :)

You can up the C on downline supply smoothing caps if you want, ie the one that feeds the low level signal area after a highish R dropper, but you should definitely not overcap the main reservoir. And you really should not overcap the smaller smoothers either, as it deteriorates sound quality. Valve radios typically have crappy bass response plus

100Hz signal modulation. This modulation restores the apparent bass and gives them that warm sound. Putting big caps on to eliminate the slight background hum will stop this and be bad news for sound quality. Far better to trust the original designer and have the radio work the way it was designed to, and get the best from it.

Overcapping valve radios is a bit like fitting upvc windows to period properties, except that theres no benefit.

NT

[Chris Bacon]

Blah blah blah.

[Ian Stirling]

You can also take the third way - carefully dissasemble the old ones, and insert new inside.

I was making a battery charger out of a 'name brand' (lambda I think) power supply, last used for testing LARGE racks of 20Mb RLL hard disks. After about 8 hours under power, the 240V resovoir capacitor went (on the 5V size) with a very loud 'Fsss'.

(car battery charger - 12V 40A)

20 years idle?

[Kate]

You never know ;-)

The OP is replacing the capacitors in a psu.

Take care to replace the old ones with new ones of the same nominal capacitance.

Putting in a higher capacitance can look good, but it isn't better.

The increased current peaks through the rectifier and transformer can overload the latter, as it wasn't desgined for that. The transformer could 'let go', never to work again.

I had a psu where this had been done. The psu was fine for a few months, then stopped working. That's how it stayed as all the wound components were sealed in pitch...no way back. The designers knew what they were doing. HTH.

[Andy Dingley]

Generally, at that sort of voltage, they'll burst. remove.

If they're not dried out then you _might_ get away with re-forming them. But if they're "valve vintage" then they're probably goners.

Keep the old cans. Some people hide the new component in the old can.

[Andy Dingley]

Bugger me ! They still make C804s !

These were the typical ones

[Andy Hall]

.. and why 465kHz (or whatever it was) for the IF?

[Frank Erskine]

Because at the time it was thought that it would minimise the risk of second-channel (image) interference.

[Andy Hall]

With what though? Is this because of the frequency range of medium wave? Why 465 rather than 420 or 480 for example? That would suggest wanting to avoid a specific frequency.