The PHILCO Phorum

OK, this safety cap thing....  I want my sets to be safe if something fails.  Please forgive me if this has been answered 100 times before.  I didn't see any references to it.  

How do I know if I need a safety cap as a replacement when looking at the schematic? 

I have a Philco Jr. 81 and I believe the 0.015uf dual bakelite block needs them from what I read in a thread on Antique Radio site.

But I'd like to know with confidence when I need to apply them, for future restorations. 

Thanks in advance.   

The place where one uses safety condensers is at the line power input to the piece of equipment to filter any RF from the line. Here is a link to a good explanation of what safety condensers are, and how they are used:
https://www.allaboutcircuits.com/technic...apacitors/

Here is a section of the schematic diagram of a Philco 38-10.

Notice the part numbered 47. That is one of the bakelite blocks commonly used in Philco radios to hold condensers and other parts of the circuit. Here you see the two condensers used to filter RF from the line. These are the condensers usually replaced with safety condensers. Since they go between each side of line to the chassis ground, one would use type Y safety condensers in this application, since, should they ever fail, they are designed to fail OPEN, and so, while no longer filtering out RF noise on the line, won't cause a shock hazard by shorting the line to the chassis.

To replace these condensers one needs to remove the bakelite block from the radio, then put it on a little tray to catch the molten tar, and heat it on the oven to about 250 F so that the tar inside it melts. You can then pull out the old paper condensers inside it, and replace them with the modern safety condensers.

Thanks, Mike. In the future, I'll look for situations where a short would cause a direct short across the AC line. This is true of the dual 0.015uf bakelite block in my Model 81. I think I understand now.

Good article Mike..thanks for posting that.

The purpose of the line caps is often misunderstood. Their main function is to provide an RF ground path for the antenna circuit. If you look at the schematic, you see the antenna coil is connected to chassis ground, in turn, the chassis is connected to the line through the line caps providing this ground path.. Try this experiment, remove the screw that fastens the line cap block to the chassis, lift the block so it no longer has a ground connection. turn the radio on. You won't notice much if any reduction in electrical noise/interference. What you find is a great reduction of sensitivity though. A few years ago, I restored a Philco 20. The Philco 20 originally had no line caps but, required a ground connection to the chassis for good performance. I decided to add line caps. After the caps were added, the radio worked equally well with or without the ground

Steve

Unless someone is running a brush type AC-DC appliance, like an old vacuum cleaner, in your house, you will probably not be bothered by RF on the power lines these days, But when these radios were made it was a more common problem. You are right that the caps do also allow RF to go the other way, and can serve as a make shift ground; however, these radios were designed to have a proper ground and antenna. That is why they have a ground terminal. The main purpose at the time of their manufacture was to filter interference from the line.

I have found what Steve said to be the greatest benefit today. Most of the noise around here is RF.

Don't make them too large (~.01). I dislike that tingle.

Sal has a good deal on a package of them (the little blue disc caps).

The purpose of the caps notwithstanding:

You need X-rated caps anywhere where a cap goes between L and N.
You need a Y-rated cap anywhere where a cap goes between L to PE (chassis) or N to PE.


Speaking of a misunderstood caps between Chassis and L/N, I had a Philco 16B, I think it was, where I accidentally isolated the screw holding the GND lug of a backelite block to the chassis. I forgot how I made it. But I did.
I spent two days trying to understand why I had such a pronounced hum.

or you can use X/Y rated caps

Hello Guys,
Yep the good old Safety capacitor and I most of the time use the ones that falcon talks about the X/Y rated caps .
Sincerely Richard

Just to add a little confusion here. If ‘safety’ caps are designed to fail open, why does it matter if you use X, Y, or X/Y types in any application? Shouldn’t they work safely in any position, across the line or from both lines to chassis?

Ron

462ron, please go back and read the information in the link I posted earlier again :  https://www.allaboutcircuits.com/technic...apacitors/  . There you will see that there is a difference between type X and type Y safety condensers. Type X are designed to go directly between the two sides of LINE, that is between hot (black) and neutral (white). They are designed to fail SHORTED, so that when they fail they will blow the line's fuse, and so cut power to the appliance. Type Y are designed to fail OPEN. They are designed to be used between line and chassis, or ground. When they fail, they fail OPEN, and so do not short line voltage to the chassis. This is why in those applications like in the Philco bakelite blocks used for filtering power input, one should use type Y.

Learn something new everyday! From previous articles I’ve read on the subject, I never saw where the X type safety caps were designed to fail in the short mode. I’ve only ever read that generally safety caps fail in the open mode. This is a new one on me but it makes sense. Thanks for the update Mike!

Ron

Makes me wonder about those caps that sellers call X2, Y2 or some variation of X and Y with 1s and 2s. What happens with those?

I found another article which has more information about safety caps which have the X1Y2 rating and such...
https://electronics.stackexchange.com/qu...or-exactly
Look under the "Answers" section. It has some deeper explanations. Basically, an X1Y2 condenser is designed to fail OPEN, but meets the continuous, surge, and pulse voltage ratings of both X1 and Y2 standards.