I need to rebiuld the receiver on my Philco 47-1230. I have a couple im sure, simple, questions. I read that mmf is the same as a pf capacitor, in other words if the schematic says 22mmf then it is a 22 pf in todays standards, is that correct? Also, it looks like you don't usually need to replace any mica capacitors when recapping a unit. This radio made through a house fire although it didn't actually catch on fire it got VERY hot and the capacitors look like they have been subjected to a lot of heat. Thing is I dont see any that look like a mica capacitor anyway, does anyone know if this set actually has any mica capacitors in it or not that I just don't reconize on this old set? I am planning on replacing pretty much all components (capacitors, resistors,etc) in it except transformers, adj capacitors, etc that seem good because of the heat it was subjected to. Any tips or opinions on this please let me know and thanks for your time replying to this.
The mica caps will generally appear as small plastic blocks with colored dots on them. Sometimes red, brown or black package. Sometimes called 'domino' capacitors because of their appearance.
As for replacement....touchy issue. They are generally more reliable than a paper capacitor but their failure rate is increasing as time goes on. They are most often found in the rf stages of the circuit and may have oddball values that are difficult to find and are critical to the circuit. I generally leave them alone unless I find a need to replace them. One of the main causes for failure these days is a ham-handed repairman twisting them over to see what is printed on the other side. That breaks the seal where the lead exits the device and often 'twists' the guts internally.
One exception when it comes to such caps. You'll see some larger values, say over 5000pf (.005) that have paper construction internally. They are as bad as any paper cap. A notorious use of them is as AC line bypass. Usually a large one, like 0.047 mf. Definitely replace that one straightaway!
If it is the line bypass (L to N) replace it only with X-rated cap. If it is Y-cap (L/N to Chassis) replace ONLY with Y-rated caps. Some caps are X/Y rated, usually Y is rated at lower voltage than X, but still OK used at 110VAC.
Not sure which would be L to N, etc. They look to me like either they are paper caps or the block type caps in this radio. Again, the smoke on the chasis etc is from a house fire not the radio itself burning up. I can power up the radio but I do not get any stations, I do get a hum from the speaker though.
Looks like the line to chassis caps are in the upper left hand corner and appear to be the domino style paper caps that exray mentioned. They should be replaced, but they are not the cause of the radio not receiving signals.
They have a bad habit of exploding when the set is left plugged in even when turned off, as they are before the power switch and always have line voltage applied.
No, replacement line caps will look different and may be either ceramic or film. It should be a Y rated cap as Morzh explained above. You may be able to cut open the original cap and remove the paper assy and replace it with the modern one so it looks original.
Are you planning on restuffing the original wax caps with modern film replacements?
Don't cut them - if you pull by the wire most probably the wire will pull the potting, whatever it is, tar or epoxy, with it. Then you pull the second wire. This may just yank the whole gut with it. If it doesn't, you just push it out. If the wire does not pull easily (sometimes it happens) scoop out the epoxy or drill it out.
Morzh, the line caps he was referring to are molded bakelite so they will have to be cut open to insert the replacement. They are the type that look like large molded micas.
Ah those....I thought you referred to those in your photos. Yes molded mica-likes may have to be cut. Not sure how convenient it is and whether it is easy to do, this is why they are molded - the outside is some sort of resin or epoxy and I am not sure it will easily lend itself to cutting. I think it will crack and go in pieces.
I would just replace it with a suitable cap. As much as I like to get as authentic as possible, I easily compromise when I see that the effort is disproportionate.
I wasnt planning on restuffing the wax capacitors because if you zoom in on the pic you can see that the smoke and heat messed up the outer coating of the capacitors so they have bumps in them and it is hard to read if not impossible to read the values on most of them anyway. Doesn't look to clean in there with those caps there.
So your saying those block like caps are Y rated, but I can use ceramic or film caps not mica? I will have to make sure I find some of those for those two then. Thanks for everyones input.
(This post was last modified: 07-04-2013, 09:54 PM by Warren.)
Those that are used as AC Line Filter. They can be any. Typically those across L and N are film and should be X-rated. Those that are across L and Chassis or N and Chassis are usually ceramic and should be Y-rated.
Rule of a thumb - Anything that touches L or N on one side and Chassis or anything user-touchable on another should be Y-rated.
I have a dumb question. I have looked up the capactitors for the radio online and no one ever asked if they are ac or dc caps. I called a local electronics store and they said they needed to know if I need ac or dc caps to tell me if they ad it or not. Consudering online it wasnt an issue I am wondering, were they just to lazy to look up a cap or is it actually an issue? The schematics dont specify. Thanks.
Now if you mean the caps some of which show DC working voltage and some AC - yes, that does exist.
Typically AC rating is shown with AC line filter caps.
