The 280 pound capacitor

I have a 280-pound capacitor, four of them in fact. Well, they must weigh something in that vicinity. They cost $500 each, including pallet shipping.

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 Thanks, 
    - Win

Hi all,

I tore down a Marconi signal generator today. It's been awaiting my attention for quite a while. Can't recall the model number off hand but it does 10kHz to 5.4Ghz IIRC. I bought it from some chap who told me it had a faulty smoothing cap in the PSU 'cos it was generating signals with ripple on it. He told me he'd been quoted GBP280 ($387 in US dough as of today's date) for a new replacement from Marconi and I bought it on that understanding. Anyway, I tore it down today and located the said capacitor. Here it is:

public/

This is the only pic that came out for some reason, but it's got most of the important info on it. You can't quite see, but it has 5 terminals for some reason, but on the board only 2 of them are connected. It's gone seriously low-res internally, BTW, so *does* need replacing.

Questions: what makes this thing so special as to cost so much? Why have the designers used such a huge capacity cap in this low current drain application? If I can source a generic electrolytic of the same spec or better for <

30 quid, why should I not use that instead of the bespoke replacement??

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I've come across these type of 5 terminal capacitor before. 3 pins are just for mounting and are not used in circuit. (Are they not stamped with an "x"?)

Gareth.

On a sunny day (Thu, 8 Jun 2017 17:28:02 -0000 (UTC)) it happened Cursitor Doom wrote in :

Nothing, it is a crappy old Philips, the contacts to the pins go wrong too. Just replace with some caps with right capacitance / voltage and sintered wires, not that crap.

At a guess - exact replacement parts might no longer being made, the equipment manufacturer has a small remaining stock, there may be no other source. Some owners of the equipment (e.g. military and some businesses) may have an "exact replacement only" policy for spare parts, to avoid the need to send equipment through a formal requalification process.

So, Marconi can charge that much for a cap, because there are people willing to pay it (rather than scrap the whole piece of equipment).

Might be "because they could". Or, possibly, some of the downstream circuitry might have poor power-supply rejection, and having a truly huge filter cap might be the only way to get ripple-related noise and sidebands down low enough to meet the device's specs. They might also have figured that this part might be prone to degrade over the years (as it apparently has done?) and they installed one of larger- than-initially-required capacity to stave off the effect of this aging and degradation.

The extra hold-down terminals might be needed in order for the device to meet its reliability specifications, when installed under conditions of high vibration and possible acceleration shock (e.g. in military installs, on boats or airplanes). Without the additional pins soldered to the board, vibration could result in the cap shaking back and forth, with all of the stress placed on the two solder joints (and the PCB traces) resulting in stress cracking.

A standard modern cap of the same capacity and voltage rating, and equal or better temperature and lifetime specs, is likely to be a good deal lighter than the original. If you can find one which fits the connection terminals, and don't mind the fact that it might break loose if you use the equipment in a bomber that's flying through intense flak explosions for months on end, I suspect it'd work out just as well for you.

Insane ripoff. Good reason to never buy Marconi.

Looks like you ripped out the hole plating on one pin. With luck, it will be one of the passive mounting pins.

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John Larkin         Highland Technology, Inc 
picosecond timing   precision measurement  

jlarkin att highlandtechnology dott com 
http://www.highlandtechnology.com

Sounds like the $ 100,000 diode for the military that could have been replaced by a diode that cost less than one dollar except for the military spec. Seems the military supply depot did not have any. The company that made them did not have any,so they had to make one. Could not make just one, had to do it in a large batch. Probably made 10 to

20 thousand of them.

Fortunately it is. :-)

Many years ago, my Tek 7603 failed to start. I pulled the power supply out and was driven nuts by a simple DC voltage regulator problem in the power supply. A bypass electrolytic capacitor would have been the obvious soluti on, except this scope used at least a half dozen extra large Mallory built capacitors in parallel, and there's no way they all died together. Adding a bit of external capacitance though brought the voltage right back and the scope to life. Turns out those big caps were dropping out one by one over the years and gave no indication of anything going wrong as they did, unti l the very last one opened when the supply went out.

Why did I mention all of this? Because I just removed those big Mallorys a nd stuck in some standard electrolytics of maybe half the total value and t aking up about a tenth of the physical area of the originals, and the scope still runs daily with a perfectly clean and stable trace.

In other words, I doubt you'll see any difference by doing what you instinc t tells you. That cap may be very low ESR, have special impedance specs or ripple current specs, but I'd be stunned if it makes any real world differ ence with off the shelf caps. If it were mine, I'd use Panasonic FR series caps.

When Tektronix had a base in Guernsey, Channel Islands, thay adopted the following spares procedure. Each year, divide the stock by half, sell off that half at auction, then double the price of what they kept in stock.

And this process led to their going out of business sale?

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Rick C

Marconi Instruments were hot on vibration tests since they're key to reliability in military use. Competitor equipment often failed their tests.

As well as what has been mentioned, a big cap would presumably help ride over an arcing mains connection, giving reliable service where a lesser device would cause malfunction.

As said if you're just using it on a bench you can put whatever cap you like there. It won't be a low ESR type on a 50Hz PSU. If you glue it down it will improve its shock/vibration resilience, but not to match the original marconi & mil specs.

NT

Probably because they got a bunch of large value weird-ass caps cheap and that's what they use in everything. Like a guy who asked me why they used a 1N4002 in this one mass-produced rack effects box when a 1N4001 would've been fine from a ratings perspective and it's cuz "that's what they use in everything"

They're like 5 bucks:

At 280 lbs, it would take several big men to move the thing. (Or a forklift). Not the kind of thing you can just replace on your work bench, because the bench would probably collapse.

The physically largest capacitor I ever saw in person was a PIO type rated IIRC for a couple of uF at several kV; it weighed about as much as a bowling ball and was about the same size

Our 170 pound energy discharge capacitors, each 70 uF at 12 kVDC:

On a sunny day (Fri, 9 Jun 2017 08:05:20 -0400) it happened bitrex wrote in :

In the sixties I worked in a company that made HV transformers and equipment for power stations, railways, etc, now the caps I have seen in the HV test room were alsmost as big as me. Soem of the transformers required a ladder to climb on those. The caps looked a bit like these:

Dangerous place...

You realize the OP was referring to cost (280 pound sterling), not weight. If you're making a joke, the second poster beat you to it.

47,000 uF 16v. You should be able to find that in a physically smaller package.

Carefully remove the base from the capacitor, preserving only the base and the can. If you're really careful, you might be able to also save the vinyl insulator. Tear out the guts and throw it away. Install the replacement physically smaller capacitor inside the can, connecting the capacitor leads to the base to match the original. Solder it back onto the PCB and you're done.

If you don't care if it looks like the original, forget the aforementioned process and just solder the replacement cap to the PCB in place of the can in any manner that will fit.

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Jeff Liebermann     jeffl@cruzio.com 
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I've worked with capacitors bigger than that, although I think they were in sections so maybe it's not technically true to say "bigger capacitor" (singular). :^) Ratings were around 100s uF, 2000V, lots of amps.

Tim

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Seven Transistor Labs, LLC 
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