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Components and Circuits For discussions about component types, alternatives and availability, circuit configurations and modifications etc. Discussions here should be of a general nature and not about specific sets.

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Old 13th Aug 2022, 2:59 pm   #1
Goldieoldie
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Default Plessey electrolytic

Hi
I recently replaced a 100 uf cap in a transistor radio
It fell apart on removal .
I was surprised how small the actual cap was
Why I’d they put them in such large cases ?
Cheers
Pete
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Old 13th Aug 2022, 3:09 pm   #2
G6Tanuki
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Default Re: Plessey electrolytic

There's a lot of benefits in standardising component profiles; if you are using automated PCB design and assembly it would be a right royal pain if you had to deal with loads of different sizes of parts.

Those Plessey electrolytics are, it has to be said, a pest though. Even if you find one with both the crimped leadouts intact and uncorroded it will still either be lying about its remaining capacity, or have developed an alternative career as a resistor.
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Old 13th Aug 2022, 3:17 pm   #3
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Default Re: Plessey electrolytic

Experience suggests that discussion on these Plessey electrolytics might best be used as a tutorial on how not to design a capacitor. Their sole merit is their colourful appearance.

Martin
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Old 13th Aug 2022, 4:02 pm   #4
mark pirate
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Default Re: Plessey electrolytic

These are a "change on sight" component, very common in late 50's onwards transistor sets.
I have had a RobertsR200 in for repair recently, all five were leaky or open.
I suppose you could restuff them, as they are easy to get apart



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Old 13th Aug 2022, 4:41 pm   #5
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Default Re: Plessey electrolytic

Plessey were well known for "budget" components in the 50's through to the 70's for the domestic radio and TV industries and although some other budget parts have lasted better they certainly were not designed to last this long.

Plessey were certainly capable of making very high quality parts for military and aerospace use.

It would be interesting to know just how they arrived at this design, which is striking to look at and contained a number of interesting features ( there is never any doubt for the standard electrolytic which way round they went).

I suspect the riveted lead attachment was less than air tight allowing the electrolyte to dry out over time.

The yellow labels tend to fall of or crinkle up and look untidy.

I am sure someone thought it a good idea at the time and perhaps during a normal lifespan they were right .

I don't remember having to replace any in the 60's and 70's but that might have been different for a service department rather than a schoolboy with a hobby like myself.

I did have to replace a few electrolytic caps causing motor boating and other faults at that time but they were conventional metal cased types.

Anyone in the service industry may have a better idea about normal service life failure .

Cheers

Mike T
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Old 13th Aug 2022, 5:04 pm   #6
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Default Re: Plessey electrolytic

I guess that the kit in which we are seeing these Plessey capacitors fail must be very much the 'Long Tail' of survivors. Sure , they could have designed capacitors differently so that they had a service life of more than a couple of decades, but that would have pushed up the cost, just as Hong-Kong and Japan were undercutting the UK electronic industry.

Value Engineering was a thing back even in the 1950s, sure, Plessey could produce high quality parts to go into military stuff or telephone exchanges but even their telephone exchanges usually only saw a service life of 20 years... So designing for a longer life was a waste.
Let's not mention the Tantalum electrolytics fitted in Plessey built Clansman military radios...
Even the Mullard / Philips blue sleeved electrolytics (C280 series) go soft after 45 years - I have just been replacing a bunch of them in a high-band Pye Westminster W15FM.
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Old 16th Aug 2022, 12:57 pm   #7
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Default Re: Plessey electrolytic

I wonder which of the constructional features is responsible for the high failure rate? According to patent GB757804 (granted to Reginald Hipperson in 1956) the novel aspect was the use of two thermoplastic housing components fusion-welded together by a metal interface ring (which we can see in the picture above) induction-heated during assembly. This is a widely-employed jointing method that offers reliable hermetic seals, however might be subject to fissuring or other defects depending on how it was carried out.

Riveted leadouts are standard in electrolytics with rubber-bung ends, which are the reference against which we are comparing the Plesseys. However the sealing of the rivets may be impaired by lack of elasticity in the thermoplastic body as compared to rubber bungs?

As for the large case with a small internal element, the question is whether there was a smaller case size that would still have accommodated it, and if so, whether perhaps the element was reduced in size compared to the design spec used to select the case size initially.
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Old 16th Aug 2022, 1:07 pm   #8
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Default Re: Plessey electrolytic

Quote:
Originally Posted by Lucien Nunes View Post
Riveted leadouts are standard in electrolytics with rubber-bung ends, which are the reference against which we are comparing the Plesseys. However the sealing of the rivets may be impaired by lack of elasticity in the thermoplastic body as compared to rubber bungs?
I've seen some of those Plessey electrolytics where the stamped lead-out tag has clearly been distorted by the rivet that goes through the plastic cap to connect to the internals, so much so that there was a clear gap in one part of the circumference of the rivet-to-ring interface - meaning a distinctly less-than-airtight seal.

I wonder if perhaps the induction-heating used to fuse the two end-caps would have transferred much heat to the capacitor's internals, thus essentially priming them for subsequent failure? "Normal" aluminium-jacketed electrolytics with the end-cap being crimped/swaged in-place would not have had their active chemistry exposed to such process-heat.
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Old 16th Aug 2022, 5:37 pm   #9
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Default Re: Plessey electrolytic

Well well - at least it makes re-stuffing a bit easier if you’re that way inclined

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Old 16th Aug 2022, 6:12 pm   #10
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Default Re: Plessey electrolytic

I wonder if the capacitor inside was larger when it was made and has shrunk to the size it is now as it dried out?
I don't remember having to change them when the sets they were fitted to were still 'current' and judging by the fact that most sets still seem to have the originals in place it looks as if they weren't a problem when the radios were still in service?
Next time I see one I will test it and take the cap apart to see what the inside looks like.
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Old 16th Aug 2022, 7:46 pm   #11
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Default Re: Plessey electrolytic

Maybe it is a 'one size fits all' in respect of the case versus capacitance / voltage rating. Rather than having 10+ different mouldings they went with the largest case and anything smaller just went inside with plenty of air.

I have removed a few recently from restorations so I can add to the impromptu 'survey'
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Old 17th Aug 2022, 8:52 am   #12
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Default Re: Plessey electrolytic

I had never before realised the two plastic halves were welded by induction heating a metal ring. Cunning!

I doubt it heated up the internals much.
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Old 17th Aug 2022, 1:35 pm   #13
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Default Re: Plessey electrolytic

It was a use for the valve getter firing coil as valves were being phased out at the time.
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Old 17th Aug 2022, 7:19 pm   #14
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Default Re: Plessey electrolytic

Quote:
Originally Posted by alfatangowhisky View Post
Maybe it is a 'one size fits all' in respect of the case versus capacitance / voltage rating. Rather than having 10+ different mouldings they went with the largest case and anything smaller just went inside with plenty of air.

I have removed a few recently from restorations so I can add to the impromptu 'survey'
Well there is my theory blown out of the water. The two I had lying around were of different sizes. The smaller one had lost its wrapper so the rating is unknown. I would still expect them to have a limited range of case sizes to economise.

The bigger one popped open quite easily but the little one was built like a tank and I haven't managed to pull it apart.

Inside the big 'un the contents were visibly damp (I didn't want to touch it to confirm) but of interest was the fact that positive lead appeared to be eaten nearly through (by the electrolyte?). Regrettably I did not test the capacitance before the investigation.
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Old 17th Aug 2022, 7:22 pm   #15
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Default Re: Plessey electrolytic

Quote:
Originally Posted by Goldieoldie View Post
Hi
I recently replaced a 100 uf cap in a transistor radio
It fell apart on removal .
I was surprised how small the actual cap was
Why I’d they put them in such large cases ?
Cheers
Pete
Seems my 350uf and your 100uf share the same case size? That's why the contents of yours look so out of place in the big body!

Last edited by alfatangowhisky; 17th Aug 2022 at 7:23 pm. Reason: Cap is 100 not 200uf on further study
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