[TheUniversalDave1]

I'm on the fence about replacing those wax capacitors. Everyone at my radio club - even the old guys - say you should leave them in there. They say that it is not worth the time if the capacitor is still good. If I'm going to replace any capacitors, it will be in the tone circuit. The first two positions sound to "tinny" for my ears. I'll be slightly raising the capacitance on the first two to give a slightly bassier sound.

[morzh]

Dave

Most old paper cap devlope leakage oved time. Matter of properties of the materials.
This translates into DC currents, small though they might be, in places where there should be none. Which changes DC biases, or load power sources.

Now, you could check every cap for value and leakage, for which you will have to take it out and test it with an appropriate tester(I do not have such a tester) or you could replace them. When replacing, you could restuff old shells with new caps so it will look authentic if this is of concern.
But simply not replacing them without checking is not a great idea.

[Mondial]

There are some locations in the circuit where replacement of the paper caps is very critical. The most important is the coupling cap between the first audio amp plate and the audio output tube grid. Any leakage here will cause the output tube to draw excessive plate current, loading the supply and overstressing the tube and output and power transformer. As the radio warms up, the leakage will increase and eventually cause a runaway condition. This can cause overheating and damage after the radio has been on for a while.


Any of the paper caps subjected to high B+ voltage and where their failure would cause additional damage should be routinely replaced.

[Arran]

 This isn't normally a concern with Philco sets because of the way they are wired, but one critical cap that should be replaced in many old radios is a tone correction capacitor across the primary of the output transformer. These caps can see as much as 900 to 1000 volt spikes across it, and if they short they can do real damage to the power supply. In Philco sets they typically connected a tone compensation cap between the plate of the power output tube, in series with a resistor, and then to the cathode of the audio output tube, so less danger there.
 The old timers talk this way because that was what was drilled into them 60-70 years ago, back then you could get away with it because the capacitors were 1/2 a century newer. Also back then if lets say you burned anything up you could got to your local parts jobber and pick up a replacement or order an exact part from G.E, RCA, or Philco, you can't do that anymore.
Regards
Arran

[Eliot Ness]

We're not out of the woods quite yet, though. I let the radio play for about 30 minutes, and when I went back into the room after breakfast, I was met with a very powerful smell. Burning plastic or something. It was still playing fine, but the whole room reeked of that burning smell

I'm on the fence about replacing those wax capacitors...........

Dave, so something is overheating in your radio and you are on the fence about replacing the highest failure items in a radio of this vintage? 

The only way you'll know if those wax caps are good is to test them for capacitance and leakage.  And even if a couple of them test OK today, that's no guarantee that those 75 year old caps won't go bad the next week, next month, or the next time you turn it on.

Capacitors are cheap and easy to replace, and like Mike said, if you're concerned about looks just restuff them so it looks original.  It's your radio and you can do what you want with it, but if you're going to play it safely you need to find out what is overheating and why.

[GarySP]

Hello Dave.  I couldn't agree more with my fellow forum member comments about the caps.  As a rule, I change all caps and resistors in any set I plan on enjoying down the road as a matter of principle.  Those old components are ticking time bombs.  They may work for awhile, but why take the chance when they are probably the least expensive components in the set.  Furthermore, when they do fail, they don't like to do so alone.  More often than not they will take out other more expensive and harder to obtain parts with them.  While vacuum tubes usually won't go bad waiting to be used, caps and resistors will.  That's a nice looking radio... don't let it fry down the road because you wanted to save a few bucks now.  Take care, Gary

[TheUniversalDave1]

Since this radio has a transformer, I'm assuming that it is okay to use a non polarized plug? I know this can be a problem with AA5's since the chassis can be connected to the hot line.

Where might I find the audio coupling capacitor?

[Mondial]

The audio coupling cap is marked 35 on the schematic. It is a .01 uF tubular wax cap, connected from the triode plate of the 75 tube (Pin 2) to the control grid of the 41 output tube ( pin 4).

If this cap is leaky it could be causing your overheating problem, so definitely replace it.

http://www.nostalgiaair.org/PagesByModel...013289.pdf

[TheUniversalDave1]

I'll be sure to replace that one next time I'm at the AHRS. This weekend is the swap meet, so it won't be happening for a while.

[Nick3092]

It's always a good idea to switch the hot side of the power cord on transformer sets. So a polarized plug is recommended. That's assuming the outlets in your house are wired correctly. Which sadly, many are not. To check, go to your local home improvement/hardware store and they sell a little plug with lights for about $5. Go around to every outlet and it will light up and tell you if your outlets are wired correctly.

And again, I'd stress that you do more than just replace C35. This radio is riddled with rubber wiring where the insulation is most likely flaking off if you look at it wrong. And if you want the radio to perform correctly/reliably, you need to replace all paper/wax caps and check all resistors. Your club members are way off base in telling you that these things don't need to be done.

[TheUniversalDave1]

This radio does not have any rubber wiring - it is all cloth. My vacuum room plugs aren't even grounded, so I wouldn't be surprised if the wiring is completely jacked up.

[klondike98]

I believe the wiring in the power transformer is rubber, at least it was in Bob Andsersen's 39-30 seen in snother thread so take a close look. Sometimes it is still flexible but often is brittle.

[Nick3092]

In the first post he noted the transformer and rectifier have been replaced. So it may not be rubber coated. But your right, my 39-30 transformer is all rubber (in addition to the rest of the chassis). As was the two 39-6 chassis I have. Odd though that it doesn't have rubber wiring in the chassis. I think every '39-'41 Philco I've ever touched was at least half rubber wiring.

Still not a reason to do a proper electrical restoration of caps and resistors though. And cloth wiring isn't 100% safe. It still cracks and exposes wiring over time. Granted not the the level the rubber stuff does. But it can still be a hazard if not inspected.

[TheUniversalDave1]

It may have some rubber wiring - I can't remember. But the transformer wiring is definitely (thick) cloth.

Speaking of rubber wiring, in the coming weeks, I will be attempting the most in depth radio electronic restoration I've done yet. I few months ago, I got a 1940 Zenith 8S563 console for $75. The cabinet is in very good shape, but the chassis leaves a lot to be desired. That thing is chock full of rubber wiring, so that will be an interesting challenge. Do y'all want me to post about it in the non-Philco section?

[Nick3092]

Why not work on the 39-30 properly (wax caps, resistors, rubber wiring) before tackling the Zenith? More tubes means more work (8 in the zenith vs 6 in the Philco). Get your feet wet in the Philco first.