[adamrobertg]

I finally started to work on my 39-40 with scary deteriorated rubber wiring coming from the power transformer.  I took the transformer apart, and was able to successfully cover the wiring in shrink tubing and put everything back again with no problems.  I've replaced just about all the capacitors, any dried out rubber wiring, and now the radio is playing well.  Great tone, lots of stations. 

I noticed that two capacitors, both .003mfd ones near the type 42 audio output tubes, were originally rated at 1000v.  From what I can understand on the schematic, there shouldn't be anywhere near 1000v in the radio.  All of the other paper capacitors removed from this radio were 200 or 400v rated.  Was this a case of Philco wanting a higher voltage rating than 400v, and the next available was 1000v?  I probably did wrong and replaced them with the 630v ones I have, but the radio is playing fine and nothing has caught on fire... yet.

http://www.nostalgiaair.org/Resources/do...013296.pdf

So the question... can I leave the 630v ones in there?  Or do I really need to find 1000v rated capacitors?

[Ron Ramirez]

Leave the 630V rated caps in there, they will be fine unless you plan to run your 39-40 at full volume eight hours a day, every day. And I am sure you won't be doing that.

The 1000V rated caps were specified to handle peak "spikes" in the audio. Most of us never crank the volume past 1/3 of the way or so up.

[morzh]

Plus, even if you ran it as Ron suggested, the high voltage is there more to prevent flashover in case of the speaker disconnect than anything else.

[nsofti]

I am also in process of obtaining caps for my Philco 39-40. I am stumped as for 30-4455 .1 mfd. What is the max voltage. Also 30-4481 .002 mfd max voltage. I only seemed to find one 1000v oil impregnated cap (30-4469 .003 mfd) did I miss another one somewhere else?

[Ron Ramirez]

Again - just replace with 600 or 630 WVDC rated capacitors. There is no need for capacitors with higher rated working voltages in these old Philco radios.

FWIW - 30-4455 is 0.1 uF, 400 WVDC. I could not find 30-4481 (.002 uF). But to reiterate - capacitors with 600 or 630 WVDC rating are just fine as replacements.

[nsofti]

looks like I made a typo the 30-4481 is .02 mfd
As for the question on 30-4469 .003 mfd 1000v I only located one on the schematics but did locate two 30-4580 .003 mfd and wated to make sure I wasn't missing another 1000 or higher volt cap.

[Chas]

There is a failure mechanism at work when a tube radio is playing and reception is during an electrical storm.

Plate current via the audio output transformer is abruptly cut off and the resultant magnetic field in the core collapses. This induces a very high peak voltage of several cycles. Without any capacitor to absorb this spike the transformer will arc internally. When that occurs windings may still conduct but there will be a carbon path and the generation of acids, that, in time, will corrode the winding and open the transformer. Factoid: copper in an arc/flash expands 67k times...

Most radio designs provided for reduction in adjacent channel (10khz whistle or "tweet") with a filter just after the detector but also with a capacitor across the primary of the output transformer or from the output tube plate to B-. The spike is suppressed but the voltage value of that capacitor must be high enough to withstand that static generated spike. A thicker paper/foil condenser works as was installed OEM. In modern times, generic metalized caps cannot propagate this high frequency spike across the metalizing even if the cap is rated for 1kv. The metalizing disconnects permanently but the insulating film does not perforate. To the user, there is little difference (in audio) until the metalized cap looses almost all of its capacity. Then, the output transformer becomes vulnerable to static destruction. A occasional phenomena of a failing metallized cap is noise, random, very high frequency spikes as the disconnected area of metalizing connect and disconnect. Often heard is 'ticks" in the speaker. The noise can be captured easily with most scopes.

That said, simply increasing the working voltage of a replacement metalized (common) cap in the plate bypass is not a method as it is not the plastic that is failing. The metallized is ill suited in that circuit, its "plates" are just not thick enough. Choose a metal foil/poly cap that has a published dv/dt rating. The metal foil will not deteriorate and the pulse current will be absorbed without damage to the dielectric. In some radio designs this same cap provides a dual function of tone control, that cap should also be of the same type with a dv/dt rating. Some radio designs have both a transformer shunting cap and a cap(s) used for tone control. Can the tone caps be plain metalized? IMHO yes, as the pulse is being absorbed effectively by the foil/poly cap.

I have read of many radio repairs failures of output and even some interstage transformers failing and though amateur transformer forensics reveal corrosion and often associated with moisture, the green dots in an OPT winding can be the result of static induced arcs and acid production.

I would give some thought after installing a Hammond (costly) replacement or preserving an OEM functioning transformer...

YMMV

Chas