Replacing 5000V paper capacitor - Page 1
Forum section: electronics / beginners

author: Thief
author_rank: Contributor
timestamp: November 21, 2020, 04:10:38 pm
content: I need to replace a 5000V 6.8nF paper capacitor in a CRT power supply, but I'm having difficulty finding anything over 3000V rating (and I don't trust the specs on those Ihavefound).Would I be ok to use series 3kv caps to build a capacitor array? As I understand it, voltage rating adds in series and capacitor rating divides, so two 3kv caps of twice the rating should be ok?Has anyone done this? Or can source a 6.8nF cap of >5000V rating in the UK?I've seen recommendations to use bleeder resistors to balance series caps, would you guys agree? Or is that not generally necessary?

author: pvsage
author_rank: Contributor
timestamp: November 21, 2020, 05:18:20 pm
content: In general, bypassing series capacitors is necessary with electrolytics, where the high tolerance and ESR can cause one cap to see the majority of the voltage and exceed its voltage rating. From what I hear, this isn't *usually* needed with film capacitors, though some would argue it's a good idea. The fact that you're dealing with a 5KV supply complicates the issue - resistors' voltage ratings aren't commonly banner specs. Years ago I was shopping 1W mil-spec resistors, and I recall their maximum voltage rating was something around 600V.Speculation: Carbon comp resistors may have higher ratings than film resistors due to their physical construction. (Film resistors have a spiral groove cut into film on the surface to increase resistance, while carbon comp resistors are essentially solid graphite slugs.) This merits further research.

author: Thief
author_rank: Contributor
timestamp: November 21, 2020, 05:34:06 pm
content: I don't think the original cap is electrolytic (I'm not too familiar with old paper capacitors) so if I use a non-electrolytic replacement I guess it'll be less necessary to add resistors.Edit: it just occurred to me that I have the original schematic so I should check if it already has a resistor across the original cap - and if so, it might work to put two and split it for balancing - depending on valuesEdit2: though as you say that could be a problem at this voltage, so it probably doesn't

author: Gyro
author_rank: Super Contributor
timestamp: November 21, 2020, 06:50:51 pm
content: No, at 6.8nF it wouldn't have been an electrolytic, just a high voltage paper one.You should be easily able to do it with 2 series Polypropylene film capacitors. These are commonly available up to 4kV (commonly used for 720V AC applications).You don't say where you are based but,for example:https://uk.rs-online.com/web/p/polypropylene-film-capacitors/6351972/Two in series would give you 7.5nF which is close enough, particularly when replacing a wide tolerance paper one.It's very debatable whether you would need balancing resistors in this case - as long as the junction is well insulated then the caps ought to have close enough leakage current to share the voltage reasonably equally (unless you put a meter on it!). If the original cap was rated at 5kV, then the actual HT voltage is probably closer to 4kV anyway.High voltage, high value thick film resistors are available reasonably cheaply, but probably still more expensive than the caps.

author: Benta
author_rank: Super Contributor
timestamp: November 21, 2020, 08:02:08 pm
content: I question whether you really need a 5000 V cap for your CRT.The only place I can imagine for that type of capacitor is in the horizontal output stage, specifically the flyback capacitor, aka, "safety" capacitor. It is named thus, because if it fails, the whole output stage literally blows up (I've seen it happen!).It normally sees up to 1000 V max. during normal operation, depending on CRT size etc.Paper types were extremely overspecified, as they are not self-healing. This explains the 5000 V rating in your CRT.Modern sets use polypropylene (PP) or PPS for the safety cap, often two or three in parallel for application reliability.The main failure mode is not capacitor failure, but solder joint fatigue (the horizontal stage is quite hot, resulting in temperature-cycle cracks at the joints).Film capacitors are self-healing, meaning you can go with a lower voltage rating without worries.I'd go with something like the Vishay/Roederstein KP1836 series at 2000 V.EDIT: it just came to me, that I've no idea whether the OP has an "oscilloscope" type of CRT, or a TV/monitor type CRT. If it's an oscilloscope, please disregard my above comment completely.

author: David Hess
author_rank: Super Contributor
timestamp: November 21, 2020, 08:04:33 pm
content: Would need to see the schematic to decide if adding balancing resistors is feasible. They make special high voltage film resistors for this sort of thing like the Vishay VR and HVR series.

author: bdunham7
author_rank: Super Contributor
timestamp: November 21, 2020, 08:42:47 pm
content: IDK about in the UK, but Mouser here shows these, for example.https://www.mouser.com/ProductDetail/Nichicon/MWS2631680KO?qs=sGAEpiMZZMsh%252B1woXyUXj%2FgcXCpI1SPdPSqT%252BADhaiU%3DThat one is miscategorized as an electrolytic, it is actually a polyester film cap.https://www.mouser.com/ProductDetail/WIMA/FKP1Y016806D00MSSD?qs=sGAEpiMZZMsh%252B1woXyUXj17xhsgMjaIf3ZzL8ETDCtw%3DThe photo says 2kVDC, but the datasheet actually confirms that it is a 6kVDC rated cap.

author: Thief
author_rank: Contributor
timestamp: November 22, 2020, 07:16:42 am
content: Quote from: Benta on November 21, 2020, 08:02:08 pmIt's an old 15 MHz oscilloscope - A Philips 3231. Absolutely huge.I looked at the schematic and the capacitor seems to be wired between 3000V and ground. So a 3000V cap might be ok? The faulty cap is C842 in the schematic. Top right of the first picture, bottom right of the 2nd.Pictures from schematics:

author: exmadscientist
author_rank: Frequent Contributor
timestamp: November 22, 2020, 09:11:27 am
content: They're a US supplier, so I have no idea how painful they are to buy from in the UK, but Allied Electronics tends to carry a better selection of HV capacitors than the bigger distributors. Their website is quite unpleasant to use, though.This partlooks like a good match for your application, though they have others. I can personally vouch for that line's quality: we've recently ordered several similar ASC caps from Allied and they've withstood moderately severe abuse without any trouble.Whether you end up paralleling parts or not, a bleeder resistor across any HV cap is good design practice. Anything that makes it harder to get shocked is worth doing! Even as much as a gigaohm can be all you need. Digikey has a handy calculator that makes things easy:https://www.digikey.com.au/en/resources/conversion-calculators/conversion-calculator-capacitor-safety-discharge

author: Gyro
author_rank: Super Contributor
timestamp: November 22, 2020, 11:42:46 am
content: Quote from: Thief on November 22, 2020, 07:16:42 amA 3kV capacitor on a 3kV rail isn't ideal, but this Kemet Polypropylene one is tested at 6kV for 10 seconds and rated for 100k hours at 3kV. Worst case, the 100k series resistor will fry (you should probably check it anyway).https://uk.rs-online.com/web/p/polypropylene-film-capacitors/3847723/It might be an idea to check the condition of C841 too (they're both Paper). 10nF will be fine for both of them.

author: David Hess
author_rank: Super Contributor
timestamp: November 22, 2020, 05:21:28 pm
content: That looks like the anode supply so there is a built in bleeder resistor in the CRT which controls the electric field gradient. Finding a suitable 4 kilovolt or higher voltage part should not be difficult, and a larger capacitance can be used; it does not need to be 6800 picofarads exactly.If you do want to add bleeder (or balance) resistors, then there are suitable inexpensive high voltage metal film and thick film resistors available.

author: TimFox
author_rank: Super Contributor
timestamp: November 22, 2020, 05:40:38 pm
content: I’m skeptical about not using equalizer resistors across series capacitors, even high-quality polypropylene film units with low leakage currents. The actual leakage could well be determined by surface leakage, and could be very different in the small amounts between two units. However, proper equalizer resistors would not need to pass high currents. As a numerical example, 1 nA leakage into a 15 nF capacitor is 1/15 V/sec, or 5 kV in 21 hours.

author: Gyro
author_rank: Super Contributor
timestamp: November 22, 2020, 06:29:08 pm
content: Agreed, I wouldn't have suggested it unless each capacitor was very close to the needed voltage rating in the first place. In practice, unless one capacitor was very dirty externally, they would be very unlikely to settle with all of the voltage across one capacitor. It would be very difficult to come up with any sort of confident rule of thumb though.As has become clear, luckily the OP can meet his requirement with a single capacitor. Otherwise it would have been a tricky solution, with high source resistance adding to the difficulty of achieving a cost effective solution involving balancing resistors, probably doubling the cost.

author: Thief
author_rank: Contributor
timestamp: November 22, 2020, 06:30:52 pm
content: Not the cheapest, but it looks like this is a 4kV cap that would do the job then:https://uk.rs-online.com/mobile/p/polypropylene-film-capacitors/6351950/10nF, 4kV, should be able to replace both high voltage supply caps (842 and 841 on the schematic).

author: Gyro
author_rank: Super Contributor
timestamp: November 22, 2020, 06:38:10 pm
content: Yes, that's the same family that I suggested in reply #3. It's a shame RS only sell them in packs of 5. I was congratulating myself in having found a capacitor that RS actually sells individually with the Kemet ones!It is a shame that the voltage rating is so close though.Farnell and CPC look like a dead loss on this one (Edit: Ironically, unless you want SMD ceramic!)

author: Thief
author_rank: Contributor
timestamp: November 22, 2020, 09:36:30 pm
content: Quote from: Gyro on November 22, 2020, 06:38:10 pmThat's part of what prompted me to make this thread in the first place! Haha.

author: Thief
author_rank: Contributor
timestamp: November 23, 2020, 02:07:31 pm
content: Hang on, would this cap work?https://uk.rs-online.com/web/p/ceramic-single-layer-capacitors/8313278/Tolerance isn't as good as the other (is that an issue given it was a paper cap before?), butit's available in singles.

author: drussell
author_rank: Super Contributor
timestamp: November 23, 2020, 02:30:48 pm
content: 6800 pF film capacitors at 5000 and 6000 volts are readily available. WIMA makes them, Illinois Capacitors makes them... They're used all the time in the vintage TV repair circles... Things like the Philco Safari are full of very high voltage paper caps that need to be replaced with modern film. You don't even need to sub in a doorknob ceramic or something... Places like Just Radios always stock stuff like this but they're much pricier than Digikey or Mouser when they stock 'em...What's wrong with this one? 6800 pF, 6kV DC, in stock at Digikey, $2.61 CAD ea:https://www.digikey.ca/en/products/detail/wima/FKP1Y016806D00JSSD/9370161A better link for you on the old Digikey system for the UK, £1.40 ea:https://www.digikey.co.uk/product-detail/en/wima/FKP1Y016806D00JSSD/1928-1206-ND/9370161

author: Thief
author_rank: Contributor
timestamp: November 23, 2020, 02:35:31 pm
content: Quote from: drussell on November 23, 2020, 02:30:48 pmNot to useful being in Canada when I'm in the UK, but it seems Digikey UK have a few in stock and only £1.40 each.Seems like a better option than rs-online.com had!

author: drussell
author_rank: Super Contributor
timestamp: November 23, 2020, 02:43:33 pm
content: Quote from: Thief on November 23, 2020, 02:35:31 pmLOL, you beat me to it... I added a direct link in the post above.You can switch countries in Digikey by selecting where you want in the upper right hand corner in the drop-down list or simply changing from digikey.com to digikey.ca to digikey.co.uk etc. They're all shipped from the same warehouse in the US where the 75 units are in stock.My point was, they're readily available for a buck or two.

author: Thief
author_rank: Contributor
timestamp: November 23, 2020, 02:49:12 pm
content: Not a buck or two when shipping for two capacitors to the UK is £12!EDIT: Mouser is the same. £12 international delivery.

author: drussell
author_rank: Super Contributor
timestamp: November 23, 2020, 03:00:48 pm
content: Quote from: Thief on November 23, 2020, 02:49:12 pmWell, obviously you would want to pad out your order. Shipping here is $8 flat rate and customs cleared to Canada, next AM, free above $100, but we're a tad closer to the warehouse than you are.What other components to you need to repair/refurb/restore the scope? There are probably other paper caps and such in there that need replacing anyway, perhaps just order those from Digikey as well to make the average shipping per part more reasonable. Ordering one resistor or a couple of capacitors in a single order is always going to be swamped by the shipping cost, no matter where you're buying from.I can pretty much always find some stuff I need to add to my order when I'm shopping Digikey, I usually even manage to make it to the $100 per batch to get free the shipping unless it is something urgent, then the extra percentage paid shipping probably doesn't matter if it is urgent.I kinda always just keep a running list of "Digikey bits" that I'd like to order atsome pointto pad out orders with. I also always call my friend Ian and he calls me when we have a need for something from Digikey to help each other save on the shipping... The last time a couple weeks ago it was him who called and said he had about $50 worth to order, did I need anything? I happened to need about $75 worth of stuff in reasonably short order anyway, so we even got the free shippin'...

author: Thief
author_rank: Contributor
timestamp: November 23, 2020, 03:05:18 pm
content: Surprisingly, those are the only paper caps in it - there's a whole bunch of electrolytics as well though.As a second surprise, it's actually fully functional, even in spite of the dead capacitor. So I was only planning to replace the two paper caps at the moment...

author: drussell
author_rank: Super Contributor
timestamp: November 23, 2020, 03:07:09 pm
content: DANG, you guys actually get a lower cutoff on the shipping, it is free shipping above $50 USD!Just pad your order out to $50 USD worth of stuff:"FreedeliverytoUnited Kingdomon orders of $50 USD or more. Adeliverycharge of $18 USD will be billed on all orders less than $50 USD."Edit:That's £33 when paying in GBP:"FreedeliverytoUnited Kingdomon orders of £33 or more. Adeliverycharge of £12 will be billed on all orders less than £33."Surelyyou can find £33 worth of stuff you could use from Digikey.

author: drussell
author_rank: Super Contributor
timestamp: November 23, 2020, 06:21:15 pm
content: Quote from: Thief on November 23, 2020, 02:49:12 pmP.S. I think those are downright reasonable fees....This sounds a lot like all the complaining going on in theBad experience with JLCPCB and DHL – things you should know before ordering PCBsthread about some promotional $2 ultra-discount PCB service special costing more than the $2 bare board cost to actually get in your hands, halfway around the globe.You just need to find £30 more worth of stuff to add to your order in addition to the two capacitors and then you won't even pay a dime for shipping.That'san even moreinsanelygood deal!

author: Gyro
author_rank: Super Contributor
timestamp: November 23, 2020, 06:40:58 pm
content: Quote from: drussell on November 23, 2020, 06:21:15 pmIt may be an insanely good deal, but I prefer the £4.95 for orders under £30, from RS for next day.Edit: Within the UK of course.

author: drussell
author_rank: Super Contributor
timestamp: November 23, 2020, 06:47:51 pm
content: Quote from: Gyro on November 23, 2020, 06:40:58 pmFair enough, but it doesn't have to be shipped all the way across the ocean.You're telling me that you think an extra £7 to get it all the way across the Atlantic in 48h is somehow unreasonable?Nobody like paying any more than necessary for something, but I don't think these fees are in any way unreasonable for the service being provided, especially with such low "free shipping" thresholds!

author: floobydust
author_rank: Super Contributor
timestamp: November 23, 2020, 07:57:44 pm
content: Quote from: Thief on November 23, 2020, 02:07:31 pmHV ceramic capacitors are dogs, the voltage coefficient of capacitance is really really bad and so at high voltages you get much less than 1/2 their rated value. You have to be careful using them and upsize accordingly.OP's 5,000V film-cap is uncommon so I would bite the bullet and pay the shipping from the disti.

author: Thief
author_rank: Contributor
timestamp: November 24, 2020, 08:15:20 am
content: Quote from: drussell on November 23, 2020, 06:21:15 pmBetter, though, would be a UK (or EU) supplier that could ship them for only a few quid because itdoesn'thave to go halfway around the world. Plus it's two bloody capacitors - you could send it in a padded envelope for a couple of bucks. I can send a large padded envelopetothe US for under £4... £6 if I want tracking and a signature.Also, I think having a .uk site with prices in pounds is a bit deceptive if you don't actually have a UK warehouse and are going to charge international shipping at checkout.That said, the free shipping threshold is a great deal, so I've been trying to think what else I'm after at the moment.What I don't understand though, is whynoUK component sellers seem to stock these - or even list them as an "unstocked" part that's nevertheless available to order.

author: drussell
author_rank: Super Contributor
timestamp: November 24, 2020, 10:25:25 am
content: Quote from: Thief on November 24, 2020, 08:15:20 amYou're lucky over there, then... I probably couldn't send themlocallyfor (£6 = $10.46 CAD), although it might be close...Our postage rates hereare pretty insane and 20mm is the maximum thickness here for Lettermail (those caps might JUST make it through the checker slot in a padded envelope, though, in this example), but if you were regularly shipping things and want delivery confirmation and tracking, it would have to be sent registered and that's an extra $9.75, plus an extra $2.25 per $100 value increment if you want it insured. Perhaps tiny regular parcel rate locally would come in under $10.50, but that's really pushing it.Strangely enough, some things could theoretically be sent down to the US, then sent back to Canada for less than I could send it directly, domestically. I certainly couldn't send something over there to you for $10.50, though. Obviously likes of Digikey could get a deal for huge postal rate discounts if they wanted.... Buuuuuut...They aren'treallyinterested in catering to the one-resistor crowd, (but it's awesome that they let us order just one of something!) Hence the nice, low minimum order size for free shipping. They have a really sweet deal with FedEx, so they're inclined to use it.Someone also recently mentioned that they still always do free shipping if you send in an actual mail order with a cheque!QuoteI strongly disagree. I find it ultra-convenient to have international sellers that make it easy for me. Charge in my local currency so I don't have to pay insane conversion fees, they do that in large batches. I don't have to worry about insane shipping fees, it's flat rate and customs cleared. It shows up right to my door the next morning if I order something by 8pm the night before, I don't care that their warehouse is located in the US and that it changed planes in Memphis.I don't mind, as long as they company clearly states where they actually are. Some places (especiallyinChina) try to completely obfuscate where they actually are, you can't find an actual address anywhere. I certainly don't like those shenanigans!Digikey says 48h typical delivery time to your door in the UK, which has to be within 24h of what it would take from someone stocking there, but a supplier the size of Digikey can't possibly have warehouses all over the planet, so instead they've just struck a deal with FedEx to cover the globe in a timely manner for reasonable fees, or free.QuoteIndeed. You just need to think up / save up a list of enough bits and pieces for each batch before you order, try to save any items on your list that take you way over the £33 mark on a given order if you don't need them right away to help pad out a future order, or they're available more locally where you might want to pad out one ofthoseorders instead.What projects do you see on the horizon? As an example, I'd always wanted to upgrade my welder to add capacitors that can be switched in after the rectifiers to add a little more heat, and extra "oomph" striking the arc when working with super-thick aluminum wire in the MIG. One time I had a smallish order, so I thought, hey, I'll order up a 50-pack (or whatever it was) of medium value, high ripple-capable caps to make into 4 capacitor banks, which padded out that order. I've since made up the 4 packs of 12 (or whatever it was) but I've never hooked them up yet (haven't got back to my custom aluminum intake manifold project again yet.) I've since decided that I want to use little circuit-breaker switches (like you find on a power bar) to do the switching in and out from the front panel, so that's now one of the many things on my list of possible items to pad orders with. Not urgent, but I'd like to get themsometimewhen I can spare a few extra $ towards the ongoing welder upgrade project.How is your stock of basic supplies? Another roll of solder is always handy, perhaps in a different diameter than you "normally" use, like extra thin for SMT or extra thick for big fat wires? A flux pen? Solder paste? Desoldering braided wick?Solder is kinda perpetually on my list right now. All my rolls of the good Multicore that I like (which I bought decades ago when they were like $9.99, then $14.99 for a 1lb spool at the electronics store and I used to always just grab another for convenience when I was there and had a few extra bucks,) are getting very low, and they're like $45 for a roll now (!!) but I need it, it doesn't go bad, and it will always get used up eventually. Things like that make a good items to pad out a smaller order with when I can afford to splurge a few extra bucks on solder.Perhaps stock up on electrolytics, get 10-packs of some common values to stock your parts drawer. Perhaps order up some of the values used in your Philips scope, in case you decide to recap it, 10 x 32uf @ 100V, 80uF @ 50V, etc. etc. Stock up. They're capacitors, you'll end up using them forsomething. You could probably order up every electrolytic in that scope and still be under the £33.Get a bag of X/Y rated safety capacitors so you can always replace a RIFA on sight as a matter of course if you're ever inside "older-ish" gear or repairing/restoring vintage gear.Connectors? Switches? Fuses? Fuse holders? Thermistors for inrush limiting? Small transformers, voltage regulators and bridge rectifiers for generic projects? Things like that are used in pretty much every device or project. Generic project boxes? LEDs?Just think of what projects you're likely to work on in the next 6 months, there's bound to besomethingyou're likely to need that you can pre-order, especially if it's generic bits that can be used for many different things. You'll end up using them sooner or later...QuoteI guess they're just uncommon enough that they don't stock them, but since they exist from a manufacturer on their line card, they can can always order from the manufacturer, but that will usually take some time. Some timessignificanttime, and would often incur a separate shipping charge if it was a backordered fraction of a larger order, though policies on that vary.

author: Thief
author_rank: Contributor
timestamp: November 24, 2020, 11:24:28 am
content: Electronics isn't my main hobby - to be honest I picked up this old oscilloscope on a whim. I solder, do some basic repairs occasionally, but it's just an occasional thing for me.You're pushing me away by trying to convince me to buy a lot of stuff that I wasn't looking to buy.I just want to toy around with my old oscilloscope I got for cheap.

author: drussell
author_rank: Super Contributor
timestamp: November 24, 2020, 11:29:53 am
content: Ah, well, YMMV then....I just made the assumption that since you had got yourself an oscilloscope you were trying to expand your electronics hobby....

author: Thief
author_rank: Contributor
timestamp: November 24, 2020, 11:48:46 am
content: The main "use" I see for an oscilloscope in my future is helping diagnose why my children's toys stop making sound. If I can get a sound waveform, I know the speaker is shot. If I can't, the problem is probably the main chip and hence unrepairable. Most of these toys don't have any more components than that, really.The number of toys with internal electronics that have had a wire break off and stop working because of that... such an easy fix if you can solder.My last actual electronics project was putting LEDs in a Christmas tree jumper - but that was a premade ATTiny board, a battery pack, and some little sew-on-able RGB LEDs with built-in regulators - just soldered on some wire and a little bit of programming, no traditional components needed, not even a resistor. The wires kept breaking at the solder joints though.I used to be more into the idea ofmakingelectronics but I'm not sure what I'd actually make now, and I don't get a lot of free time these days anyway.

author: drussell
author_rank: Super Contributor
timestamp: November 24, 2020, 03:08:45 pm
content: Quote from: Thief on November 24, 2020, 11:48:46 amYes, those two things will tend to go hand in hand!QuotePerhaps someday, but indeed it makes no sense to stock up on actual parts if you're not going to be actively doing any electronics anytime soon.Anyone else over there in the UK that needs some stuff from Digikey want to chime in and stick a couple capacitors on their next order forThiefand then send them along?

author: Thief
author_rank: Contributor
timestamp: November 25, 2020, 09:22:57 am
content: Haha this should make you laugh.I just found the 10nF Wima caps on Farnell UK - but they charge a £10 handling charge on orders under £30. Coupled with them selling the capacitors for twice as much as digi-key, it's actuallymoreexpensive to order those capacitors from Farnell UK than digikey US...That's just silly.

author: wiesl
author_rank: Contributor
timestamp: March 16, 2021, 02:49:22 pm
content: Get X or X2 Rating caps.

author: Gyro
author_rank: Super Contributor
timestamp: March 16, 2021, 04:33:31 pm
content: Quote from: wiesl on March 16, 2021, 02:49:22 pmNo, It's an internal high voltage rail, not mains (he's hopefully well sorted now as this was back in November).