Live voltage on a capacitor can - why? - Page 1
Forum section: electronics / projects

author: lasseo
author_rank: Contributor
timestamp: December 06, 2023, 10:56:56 am
content: It is not broken, it seems very much intentional. Reasonable amount of effort seems to have been spent on connecting the capacitor minus leg to the protective canister. What's the benefit of that? A stab at reducing EMI?Here's a funky looking Sprague capacitor off a Tektronix 2213 oscilloscope power supply where exactly this has been done, complete with warnings about dangerous live voltage stamped on the can. A mounting ring with solder lugs at the bottom of the case is soldered to PCB in three locations around the bottom, and a strip soldered to the - (minus) pin of the capacitor. The + pin is connected to the positive rail. It appears to be a filtering cap, connected right after a bridge rectifier feeding a switch mode power supply. This scope was first released in the early 1980's, so it would have been designed in the late 70's. That's not old enough to be hangover from days when multiple capacitors were stuffed in a single can. It also was a product that was designed to a price point, so going away from standard parts would have been done out of necessity; the bean counters would have seen to that.So, to what end would anyone want to do this?

author: lasseo
author_rank: Contributor
timestamp: December 06, 2023, 10:59:26 am
content: I'll post some photos once I can, for some reason I can't seem to upload them right now. Yes they are below 4 MB in size.

author: tggzzz
author_rank: Super Contributor
timestamp: December 06, 2023, 11:02:41 am
content: Quote from: lasseo on December 06, 2023, 10:56:56 amYour first port of call is to get the service manual (from any of the usual places), identify the capacitor, and locate it on the schematic.That will show you the all-important context, and should give big hints about why it is connected that way.Feel free to post the relevant bits of the schematic, and ask further questions.

author: tggzzz
author_rank: Super Contributor
timestamp: December 06, 2023, 11:05:52 am
content: Quote from: lasseo on December 06, 2023, 11:00:23 amI don't see any warnings about dangerous live voltages on the can.As with all capacitors, I do see a statement of the maximum rated voltage across the capacitor's pins.

author: ArdWar
author_rank: Frequent Contributor
timestamp: December 06, 2023, 11:17:45 am
content: Quote from: lasseo on December 06, 2023, 10:56:56 amWhile this particular one isn't a multiple caps it's definitely manufactured in similar form factor. CAN NEG is quite common for that style (originally/usually COM NEG or CAN COM NEG).

author: coppercone2
author_rank: Super Contributor
timestamp: December 06, 2023, 11:52:51 am
content: I don't get it. It looks like a RF ground for the capacitor can. That seems normal.Are you saying that tab is connected to mains and the pin to ground? If its a +- rail, then pos would be ground

author: SeanB
author_rank: Super Contributor
timestamp: December 06, 2023, 01:56:44 pm
content: Most electrolytics have the can connected via the electrolyte to the negative end. For axial capacitors there is a hard connection via the internal tab of the negative foil, but for radial capacitors this is mostly incidental. For this one the manufacturer decided to bring out the negative lead separate from the case, likely because they were reusing an old 3 terminal leadout gasket and phenolic base, and wanted to not have a long tail that was folded into the capacitor, which reduces the volume internally available for the paper and foils, and thus decided to have the extra volume filled, and use short tails to connect. However the OEM wanted a capacitor with case connected to negative, so they simply went and modified the outer washer used on the crimp, there to provide an extra layer of rigidity to the seal during closing, and which also has the case mounting pins or tabs on the steel. So now it has a tab that is soldered to the negative pin, so connecting to firmly to the aluminium case, but the small steel tab also connects negative to the 3 mounting pins as well. After sealing they simply pressed the tab down over the small length of pin in the insulation, soldered it down, then trimmed it off, looking like it was ground off automatically with a small carbide wheel.Just a decision made by the OEM, on order from the buyer, who wanted a semi custom capacitor with high capacitance, high voltage withstand, but in a smaller can than standard for that value and voltage, and also wanted to have lower ESR and higher ripple current spec than standard. Sprague was quite willing to do semi custom orders, especially for the manufacturers who had large exclusive order volumes with them, as this also menat a large degree of vendor lock in, and as well the OEM had a now non standard part they could sell into the repair market, knowing they could charge a premium for a part that was, to them, a very low cost item.

author: CaptDon
author_rank: Super Contributor
timestamp: December 06, 2023, 02:20:58 pm
content: Yes, can as negative is very common! As for voltage warning, That capacitor could be part of a line operated voltage doubler circuit common in older 'line operated transformerless' television sets. Usually the 'hot' can had a cardboard sleeve around it and these were also classed as a 'hot chassis' television. The capacitor was in fact at line A.C. voltage as well as a D.C. voltage referenced to chassis. Those 'cans' were also usually mounted on a bakelite wafer to insulate them from the chassis. A very interesting thing about the 'series capacitor voltage doubler' on the Sylvania / G.T.E. D19 chassis is that all of the line current to operate the T.V. was going through the capacitor and being of a very sturdy construction with a vent hole in the bottom which got occluded from crystallized dry electrolyte would cause the can to explode violently due to heat from E.S.R.! I have seen them snap the neck off of the C.R.T. and a saw one where the can broke near the bottom flange and shot the empty can straight up and completely through the plastic back of the television cabinet!! The owner (a family friend B.T.W.) carried in the T.V. and the 'missile' wondering what in the hell happened and would it happen again? He said the bang was so loud he nearly crapped his pants and was afraid to go near the T.V. due to the smoke and stink!

author: lasseo
author_rank: Contributor
timestamp: December 08, 2023, 07:38:32 am
content: Quote from: tggzzz on December 06, 2023, 11:05:52 amYou are right of course. I must have been staring at the high voltage warnings on the main board for so long that the component specs on the side of the can read like "Danger Will Robinson!" in my head.

author: lasseo
author_rank: Contributor
timestamp: December 08, 2023, 08:15:50 am
content: Quote from: tggzzz on December 06, 2023, 11:02:41 amYes Sir. Unfortunately, none of the usual suspects seem to have scanned the portion where the schematic for this sub-board would be shown. It is, however, shown in the block diagram of the scope, and the service manual states that either the current limiting board is fitted, or the preregulator board that my scope has, but not both. I've highlighted the section in the excerpt of the block diagram:Another clue comes from a field service bulletin that describes retrofitting power supply improvement ("Option 48") to existing equipment, to "make it less sensitive to power line transients". That makes sense, at 6.1 kg this is a relatively light scope that service personnel was meant to be able to take with them, so power line quality may have been compromised. Looks like mine came fitted with that option straight from the factory.

author: lasseo
author_rank: Contributor
timestamp: December 08, 2023, 11:27:27 am
content: Here's a pretty good photo of the location. The Sprague is located at the very back, lurking in between the two pink connectors can be seen at the top of the image on rkagerer's scope. The preregulator board is installed above the main board, with the components hanging down:https://www.eevblog.com/forum/repair/tektronix-2213-repair-leaked-capacitor/msg2685186/#msg2685186As you can see it sits relatively close to a transformer. According to the schematic, it's T942, which is "Transformer,RF:driver saturating pot core". I don't know what that is, but maybe that could be something that the cap has to be protected from like coppercone2 suggested?

author: coppercone2
author_rank: Super Contributor
timestamp: December 08, 2023, 10:00:10 pm
content: if it saturates that means that the field is at maximum. I think they leak more and more when they get saturated because its not like the magnetic field disappears, it just can't go through the core anymoreLike you increase current past saturation... its still increasing the current through the loop right? then its like a over heating air core inductor I think.

author: lasseo
author_rank: Contributor
timestamp: December 13, 2023, 11:30:30 am
content: Quote from: SeanB on December 06, 2023, 01:56:44 pmThis explanation seems quite plausible. As you can see it's pretty tight on the board, with the can very close to the rectifier bridge:

author: lasseo
author_rank: Contributor
timestamp: December 13, 2023, 11:49:59 am
content: The question follows, should I try to replicate the set-up when substituting the Sprague for a modern capacitor? Of the reputable suppliers, at least Chemi-Con and Rubycon have suitable replacements, and the can size is considerably smaller than this one. But how best to go about it.First option, I could conceivably take a leaf off vintage radio enthusiasts' book and stuff the old can with a modern cap, still utilising the old can as the presumed RF shield. There would be an air gap between the old and the new can, so there is at least some risk of overheating. If I fill the gap with potting compound, I will unlikely be able to re-use the can as a shield again. That could well be an in-fifty-years-time type of problem, but stillSecond, I could just recycle the phenolic base of the old cap as a socket, making for easy connections to the double-sided board, and figure out some other way to shield the cap. Suggestions how to do the shielding are most welcome.Third, my least favourite for the time being, would be to just drop in a modern capacitor, add a couple of jumper leads and hope that the resulting EMI or noise degradation will not be noticeable in use. It's just thathoping for the bestdoesn't ring like a robust fix to me...Thoughts?

author: tggzzz
author_rank: Super Contributor
timestamp: December 13, 2023, 05:29:05 pm
content: Quote from: lasseo on December 13, 2023, 11:30:30 amYou probably want to replace that RIFA, before it lets everybody in the room know it needs replacing.

author: lasseo
author_rank: Contributor
timestamp: December 14, 2023, 01:48:53 am
content: Quote from: tggzzz on December 13, 2023, 05:29:05 pmYes! There's four of them on that board, all in various states of developing cracks, and a fifth slightly bigger one on the main board. From what I gather the flash-bang can be quite spectacular.

author: MathWizard
author_rank: Super Contributor
timestamp: December 15, 2023, 11:02:37 am
content: This is a reminder to me to not touch anything inside mains powered stuff with hands or fingers, or gnd clips, etc. You can't just assume it's safe. At some point you might, but better check 1 or twice even on working stuff.And with old vintage equipment, watch out for previous DIY repairs that might be dangerous.

author: lasseo
author_rank: Contributor
timestamp: December 18, 2023, 08:04:02 am
content: Quote from: MathWizard on December 15, 2023, 11:02:37 amYes. I have seen some - luckily not dangerous, let's just say craftsmanship-challenged.Curiosity got the better of me, so I opened the capacitor can. Carefully pried open the edge and removed a retainer ring, the three-pronged leg, and the phenolic base, to be treated to a whiff of something that smelled like a combination of sulfuric and citric acids.

author: lasseo
author_rank: Contributor
timestamp: December 20, 2023, 12:17:52 pm
content: Does anyone have experience with this or a similar product? It is a "re-enterable" potting compound, which supposedly means it is possible to get components out of it if a need arises. I should think using a filler is much better thermally than leaving a couple of mm air gap between the can and the infill replacement capacitor.https://www.raytech.it/en/product/low-voltage/fillers/gel/magic-gel

author: coppercone2
author_rank: Super Contributor
timestamp: December 20, 2023, 05:08:52 pm
content: what we need is capacitors that have a line voltage switch on top. so you twist it to wind it tighter or looser to set the voltageit might be possible

author: lasseo
author_rank: Contributor
timestamp: December 21, 2023, 12:24:35 pm
content: Quote from: coppercone2 on December 20, 2023, 05:08:52 pmOohhh, a bit like the tuning capacitors on vintage radios, but with a screwdriver slot for adjustment?

author: Gyro
author_rank: Super Contributor
timestamp: December 21, 2023, 12:59:19 pm
content: Quote from: lasseo on December 20, 2023, 12:17:52 pmIt's being used increasingly in outdoor mains jointing boxes. You can get them pre-filled so that the connector sinks into the top and bottom halves when clamped closed, but here's an example of using the raw fluid compound (scroll forward to 13:10 if you want to save time)...You could always do what the Chinese fakers do and just fill the space with dry sand.To be honest though, just inserting enough non flammable packing, or a couple of blobs of neutral cure silicone, to prevent the new cap from rattling is probably fine.

author: coppercone2
author_rank: Super Contributor
timestamp: December 22, 2023, 02:13:11 am
content: I thought the sand dampen microphonics?I imagine the ultimate capacitor would have a damper of some kind around it, like a rubber shell.

author: amyk
author_rank: Super Contributor
timestamp: December 22, 2023, 02:33:28 am
content: Quote from: lasseo on December 18, 2023, 08:04:02 amLikely boric acid.

author: Gyro
author_rank: Super Contributor
timestamp: December 22, 2023, 12:52:22 pm
content: Quote from: coppercone2 on December 22, 2023, 02:13:11 amIn the faker situation it is there to add convincing weight and make the can sound solid with the tiny cap that they put in there. In this case it would just be something easy to settle into the narrow gaps and pour out if the OP needs to replace the cap again in future. It would probably add some degree of thermal conductivity [better than air anyway], that he mentioned, too.

author: lasseo
author_rank: Contributor
timestamp: January 12, 2024, 04:05:37 am
content: Quote from: Gyro on December 21, 2023, 12:59:19 pmCheers! Encouraged by this video, I ordered a 300 ml kit of the Magic Gel. While opening the box, I was a little disappointed to see it didn't come with its own beaker or a mixing spoon like shown on the brochure pictures. Presumably the accessories only come with the 1000 ml kit, the smaller is all BYO. Oh well.I decided to do a little test before putting the gel to use, so I mixed a small amount in a paper cup and stuck in a plastic sleeve I had lying around. I let it set for a bit longer than the instructed 10 minutes - et voila! The consistency of the cured product is reminiscent of Aeroplane Jelly - perhaps made with a little less water than usual - and the colour is close to the Blue Heaven flavour. There is no shrinkage, no odour, and it stuck to the sleeve well enough to keep it in check while I was poking around, but not so well as to be impossible to pull it out. Just like a spoon stuck in Aeroplane Jelly - perfect!

author: lasseo
author_rank: Contributor
timestamp: February 03, 2024, 07:26:18 am
content: I gutted the old capacitor and cleaned the can and drilled holes through the connectors on the existing base. I pushed the legs of the replacement Rubycon replacement capacitor through the old base, and glued it down to the base using neutral cure RTV silicone. I let it sit overnight just to be sure, and in the meantime figured out the volumes of the old and replacement capacitor cans. This was so I knew how much to pour in. I wasn't keen for the gel to be squeezed out while I'd be pushing the new cap and the base back into the can. I filled the old can to some 60% then quickly inserting the replacement and clamped down the base to the can. Once the Magic Gel is mixed its open time is only one minute, so no time for stuffing around - literally and figuratively.

author: lasseo
author_rank: Contributor
timestamp: February 08, 2024, 11:01:25 am
content: Some more photos. Once the gel had set I placed the mounting ring back on the base, carefully observing the orientation so that the capacitor legs would fit through the hole pattern on PCB - and that the cap had the correct polarity before soldering the negative leg onto the ring. I then rolled (well mangled more like) the lip of the canister back around the base, before fitting what looks like PP sleeve and cap back onto the canister. I had to resort to a heat gun while getting the guts of the old capacitor out, and the sleeve expanded just enough to slip off but luckily no damage. I then used a soldering iron set to low temperature to fold the edge of the sleeve back over the lip. The craftmanship here is not my best work, but the end result looks pretty reasonable, and if the can is going to do anything for the EMI protection I think this will be fine.

author: coppercone2
author_rank: Super Contributor
timestamp: February 12, 2024, 08:43:02 pm
content: I was scared of it not venting properly if it was seated in the old cap like sealed in. What I did with mine is drill a hole in the base of the old cap,solder the new cap in place by wrapping wire the through hole leads on the solder posts on the bottom, then I put epoxy putty around the perimeter from the bottom, and poured liquid epoxy in maybe 1cm deep from the top (so a segment of the top was flooded with epoxy). This way the new cap was firmly anchored in the base of the old cap (with glue on the swaged area) but the top was completely unrestricted so it can vent properly.Granted you lose most of the can if you do this and it looks weird. If I had to put the old cap in the new cap and have it look original, I would cut a big hole in the top of the old cap and glue in a stainless mesh from the bottom, so if it vents it can vent nearly freely.but mine had a solid metal top (it was a solid metal can). IDK if you have some kind of plastic baffle there?I know for a fact that potting capacitors totally is a recpie for shockwave during failure, you are supposed to put a tube over it so the vent can breach freely. The burst disk is designed to vent it gracefully, if you obstruct that it goes boomI am curious how well a tinned copper mesh would solder to the capacitor body (mine was not shielded like yours) if you actually wanted EMI shield. It should be good to the GHZ with a fine mesh that is soldered or conductive epoxy on.

author: lasseo
author_rank: Contributor
timestamp: February 23, 2024, 04:43:18 am
content: Quote from: coppercone2 on February 12, 2024, 08:43:02 pmThe cap is solid, the plastic baffle is part of the PP sleeve that goes over the aluminium cap. The new cap is potted over where it's supposed to vent, so yes there probably is some risk of it going boom after the rated 12000 hours of use. I'm rather counting on (hoping?) the jelly-like consistency to give up and the bottom plate to be fitted loosely enough so it'll go PFFfffftttt... rather than BANG!!!, but it is not guaranteed. I didn't really want to drill holes because I had no way of knowing whether that would change the EMI protection.Quote from: coppercone2 on February 12, 2024, 08:43:02 pmI had the same exact thought - pull a mesh sock over it like you have the shielding in a coax cable. However, I was worried that the copper mesh sleeve would end up unruly. There's a bridge rectifier right next to the capacitor so the fit had to be pretty tight.

author: DavidAlfa
author_rank: Super Contributor
timestamp: February 23, 2024, 04:58:56 am
content: The multiple legs are just to provide mechanical support.

author: lasseo
author_rank: Contributor
timestamp: February 24, 2024, 07:33:40 am
content: Quote from: DavidAlfa on February 23, 2024, 04:58:56 amOne would think so, yes. However, it is a double-sided board, and each of the ground ring legs also had a trace coming off them leading elsewhere on the board. Perhaps ground planes weren't a thing back then.

author: DavidAlfa
author_rank: Super Contributor
timestamp: February 24, 2024, 10:19:35 am
content: Yes, I've seen a lot of that on 80s equipment, using the cap legs as part of the trace. It's a free jumper after all.