Capacitor voltage ratings when not provided in schmematics - Page 1
Forum section: electronics / beginners

author: Skylineview
author_rank: Contributor
timestamp: April 25, 2020, 11:56:31 pm
content: Hi everyone! Hopefully this post is in the right place.A little back story so you know where I'm coming from. I've been interested in electronics for a long time, watching Dave, Clive and quite a few others for a while now and I decided to finally break down and do my first cheap tube radio restoration. I bought a Zenith K731 that has the 7K07 chassis from around 1961. The cabinet is in very good shape, minor issues and the chassis is in pristine condition, no corrosion. So for my first I think it will be a good place to start.I have been reading schematics for a while but I have a question I vaguely remember another Youtuber answering, but I can't seem to find that video anymore.My questionis about capacitor voltage ratings. So five of the listed capacitors give voltage ratings, but the rest do not. Is there a rule of thumb to follow when purchasing replacements if a voltage rating is not given? Should I measure the voltage across those caps to get a rating? Can I put any voltage rating in there as long as the capacitance is the same/close to the same?I've attached the schematics for this chassis so you can see what I'm referring to. Page four is the parts list and descriptions page, the capacitors are in the first column.I appreciate any help that can be provided!7K07 Chassis Schematic:0653_12n.pdf(1174.55 kB - downloaded 471 times.)

author: james_s
author_rank: Super Contributor
timestamp: April 26, 2020, 12:05:25 am
content: It comes down to understanding the circuit well enough to know what voltages are present.In the case of a vintage radio most of the capacitors will be low value film types so you can just use parts rated at 250V, 400V or more in many cases and they'll still be physically smaller than the original parts and it doesn't hurt to use a higher voltage rated capacitor.Looking over the attached manual, there's a chart that has part numbers for the capacitors so you can probably do a web search of each of those and find a datasheet that will list the voltage rating.Also I'm not sure how much experience you have restoring antique radios so this may be obvious but the ceramic and mica capacitors tend to be very reliable so don't shotgun replace those. You only need to replace the electrolytic and wax paper types.

author: helius
author_rank: Super Contributor
timestamp: April 26, 2020, 12:07:51 am
content: The "C1{A,B,C}" in the "Electrolytic Capacitors" table is a multiple pin can. These types are mostly not made anymore and you will need to carefully study the circuit to see where to put separate replacement caps.There is another component labeled "C6{A,B}" which may also be a multiple pin can, but it's not clear from the photo. It's shown wired to the tabs of a rotary switch. In general the rows in the "Fixed Capacitors" table that are greater than 1 uF would be electrolytic, the others are film, ceramic, or wax paper. Film and ceramic don't die from age but wax paper does. The capacitances are all listed in microfarad (or "mmf": picofarad).The table lists the part numbers of substitute parts from six different companies, it should be possible to find the voltage ratings from those, or by looking at the schematic where the supply voltages to each tube are labeled.

author: james_s
author_rank: Super Contributor
timestamp: April 26, 2020, 12:13:38 am
content: When I come across those multi-section capacitors I usually gut them and stick multiple modern capacitors inside them, modern capacitors are usually small enough to fit.

author: floobydust
author_rank: Super Contributor
timestamp: April 26, 2020, 12:29:08 am
content: One danger is the power supply voltage skyrockets to over 170VDC until the tubes warm up and then they load it down to between 140V to 95VDC.So for the filter capacitors, the originals C1 are 150VDC rated and they can take a surge voltage for the few seconds.I would use nearest standard voltage of 160VDC best 250VDC parts for C1A,B,C. Modern parts are quite small anyhow and can fit in the big chassis.

author: james_s
author_rank: Super Contributor
timestamp: April 26, 2020, 12:39:28 am
content: Does it? Usually the rectifier is a tube as well so unless it warms up faster than the other tubes there shouldn't be any B+ until all the tubes are getting warm. Not that it ever hurts to have a bit more overhead on the voltage rating.

author: Skylineview
author_rank: Contributor
timestamp: April 26, 2020, 01:14:47 am
content: Thanks! I am aware of the mica and ceramic caps. No plan to touch those unless I have to. I have the schematic printed out and plan on studying it quite thoroughly before doing any actual work. I have the chassis out so I can easily reference components rather than using the poor quality images.Like I said this is my first vintage radio restoration/restoration in general. But I've read quite a bit on them, watched a lot of videos and i'm not afraid of doing any damage to anything.I will do the web searches you suggested to see what I can find.Thanks!

author: Skylineview
author_rank: Contributor
timestamp: April 26, 2020, 01:18:31 am
content: The multi-section cap that's in there is quite small, so I don't think I'll be re-stuffing it. I do, however, plan to leave it in for aesthetics. It looks like I have enough room to put in a short terminal strip to route those tie points to and will use new electrolytic's. I'd love to be able to re-stuff but, because of its size I don't think i'm up to that challenge just yet.

author: Skylineview
author_rank: Contributor
timestamp: April 26, 2020, 01:23:40 am
content: Quote from: james_s on April 26, 2020, 12:39:28 amIn this radio the rectifier is selenium. So that's another question I forgot to ask about.I've read conflicting info on replacing those. Some say if its working to leave it. Other say it MUST be changed. What's your opinion, or guidance, on working with the selenium rectifier? I am aware that modern rectifiers/diodes have a lower voltage drop than the selenium does and would require a power resistor to get the voltage closer to what the selenium puts out.

author: Skylineview
author_rank: Contributor
timestamp: April 26, 2020, 01:26:17 am
content: Quote from: Skylineview on April 26, 2020, 01:18:31 amIt's size (circumference) is somewhere between that of a quarter and a nickel. and about 3 or 4 inches tall.

author: greenpossum
author_rank: Frequent Contributor
timestamp: April 26, 2020, 01:26:53 am
content: I'd say change it. Si diodes are cheap and one less thing to worry about. Selenium rectifiers stink when they burn out.But you're right about adjusting the resistance for the lower voltage drop. Lots of docs about that.

author: Skylineview
author_rank: Contributor
timestamp: April 26, 2020, 01:31:07 am
content: Quote from: greenpossum on April 26, 2020, 01:26:53 amOkay, I will research that as well before changing it. I'm assuming that it would't hurt to do that last? Get everything else running right and then swap that out. I'm just trying to work my plan of attack and working things out in stages. I plan on starting with those black beauties then moving on to the multi-section

author: greenpossum
author_rank: Frequent Contributor
timestamp: April 26, 2020, 01:39:47 am
content: Yeah that should be ok to do last. Has it been powered on yet though? I'm wondering if it has degraded such that it won't provide enough B+ voltage. Well a measurement will tell.

author: Skylineview
author_rank: Contributor
timestamp: April 26, 2020, 01:42:57 am
content: Quote from: greenpossum on April 26, 2020, 01:39:47 amYes, I powered it up today after it was delivered and unpacked. It seemed to work okay except for the hum from the bad filter caps. It either has low hours or has been very well taken care of. I can always do it again and measure what it's putting out.

author: greenpossum
author_rank: Frequent Contributor
timestamp: April 26, 2020, 01:49:45 am
content: You have a good chance of success then if the only symptom is hum. BTW I guess you know that this circuit has transformerless PS not isolated from the mains so please take care if poking inside while it's plugged in.Edit: A bit of background. I now live in a country with 230VAC mains, but when I was in the US decades ago, I used to pick up transformerless tube radios for a buck or two at garage sales. Not all as pretty as yours. Nearly always they worked right off perhaps with some replacement of filter caps to fix hum. Those critters were tough. I had a fascination with transformerless designs because I had an old RCA tube databook which listed many tubes designed for series heater configuration, i.e. equal current but voltage drop depending on the heating required, and enough tubes for the voltage to total to 117VAC. There was even a rectifier cum power stage tube for 117V IIRC. As a matter of rule, the sensitive tubes. e.g. detector/first audio stage, have heaters at the bottom of the chain.

author: Skylineview
author_rank: Contributor
timestamp: April 26, 2020, 01:58:04 am
content: Quote from: greenpossum on April 26, 2020, 01:49:45 amYes, I am aware. Thank you for reminding me. It never hurts to hear it multiple times! I'm on the look out for an isolation transformer to work on this old stuff toreducethe chance of getting a nasty shock. I'm definitely no stranger to electricity, but I still don't want to get zapped.

author: james_s
author_rank: Super Contributor
timestamp: April 26, 2020, 03:57:21 am
content: Quote from: Skylineview on April 26, 2020, 01:23:40 amWhen I was a kid I plugged in a little power supply my uncle had that used a selenium rectifier. A few moments later it started spewing colored smoke that was the worst smelling stuff I've ever encountered. After that experience I've replaced selenium rectifiers on the rare occasions that I've encountered them. Usually you can leave it in place and tuck a modern diode nearby. Depending on the circuit you might need to add a series resistor too.

author: vk6zgo
author_rank: Super Contributor
timestamp: April 26, 2020, 03:59:40 am
content: Quote from: james_s on April 26, 2020, 12:39:28 amNot in this case----- the rectifier is a selenium one.I had never thought about it in the past, but have been wondering recently, how transformerless radios using the 120v Mains got higher voltage HT for more efficient output stages, etc.Looking at this schematic, the answer is--- they didn't.!Transformer power supplies can have HTs of 300 plus volts or above, although, in most cases, it is more around 250v, & transformerless radios from 220/240v countries can have voltages similar to the latter.In those cases, 400v & 600v rated components were usual, back in the day, & were mass produced.For the OP's radio 200v or better rated capacitors will do the job, but 160v may be a bit marginal.

author: bob91343
author_rank: Super Contributor
timestamp: April 26, 2020, 04:05:27 am
content: A foolproof way of determining voltage rating is to look in the parts list for the various manufacturer part numbers and check their catalogs.In general, ceramic capacitors in those days had voltage ratings well above what most circuits applied. Paper capacitors not so much. Electrolytic units are very specific. So look up the old catalogs if you can find them and that will answer your question.

author: james_s
author_rank: Super Contributor
timestamp: April 26, 2020, 04:58:15 am
content: Quote from: vk6zgo on April 26, 2020, 03:59:40 amThere were even "farm radios" which were designed to run off 32VDC. Prior to WWII there were still many rural parts of the US that did not have electrical service, they would use a 32V battery bank and run a generator to charge the batteries. These radios ran the plates at 32V, it was not optimal but it did work. I don't recall whether they used special tubes or not, I've never personally encountered one, I've only seen pictures.

author: Mp3
author_rank: Frequent Contributor
timestamp: April 26, 2020, 05:22:49 am
content: There is plenty of good information here, but i just wanted to share i've been replacing some can-style multi section capacitors lately, they are a pain to desolder but when you get them out I 've had absolutely no issues replacing those with plain 10k hour electrolytics (obviously make sure they're suitable for the circuit, never just blindly toss in anything)The worst problem I have run into is pulling 3000uf cans and having to put 3300 in place. Not the end of the world but hate not matching the capacitance exactly.

author: vk6zgo
author_rank: Super Contributor
timestamp: April 26, 2020, 05:57:00 am
content: Quote from: james_s on April 26, 2020, 04:58:15 amIn Oz, this situation remained till the mid 1960s.Obviously, by that time, only the more remote places needed to use such power supplies, & as 240v ac generators became more affordable, even those succumbed, well before being connected to the grid.Some 32v radios used vibrator power supplies for HT, but others were like the ones you describe, using 32 volts for HT supplies too.As the development work had already been done in the USA, we probably used the same tubes used there.Once fairly reliable audio power transistors became available, the audio power output was no longer limited.Other farmers used AC sets powered from vibrator inverters.The vibrators operated at 50Hz, & were much larger than the car radio ones, or the ones used earlier for tube HTs.Poorer farmers used either battery portables, full size radios also run from primary & secondary "dry" batteries (large "farm pack" ones in their case), or vibrator type ones run off a normal car battery, which was either swapped from car to radio service, or taken to the nearest town for charging.Of course, in the early 1960s, large transistor portable radios took over from these, as they had a very long battery life if operating from a reasonable sized battery.

author: Zero999
author_rank: Super Contributor
timestamp: April 26, 2020, 10:48:19 am
content: Quote from: Skylineview on April 26, 2020, 01:42:57 amIf it works, then why completely recap it? Although I can see why people do that, I'm never a fan of just blinding replacing components. Keep the originals, if they're good.Just replace the bad filter capacitors. As mentioned above, choose capacitors with the next µF value up, but I'd go for voltage rating of at least 200V.

author: JimS
author_rank: Contributor
timestamp: April 26, 2020, 11:25:48 am
content: Thanks for posting that Sam's Photofact, brings back memory's of day's gone bye.Along with my RCA receiving tube manual from collage on my book shelf.Did a quick google search and they are still available, if you want to keep the radio original.With the growing popularity of tube audio amps, getting a replacement should be possible.

author: james_s
author_rank: Super Contributor
timestamp: April 28, 2020, 05:25:26 pm
content: Quote from: Zero999 on April 26, 2020, 10:48:19 amBecause those wax paper capacitors get leaky and that kills tubes and burns out irreplaceable parts. In the case of antique radios it's prudent to replace that particular type of capacitor rather than find yourself looking for a replacement part that hasn't been made in decades. I've never seen one of those capacitors test good even when the radio did still work.

author: james_s
author_rank: Super Contributor
timestamp: April 28, 2020, 05:29:18 pm
content: Quote from: Mp3 on April 26, 2020, 05:22:49 amThose old 3,000uF cans were probably +/-20% tolerance, so a replacement that is anywhere from 2400-3600uF matches it "exactly". No point in getting too OCD over it.

author: Skylineview
author_rank: Contributor
timestamp: April 28, 2020, 08:53:57 pm
content: Quote from: Zero999 on April 26, 2020, 10:48:19 amI have to at least replace the can filter cap. because the hum makes it almost unusable. I"m not planning on blindly replacing all of them, only the electrolytic can and the black beauties/bumblebees. I hope to get an order in for the caps this week.

author: Skylineview
author_rank: Contributor
timestamp: April 28, 2020, 08:58:19 pm
content: Quote from: JimS on April 26, 2020, 11:25:48 amYou're welcome. It was the easiest place for me to grab a schematic from. Yeah, I probably could have found someone with one, but I didn't want to wait. I wanted to start studying it.I did notice that can electrolytic's are available, but they are all too large for this radio. So I will most likely leave it in for aesthetics and just put some new ones on the underside.

author: Skylineview
author_rank: Contributor
timestamp: April 28, 2020, 09:01:42 pm
content: I just want to take a minute to thank everyone who has already or will contribute to this post. I got a lot of good information and I'm even more confident now that I can do this.Once I get it done I plan on posting a video to YouTube. showing it working. I might try to shoot some video of the repair process when I'm able to get to it.

author: Zero999
author_rank: Super Contributor
timestamp: April 28, 2020, 09:25:59 pm
content: Quote from: james_s on April 28, 2020, 05:25:26 pmYou have a point about the wax paper capacitors, but I'm talking more about the electrolytic capacitors, especially those which are a pain to get hold of, such as those multiple can units. I see very little point in replacing good parts. At least check them with an ESR tester, which can normally be done without removing them from the circuit.

author: Skylineview
author_rank: Contributor
timestamp: April 29, 2020, 01:02:19 am
content: Quote from: Zero999 on April 28, 2020, 09:25:59 pmI don't have an ESR meter, I do have one of those cheep Chinese component testers, that seems accurate, within reason. I'll give that a shot and see what I get.Also, does anyone have a good source in the USA for parts. I know a few use Mouser and Digikey, but I was wondering if there were any other outlets.

author: Jwillis
author_rank: Super Contributor
timestamp: April 29, 2020, 03:00:16 am
content: The voltage ratings are shown for many of them in the parts list. I also looked up the Sprague numbers for a few and the numbers are still valid. For example the 10TS-D10 for C2A and C2B 1kV , 5HK-D10 for C4 1kV , 10TCC-Q12 for C6A and C6B 1kV , 10TCC-V68 for C10 1kV .Many of the old paper wax caps can be exchanged for modern polys or ceramics without any problems . Unless it's for aesthetics.Don't be afraid to use the next highest voltage rating or value if the exact match can't be made.Many times the values of the caps will be odd so you can use the closest equivalent . for example C10 is a 7mmf so 10pF would work . mmF = pFI added a PDF of some vintage Caps for cross referencing to help out.https://www.americanradiohistory.com/hd2/IDX-Site-Technical/Archive-Catalogs-IDX/IDX/Allied/Allied-Catalog-1958-OCR-Page-0123.pdfIf you need further help with cross references and supply you can try these guys. The products are awesome .https://www.justradios.com/Hope this helps out.

author: CatalinaWOW
author_rank: Super Contributor
timestamp: April 29, 2020, 03:46:25 am
content: Want to add a vote for bypassing the selenium rectifier. That nasty smelling smoke is very toxic and the odds of the rectifier emitting that smoke is uncomfortably high.

author: helius
author_rank: Super Contributor
timestamp: April 29, 2020, 07:14:37 pm
content: Quote from: Skylineview on April 29, 2020, 01:02:19 amwww.arrow.comwww.newark.comwww.jameco.comwww.all-spec.comwww.anixter.com

author: james_s
author_rank: Super Contributor
timestamp: May 01, 2020, 01:28:40 am
content: Quote from: CatalinaWOW on April 29, 2020, 03:46:25 amAm I remembering correctly that it's green or orange colored, or am I imagining things? I was probably 10 years old at the time and I can still smell it when I picture that little power supply I plugged in on my uncle's workbench.

author: Skylineview
author_rank: Contributor
timestamp: May 01, 2020, 09:29:15 am
content: Quote from: james_s on May 01, 2020, 01:28:40 amIn this radio it’s blue.

author: CatalinaWOW
author_rank: Super Contributor
timestamp: May 01, 2020, 09:57:29 pm
content: I believe he was referring to the smoke color. I have seen a wide variety of colors for the rectifiers themselves. The ones I remember are brown, green, blue, yellow and red. Don't know if there was ever a code or if that just reflected the cheapest color of paint.I don't remember the smoke color, haven't set one off for decades and left the area very quickly when I did. I suspect it may vary with coatings, failure mode and any number of other things.

author: floobydust
author_rank: Super Contributor
timestamp: May 01, 2020, 10:16:25 pm
content: I think selenium diodes are about 30PIV and Vf 5V each, OP's stack about 5 plates. so OP will get higher B+ voltage in the radio if a 1N4007 is put in. Not likely to be a problem but R35 22R 1W resistor might need to be increased in value. It really should be a flameproof, fusible part in the first place.An old boss told me selenium rectifiers fail with age due to a fungus that forms. It is much worse in humid, tropical climates.I thought selenium rectifiers spew toxic brown smoke, smells like rotten eggs. This was a big problem using them in airplaces, back in the day.

author: bob91343
author_rank: Super Contributor
timestamp: May 01, 2020, 10:52:59 pm
content: I think the color reflects the manufacturer. As I recall, Sarkes Tarzian were blue, ITT Federal were orange maybe, one brand I can't recall was silver gray.

author: Skylineview
author_rank: Contributor
timestamp: May 02, 2020, 12:22:39 am
content: I've managed to get it working! Selenium rectifier is removed and replaced with a 1N4007 and a 75ohm resistor. All paper/wax caps were replaced with modern film caps. I disconnected the first 80 uf section of the can electrolytic and replaced it with a nichicon 83 uf @250 v. The other two sections were testing okay for now, so until my new terminal strips come in they'll stay in.Here's a short vid of it working. When I pull it back onto my bench i'll snap a few pictures of my work to share. I was in a hurry and forgot to take pictures. LOL I still need to fine some new grill cloth and refinish the cabinet, but its very presentable in its current state. It will live on top of my refrigerator and used to play the local country music station that plays polka music everyday for an hour and a half.https://youtu.be/Rge3ezHAt_A

author: james_s
author_rank: Super Contributor
timestamp: May 02, 2020, 02:12:26 am
content: Quote from: floobydust on May 01, 2020, 10:16:25 pmYes I was referring to the smoke color, it must have been orangish-brown because I remember being surprised to see colored smoke. I mostly remember that smell though, blech.