"If it ain't broke"

[catch29]

So, we are told on vintage equipment to automatically replace the low value wax cap's etc, I can see that, they're unreliable and cheap to buy. Working on my Heathkit MA-12 there are two dual cap's, 250uf+60uf 350v and 50uf+50uf 350v which are proving a pain to procure. I have metered them at 270uf+68uf & 65uf+60uf, not bad for 50 years!. Should I leave them or will they get 'stressed' by renewing the others?. Also could anyone recommend a supplier who deals with small qty audio bits. Cheers chap's.

[ppppenguin]

The tolerances on electrolytics are very wide, sometimes as bad as -25% and +100%. If they reform OK, with adequately low leakage, and don't look physically distressed then can be left in place. It's often hard to get exact replacements for big electrolytics, the sort mounted in cans above the chassis. Replacement methods include restuffing with modern components and fitting new parts beneath the chassis.

The 250u would typically be replaced with a modern 220u or 470u. The 60u with 47u or 100u etc.

Measuring capacitors with a simple meter can give unexpected errors. A leaky cap will read high, so you could have a cap that is leaking and has lost capacitance reading very close to its marked value.

When you say "audio bits" what do you mean? There aren't many parts that are specifically audio. Unless you're in "audiophool" territory which I hope you're not.

[Mike Watterson]

I test electrolytics with a current limited PSU. At battery voltage, rated voltage for cathode caps up to 30V or at 30V for HT types. Disconnect the non-chassis end. Note FM detector caps are +ve to 0V/Chassis and Germanium and some other old transistor sets use +ve battery to 0V

I've hardly ever had to replace HT electrolytics, except 1950s 120V parts on the 90V HT of Vidor battery valve sets (but not others) and the two colour plastic case ones on old transistor radios. The FM discrimator (often 4uF) next most common and then sometimes audio output valve cathode capacitor.

If the current doesn't drop to a fraction of a milliamp the leakage is too high. I might give them overnight.

If the voltage on volt meter drops to zero straight after disconnection then it's dried out.

If those pass, then I'll short it and test on a capacitor meter that shows ESR. The caps range on DVM isn't much use. Ordinary bridges can be misleading due to leakage or high ESR (drying out).

There are a few low noise audio pre-amp ICs and some low voltage headphone ICs, some dolby ICs and various IC stereo decoders. There are no other parts specifically for audio, except maybe some inductors and non-polarised Electrolytic caps in cross-over networks. I often replace non-polarised Electrolytic caps with metallised plastic foil types as they tend to dry out.

Obviously some amps or radios have audio transformers and can be burnt out (O/C) due to leaky anode to grid caps or a dead grid bias battery on really old sets.

[Murphyv310]

Hi.
I agree with Jeffrey regarding Electrolytic capacitors. Slowly reforming them is the way to go and once done either check the leakage current or make sure they don't get warm (other than radiated heat from close by hot components, valves etc)
Replacing wax capacitors cannot stress the Electrolytic capacitors in any way. Don't do blanket replacement it's a big mistake do them one at a time, I've come across people that just snip them all out in one go and land up in a huge pickle. Also the flat wax covered capacitors below 500pf are generally OK so leave these until you know for certain there is an issue with them.

[Diabolical Artificer]

Hifi collective do F&T dual caps,you should be able to get a 50u/50u 500v, a 220u can and a 47u single cap - https://www.hificollective.co.uk/compone...itors.html if in doubt ring em,they're helpful lads. Also here - https://www.hotroxuk.com/f-t-radial-capacitors.html

Andy

[CambridgeWorks]

There is a TCC 8-16uF in a radio I am restoring. A label says reformed in 1960. I brought it up to working voltage with the Variac over 3 or 4 hours. It is fine and been working for a few hours now, no distress.
Rob

[Nick]

Hifi collective do F&T dual caps,you should be able to get a 50u/50u 500v, a 220u can and a 47u single cap - https://www.hificollective.co.uk/compone...itors.html if in doubt ring em,they're helpful lads. Also here - https://www.hotroxuk.com/f-t-radial-capacitors.html

Andy

You might consider using Ask Jan First - http://www.askjanfirst.de/eindex.htm 

They have an astonishing array of great stuff, including F&T and other capacitors that they have especially made for restoring vintage equipment.

I've used Jan for many many years - he's excellent and very helpful.

[catch29]

Thank's for all the info' guys. I will let you know how it all pans out.

[Craig]

Because I'm a bit of a head case, I've been restoring a huge CT218 RF signal generator (AKA boatanchor). That had very many leaky paper capacitors, replaced with axial polyester. The CLCLC main filter had two capacitors totally kaput; fortunately none of them duals. I managed to find 32uF500V caps that fit the chassis clamps. Other 4uF cracking seal electrolytics were replaced with (oddly cheap) audio axial polypropylene, which also fit the original clamps.

There were other issues, now all solved. Including a cathode resistor in the modulation oscillator that had never been soldered, was near to but not soldered to the ground tag, and was not even tinned! So modulation could never have worked, or checked in manufacture.

Performance is (now) bang on - which is going well for a 56kg animal that was built by Marconi in 1956. It is a two person lift.

7 turret tuned ranges from 85kHz to 30MHzwith fine tuning on a 2.5 meter film scale, AM, CW, FM. Calibrated output down to 0.1uV.

Craig

[Exiled Yorkshireman]

I agree with the "if it ain't broke, don't fix it" approach to electrolytics. A bit of physics and chemistry helps to  understand what's going on. The dielectric in electrolytics is a thin layer of aluminium oxide (the dielectric) on one of the aluminium foils which is one electrode. The electrolyte (in conjunction with the other foil) is the other electrode. If the electrolyte goes AWOL, then the ESR will go high, and that's a sign that the cap is knackered and should be replaced. The more electrolyte that goes missing the lower the capacitance, too.  Now most people think of aluminium oxide as being insoluble, but actually over time, without any polarising voltage applied to the cap, the thin layer of oxide does dissolve back into the electrolyte. Of course, the thinner the dielectric, the higher the capacitance, which is why we often measure the caps as being high. Also, the thinner the dielectric, the lower the breakdown voltage. If the oxide layer disappears entirely, the cap will go short and is beyond retrieval. Caps that have not been used for some time need to be reformed slowly, without exceeding the present breakdown voltage. The gradual application of the polarising voltage will slowly re-form the oxide layer by electrolytic deposition, the leakage current will decrease, and the capacitance will drop. The key is to keep the current down to avoid local heating and to keep an eye on it. I generally find that if the leakage current is stable and within reasonable limits at the working voltage then the cap is good to go, other measurements being OK, of course. 

Another reason I do this is 1) I'm a cheapskate and 2) I'm cackhanded at restuffing.