Safety Capacitor question - Page 1
Forum section: electronics / repair

author: ttx450
author_rank: Regular Contributor
timestamp: November 15, 2021, 09:37:34 pm
content: Hi, I am restoring an RCA 35x AC/DC radio. Here a link to the schematichttp://www.nostalgiaair.org/PagesByModel/914/M0014914.pdfMy chassis is a RC-1001-A, the link shows RC-1001-C. My schematic shows both RC-1001-A & C as the same schematic.Capacitor C2, (0.20uf) with a 220k resistor shunting the 2 leads of C2. This is connected between chassis and -B. This should be a safety cap? I don't have any 0.20uf, can I sub this out for a different value? Is the resistor necessary if a safety cap is installed?Capacitor C1 (.035uf), should this be a safety cap? If so, can I change the values if I dont have the cap?Thks..

author: wraper
author_rank: Unknown
timestamp: November 15, 2021, 09:47:21 pm
content: C2 is 0.1uF according to the schematic. They are not safety caps, they did not exist back then AFAIK. Nor their application asks for safety cap (antenna connection is unsafe with any cap). You can replace 0.2uF with 0.22 uF.QuoteYes

author: ttx450
author_rank: Regular Contributor
timestamp: November 15, 2021, 10:36:06 pm
content: Quote from: wraper on November 15, 2021, 09:47:21 pmHi, I had to take a look at that again. It is C21 not C2, 0.2uf across the chassis and -B. I know that safety caps were not used then, it is recommended to use them across the chassis and -B? Also, I believe if the tuning cap is grounded on a AC/DC hot chassis, safety cap is often used. I know the bottom of the tuning cap is isolated from chassis, but not clear if its connected to -B.

author: TimFox
author_rank: Super Contributor
timestamp: November 15, 2021, 11:17:18 pm
content: In well-designed "AC-DC" receivers, the tuned circuits would connect directly to the chassis, feeding the next grid through a capacitor, with a grid resistor to B-. The cathode resistors would also go to the B- supply (near ground, but don't trust it), but the bypass capacitors would go to the chassis, along with the screen bypass capacitors and plate decoupling capacitors. Of course, the speaker was galvanically isolated by the output transformer.A good example from WW II is the E.H. Scott SLR-M short-wave receiver for Merchant Marine service, to work with either AC or DC power. It was designed to minimize local-oscillator radiation to avoid detection by enemy radio-location.(The input RF amplifier was a 6K7 with grid cap, while the IF amplifier used 6SK7, to improve further the shielding from the LO at the mixer.)

author: ttx450
author_rank: Regular Contributor
timestamp: November 15, 2021, 11:51:48 pm
content: Quote from: TimFox on November 15, 2021, 11:17:18 pmHi Tim,I am learning here, so please excuse my ignorance. So C21 should be a safety cap or not? Chassis to B- I think they are used? The 220k resistor shunted across the C21 cap is for the Grid to B- and not a fuse/resistor?C1 .035uf to B-? Should this be a safety cap or no? I believe the tuning capacitor is isolated from chassis, but C1 goes to B-.Thanks, trying to understand & learning here.R

author: TimFox
author_rank: Super Contributor
timestamp: November 16, 2021, 12:27:46 am
content: This commercial receiver is built the old fashioned way, where B- is connected directly (through the off-on switch) to one side of the power line, and the chassis is only a shield that also connects to the metal shells of the metal-octal tubes. It doesn’t use the method I described above for a military receiver.If I remember correctly, there was a UL spec for the maximum AC current (60 Hz) that could flow to the chassis, and C21 in parallel with R8 were chosen to meet that spec when the B- “ground” network was connected to the hot side of the power line (110 V). They didn’t have modern safety-spec capacitors back then, and the typical paper capacitors by now have probably absorbed moisture, resulting in increased leakage current. (The E H Scott unit used only hermetically-sealed metal-case “bathtub” paper capacitors that are moisture proof.) I have replaced old paper capacitors in tube equipment with modern polypropylene capacitors: for example screen bypass caps can depress the screen voltage with excess leakage. You can get safety-rated polypropylene caps. The parallel resistor is to bleed any residual charge from C21 when the unit is switched off.

author: ttx450
author_rank: Regular Contributor
timestamp: November 16, 2021, 12:59:23 am
content: Quote from: TimFox on November 16, 2021, 12:27:46 amThanks for the explanation. I rewired my set to B+ through switch. I used polypropylene originally, but always wondered about C21 and possible C1.. I seen all pin 1 are to chassis except 50L6GT & rectifier thinking it was some sort of shielding.ThksEdit- I might move the resistor a little further away from the cap, not sure its necessary.

author: TimFox
author_rank: Super Contributor
timestamp: November 16, 2021, 03:47:49 am
content: The metal tubes all have pin 1 to the shell, which can be touched easily, so safety is important. The 50L6 and 35Z5 are "GT" glass-envelope tubes.

author: ttx450
author_rank: Regular Contributor
timestamp: November 16, 2021, 10:23:54 pm
content: Quote from: TimFox on November 16, 2021, 03:47:49 amThanks for the info. I was not sure if they had some coating on the insides of the glass. I am new to tubes, learning here.I am still not clear if C21 should be a safety cap, I believe it should be a safety cap and also C1, not sure if it should be?Thanks for your help

author: TimFox
author_rank: Super Contributor
timestamp: November 16, 2021, 10:43:47 pm
content: "Safety capacitors" are now required for capacitors connected to the power line. They are defined by their failure mechanism: see the reference below."Class Y" capacitors are required for line to chassis, i.e. anything that can be touched by the victim (electrocution hazard)."Class X" capacitors are required for line to line or line to neutral, where neither end is touchable (fire hazard).Seehttps://www.allaboutcircuits.com/technical-articles/safety-capacitor-class-x-and-class-y-capacitors/for the different sub-classes, related to voltage ratings, such as "X1"

author: Gyro
author_rank: Super Contributor
timestamp: November 16, 2021, 10:51:22 pm
content: You could save yourself a lot of worry if you just use an external mains isolation transformer in the supply lead. It probably doesn't need to be that big (maybe 50W max?).

author: ttx450
author_rank: Regular Contributor
timestamp: November 16, 2021, 11:55:38 pm
content: Quote from: Gyro on November 16, 2021, 10:51:22 pmThats an interesting idea. I want to restore this close to new with schematic improvements and some safety updates. It's fun learning project.

author: BrokenYugo
author_rank: Super Contributor
timestamp: November 17, 2021, 03:17:13 am
content: An isolation transformer is overkill unless this is a true "hot chassis" radio (one side of the cord or DC supply straight to chassis), which it does not appear to be. Just use safety caps where warranted (definitely C21, arguably C2 and C18 as well), keep the wiring tidy, and call it good.

author: amyk
author_rank: Super Contributor
timestamp: November 17, 2021, 05:11:37 am
content: From the schematic, this look like a standardhttps://en.wikipedia.org/wiki/All_American_Fivewhich usually has a hot chassis.

author: Gyro
author_rank: Super Contributor
timestamp: November 17, 2021, 09:34:54 am
content: I though all AC/DC sets had a hot chassis - or at least a mains cord connected one.

author: TimFox
author_rank: Super Contributor
timestamp: November 17, 2021, 02:58:45 pm
content: The wiring diagrams for AA5 and similar AC-DC sets can be confusing. The RCA diagram he posted uses the common three-parallel lines for circuit ground, and another symbol for chassis ground. The chassis ground appears once at one end of the RC circuit the OP was asking about, at the other metal tube envelopes, and on the shield cans. All other connections are to circuit ground, which is B-, presumably a wired network, which does go (through the power switch) to one side of the line.

author: ttx450
author_rank: Regular Contributor
timestamp: November 17, 2021, 11:27:34 pm
content: Quote from: BrokenYugo on November 17, 2021, 03:17:13 amThanks for your response. What is the draw back of using a safety can where it might not be warranted? High ESR? Or does it matter much or not? I have .1 and .01, looks like I will need to order some 0.2uf.

author: ttx450
author_rank: Regular Contributor
timestamp: November 17, 2021, 11:36:46 pm
content: Quote from: TimFox on November 17, 2021, 02:58:45 pmHi, thanks.. I rewired the switch to take B+ and fused it. Been re-wiring to the schemtic, there seems to have been a few things miss-wired. One was supplying B+ to the 12SQ7 and feeding it to the 50L6GT. It was backwards. It possibly looks like they had capacitors going to chassis ground and not B- too. The radio looks to have been passed around before I got it. So is this a semi hot chassis or hot chassis? Here's a pict of the rats nest.

author: BrokenYugo
author_rank: Super Contributor
timestamp: November 18, 2021, 12:31:19 am
content: Where are you getting 0.2 from? Schematic shows C21 and C2 as 0.1 and I wouldn't go any higher as with 0.1 you're already looking at near 5mA (a solid tingle, bordering a shock hazard) going through if circuit ground is connected to live (plug reversed or polarized plug in a miswired socket), some AA5s used a .05 (.047) in that position.Other than cost I'm not sure of any drawbacks to using Y capacitors, other than them being bigger and more expensive.

author: ttx450
author_rank: Regular Contributor
timestamp: November 18, 2021, 12:49:44 am
content: Quote from: BrokenYugo on November 18, 2021, 12:31:19 amC21 on my RC-1001-A-RC-1001-C schematic shows C21 as 0.2uf. The schematic I have shows RC-1001-A and RC-1001-c as the same. I have a RC-1001-A. I been following the schematic, maybe I am following the wrong schematic?My OP questioned if a 0.20uf could be lowered, I was wondering if could lead to a shock hazard and if it was necessary to be that high. So do you think .047 I have a few of those. thks

author: BrokenYugo
author_rank: Super Contributor
timestamp: November 18, 2021, 05:27:16 am
content: I'm honestly not quite sure what drives the fairly large size of C21 (the B- to chassis cap) in floating chassis AA5 radios. To the best of my knowledge it just bypasses the 220k resistor at high frequencies, basically a dead short to the IF/RF. I would think .01 (35 ohm reactance at 455kHz) would just as well, but I am far from a RF engineer.C2 would probably work ok with a lower value too, if you even intend to use an external antenna.

author: TimFox
author_rank: Super Contributor
timestamp: November 18, 2021, 02:56:43 pm
content: According to p. 1268 of theRadiotron Designer's Handbookby F. Langford-Smith (Wireless Press, 1953), the regulatory limit for the current through the RC network in such an AC-DC receiver was 5 mA (the Australian specification from 1937). I couldn't find the UL spec quickly for the US, but I believe it was basically the same. Not normally lethal, but palpable. With 117 V, 60 Hz, the 5 mA limit corresponds to 0.11 uF. The examples below with 0.1 uF in parallel with 220kwould give a total current magnitude of 4.95 mA at 117 V, 60 Hz, dominated by the capacitor.A later version of the same circuit, found in the 1965 edition (RC-24) of theRCA Receiving Tube Manualdid not have the leakage current specification, but uses 0.1 uF in parallel with 220 kin that location. The circuit on page 522 replaced the octal tubes of the older circuit with glass-miniature tubes (12BE6, 12BA6, etc.). A more complicated AM-FM circuit on pp 524-5 employed a silicon rectifier. The power-line and switch connections used the same RC network, and included a special line cord with an internal third wire for the antenna.The connection of the power switch to circuit ground = B- may look counter-intuitive to the IEC generation, but was done since the power switch is immediately behind the audio volume control (full CCW = OFF), and that connection reduced the 60 Hz hum from the power line.Note that AA5 sets are "AC-DC", due to half-wave rectifier and lack of power transformer. Going into the second half of the 20th Century, there were still DC urban power distribution (at least in New York and Boston) for household loads. Con Ed in NYC finally eliminated the last DC distribution in Manhattan in 2007.

author: ttx450
author_rank: Regular Contributor
timestamp: November 19, 2021, 12:02:13 am
content: Quote from: BrokenYugo on November 18, 2021, 05:27:16 amQuote from: TimFox on November 18, 2021, 02:56:43 pmThanks guys.. very interesting, learning here. Enjoy reading everyone's thought's. If shipping was not 10 bucks or a local part store was around I would just replace with original size safety caps. I need to make sure I am using the correct schematic now. Broken thanks for pointing out the phone jack. Now have a better understanding how they are used. I wish I had better knowledge on how to substitute. Every time I turn around I need a couple more parts.. I will have more in shipping fee's then parts

author: ttx450
author_rank: Regular Contributor
timestamp: November 19, 2021, 12:34:41 am
content: Quote from: TimFox on November 18, 2021, 02:56:43 pmThe schematic I been working off of is slightly different in places then the link I posted. Most everything else looks to be the same unless I missed something. This is maybe a more updated schematic?C21 0.20uf, to link - 0.1uf both with 220kC3 .005, schematic upgraded to .015, link .02ufC5 .025 to link .02Filter caps20uf and 30ufLink 30 & 50uf.

author: ttx450
author_rank: Regular Contributor
timestamp: November 19, 2021, 12:39:57 am
content: Quote from: BrokenYugo on November 18, 2021, 05:27:16 amI would be temping to try, maybe I will after I get it together. I do have the original antenna, so plan on using it. thks

author: BrokenYugo
author_rank: Super Contributor
timestamp: November 19, 2021, 02:12:26 am
content: I assume you know it's generally safe to round to modern values.C3, I'd probably use .022, this couples audio into the first amplifier stage, higher value=stronger bass, so may as well use the biggest the engineers saw fit to use.C5 I think is a feedback capacitor? Probably another .022, remember that anywhere they used paper caps isn't terribly critical.33 and 47 (or better yet 68, old electrolytic caps were usually toleranced at -20%, +80%, so you can safely go up a bit) for the B+ filters, also go up on the voltage rating as often old stuff ran near the limit. Higher voltage rating also often means a larger, higher ripple rating, cooler running, longer life part. It is possible to get into trouble there with considerably smaller modern capacitors running hot.

author: ttx450
author_rank: Regular Contributor
timestamp: November 20, 2021, 02:30:06 am
content: Quote from: BrokenYugo on November 19, 2021, 02:12:26 amHi, yes. Thanks for explaining how the caps influence the sound. The RCA service repair book (I don't have)has upgrade and modifications. I wish I had a copy of that.So upping the 2nd cap is safe? Newest schematic version is 50uf. See what I can find in 68ish going to order some stuff soon.Thanks for the info.