Bush Set's capacitors

[Oldelectronics]

I have found that capacitors in Bush valve sets as advised are generally good if the set is woken up slowly and carefully in this case an AC41. But this one looks as though it was about to go bang although it seemed OK at first the symptom was over heating measured on my laser thermometer (45c) I don't know what I would do without it and it was inexpensive off of eBay. As I increased the voltage ie changed the bulb in my lamp limiter the temperature climbed. The bulge seemed develop after switch off. or I just didn't notice it? Glad I was checking. Have had the set on earlier for about 3 hours and it seems OK now the capacitor (32uf + 32uf) has been changed.

[Nickthedentist]

I agree, the BEC dual/triple electrolytics in Bush 1950s/60s sets generally fair very well and only need to be re-formed to perform as-new.

Yours is the expception!

[Nuvistor]

I don't remember the make but I have had to repair some Bush sets from the early 60's with the original smoothing block spread all over the chassis, those sets would only be a few years old at that time.
That capacitor looks like the bottom is bulged, it's not flat on its base unless that's the surface it is on.

Frank

[Oldelectronics]

Its the surface Frank the other end to the one shown with the bulge in the rubber has a solder tag and star washer on plus the perforated MDF makes it look even more wobbly!Interesting that some went pop early in there working lives I wonder why?

[Nuvistor]

Ok on the base having a solder tag. I don't know why they went pop, usually fitted replacements from RadioSpares if I remember correctly but its over 50 years since, it didn't reoccur after replacement. I don't have many radios but I have replaced the electrolytics in them all, saw the aftermath of more than one go bang in radios's and TV's, not common but never the less not good. They make quite a mess and I would not one go off in my face while repairing one.

[Argus25]

Oldelectronics,

I'm not going to beat about the Bush here (no pun intended) or tiptoe around it. You will notice that many folks try to re-form old electrolytic capacitors and claim some degree of success, but there is a paucity of published scientific data on the re-formed components to support these claims. There have even been capacitor re-former kits published which are about as useful as picture tube rejuvinators or taking a piano accordion to a deer hunt.

Its all a nonsense I'm afraid, that is if you are coming from a position of three important considerations or factors, three basic ones expanded to 6 features:

1) Accurate circuit functionality intended by the designers (for this to be true the capacitor must have the same uF capacity, leakage current and ESR, or better with these specs, as it did when new).

2) reliability (not going to unexpectedly let you down and cause a fault)

3) safety (doesn't physically leak electrolyte or explode in your face).

(you can think of more I'm sure)

Greater than 40 year old electrolytic caps in valve equipment simply are past supporting all the above 6 features. Just because a re-formed electro might seem to work and satisfy one or more of them doesn't mean it will satisfy all 6. To say it does is an example of the Emperor's New Clothes...in my opinion (though obviously others might disagree and that's ok).

Electrolytics were only ever rated for 1000 -2000hrs continuous use , though some modern ones can get to 10,000 hrs.They are prone to multiple types of internal degradation and failure modes as they age.

I have restored many radios and television sets over the past 40 years. I started doing it in the mid 1970's long before it was a popular hobby and I would recommend all electrolytics be replaced especially if over the 40 year age range especially high voltage ones in valve equipment.

Obviously there are issues of originality. I always keep the same canister. One good way is to carefully run a file around the folded over lip on its curved edge until the folded lip is just cut through on the base. Then the base can be withdrawn, the can kept, with the edge cleaned up, and a new base with connectors machined to fit it. This has been done with the large electrolytic in my Bush TV22 as an example of the technique, have a look on about page 5 of this article:

http://worldphaco.com/uploads/BUSH_TV22.pdf

The replacement internal capacitors can be replaced again later, after all even the new capacitors could fail in the future if you are lucky to own the TV or radio long enough.

['LIVEWIRE?']

I usually replace reservoir/smoothing capacitors in valve radios, etc., for the reasons stated by Argus 25, and have never tried to reform any. After all they will have (far) outlived their design life by now. Have to admit that I don't restuff old cans -maybe if it was a particularly rare set I would - I replace them with modern dual can electrolytics whenever possible.

[Station X]

When I was in employment one of the "routines" was to regularly power up spare PSUs to reform the electrolytic capacitors.

I have successfully reformed many electrolytic capacitors in vintage radios, a process that can take some time. After reforming I check their capacitance and ESR. The ability to hold charge or not is a good indicator as to whether an electrolytic is any good.

I would add though that:-

Some caps won't reform, remaining leaky for ever.
One exploded during reforming, despite the current being limited. Since then I've always covered them with an old towel during reforming.
One failed open circuit shortly after reforming. I assume the corrosive electrolyte dissolved away an internal connection.

[G8HQP Dave]

Reformed electrolytics can be fine. However, it needs to be made clear that running a set via a lamp limiter is not a good way to reform old electrolytics. A lamp limiter reduces the risk of explosions and fire. It will not reform an old cap, but it may push an old reformable cap over the edge before it has been given a chance to reform.

There seems to be a wide spectrum of opinion on this. Some just plug in old sets directly, and learn to duck. They find that some old caps are fine and some are not; they probably don't believe in reforming. Others use a lamp limiter and wrongly believe that they have reformed a cap. Some replace all old caps on sight. Some reform carefully, and find that some caps are fine after this and some are not - which is exactly what we would expect.

[Argus25]

Originally Posted by G8HQP Dave

Reformed electrolytics can be fine.....Some reform carefully, and find that some caps are fine after this and some are not - which is exactly what we would expect.

Actually, no reformed caps over 40 to 50 years old are "fine" at all.

They just might seem fine on limited testing.

They are still an accident or failure waiting to happen.

One reason I know this for sure is that while most service people discard electrolytics after they replace them, they do not perform a forensic investigation of why the capacitor failed as I have been doing.

Due to the fact I have been restoring valve gear for over 40 years, and I have insisted on trying to preserve the outer casings of the capacitors, I have had the opportunity to inspect the contents of many dozens of vintage electrolytic capacitors.

I have posted some photos before, suffice to say what goes on inside these capacitors is shocking, with multiple failure modes which include insulation failure, extreme corrosion directed toward the inside of the canister and the interconnects, drying out and leaking seals.

If you think a reformed over 40 year old electrolytic capacitor is a satisfactory electronic part, I'd suggest think again. The only cure for this belief is to do what I have been doing and open these old capacitors up for examination. You won't like what you find and after a while you would share my opinion.

[PsychMan]

I suppose the proof in this is whether anybody has found their reformed capacitors subsequently failing?

A little while back I restored a DAC90A and used my capacitor reformer circuit for the first time. Leakage readings read high initially, and after around a day or so read very low indeed as the guide suggested.

Its been in the set and working fine with regular use for a while now, sometimes for extended periods of time, but like a lot of my old gear I don't wander off and leave it running. Whether that component is going to last for years to come, who knows, but I'd call that a result. I wouldn't bother with other caps, but I find re-stuffing dual electrolytic cans a pain. Each to their own

[ionburn]

I think it is the same with many of the component parts of old equipment. The microstructure of the components (whether capacitors or [eg] brittle plastic fittings and whiskering in transistors) degrades and, although a set may well work apparently well, it may be increasingly fragile in a number of ways. We find that, perhaps with increasing frequency, in more modern (throw away) equipment. At least with valve equipment the valves are fairly robust so handle a degree of abuse, both physical and electrical.

[Argus25]

To a great extent I'm not very well disposed towards electrolytic capacitors.

Here is an electrolytic horror story that I just worked through, it only took a couple of days work to figure out. Last week I pulled a mid 80's vintage color pal video monitor from storage for a project. The unit had hardly ever been used. The monitor is all transistor and IC based. The pcb's have a flat or matt looking green conformal coating. I think it is a little porous.

All of the boards looked brand new, as did all the electros physically, no visible leakage or domed tops. When I powered the unit up I found that the pal decoder circuit had malfunctioned. Testing revealed a 10uA leakage current between two tracks. This was affecting a high impedance circuit. Not only that, when I isolated the tracks, the behavior of them on a resistance meter was like that of a leaking electrolytic capacitor with an initial lower value resistance falling back with time, despite the board looking absolutely perfect to visual inspection.

I realized electrolyte must have leaked out of a capacitor nearby and have impregnated the matt coating of the board surface, but optically invisible. I removed a nearby 100uF 25V electro, is rubber base was shiny looking and putting the meter probes on the rubber , the rubber base conducted proving there was electrolyte on the outside of the capacitor body !

So I washed the pcb with a trichloroethylene like contact cleaner, no result, still inter- track leakage.

Then I remembered that more things dissolve in water than any other solvent, and to dissolve the electrolyte it would need a polar solvent. So I ran hot water over the pcb for 15 min, then dried it. Success, the ionic molecules in the pcb coating now leached away and the fault fixed.

I think one can never overestimate the destructive effects of old electroltyic caps in all types of gear, even if they haven't had much use. I didn't recap the entire monitor though, just all of the same brand of 100uF 25V units, there were 5 of them, which I replaced with high temp Panasonic parts. I found I could buy radial lead Tant caps of this value, but they were $12 each compared to $1.20 for the electros.