[morzh]

Roy,

I will be very, very blunt about it, even if it does not sit well sith some folks (it seems here on Philco Phorum most of us understand this).

People who advocate against "shotgunning" (and let's be clear what I consider "shotgun repair": in my book it is "yank ALL capacitors and old resistors", which is ... not optimal, let's call it that) in the sense "only replace paper caps that are well out of range etc" are purists who let their purism get ahead of common sense and sound engineering / circuitry understanding judgement. Worse yet, they might have no such understanding in the first place.

Now. A capacitor, unlike resistors, ages due to chemistry change. Resistors age due to oxidation, contact of the carbon rod to the leads deteriorating etc. It is not dangerous but could unimprove the radio performance or stop it from performing altogether (like that - 10K, is it?- resistor in the oscillators) or if it is a bias-forming resistor that drifted too far. But it is unlikely to redplate your tube, blow your power supply, or just short the B+ and get something smoking. Which is what an old paper capacitor can do.
The chemistry change in them results in two things:
1. Most important: leakage. Meaning, the capacitor develops excessive conductivity at DC, which an ideal cap does not have and a real world cap has so little (if it is ot an electrolytic) we do not need to worry about it. The old paper cap most often has quite measurable leakage. It depends on brands, some (like German ERO), are real bad, some like old american caps, are better. But still leak.
This results in, for example, in case of DC blocking cap, in not fully blocking that DC, which will change the bias of the tube that is after that capacitor.
In case of bad leakage and he cap being the DC filter, it could smoke a resistor, or, if used in parallel to the electrolytic Power supply filter, killing the rectifier tube, filter choke / field coil if used as such.
They also used them as input DC line filters, and this is just plain dangerous (death capacitor).
Now, is there a mathematical probability that a capacitor somehow survived and is still good?
Sure. 0.1% at best.
Is it worth the risk? No.
Is it worth extra troubleshooting when you do not know what is wrong, and one of these is the culprit? No.

2. They also drift quite a bit. Usually it shows up as an increased capacitance. From 100% increase in american caps to 600% increase in those aforementioned EROs. This, simply speaking, results in some circuits not performing according to their intended operation.
Part of this is also the leackage: it does show as the capacitance increase.

There are other things like the loss tanget increase (same reason, chemistry change), etc etc.

And I intentionally omit the electrolytic caps. I hope we have an understanding about these: they just have to go without discussions and explanations. Because the electrolytic capacitor by definition is a limited lifetime device, with the typical today caps (at the max ripple/voltage) being 1,000hr and the best are 10,000hr. Today's. Not those made in 1930s, which they were just learning about how to make'em.

The ONLY type of capacitors to be left alone and not touched are the mica capacitors. They do not age. They do not change values or parameters. The only reason to change one is as the result of troubleshooting confirming the cap is bad, and I am yet to see one fail.

Now, because in this country we have the 1st Amendment and such, people have the right to be wrong.

My experience (both as a restorer-hobbyist and an electrical engineer (I am and still practice)) can be formulated like so:

Without known confirmed working components any troubleshooting is a waste of time.

I make sure all my electrolytic caps and paper caps are replaced.
I make sure all of my resistors are within tolerance or, in non-critical circuits, close enough to the specs.
I check all my tubes prior to inserting them back.
I make sure my transformer is good (the first thing I do, in fact, because otherwise it is like reviving a patient whose heart has been removed).

Then (I might forget less important things I do prior to the power up) I can do the Variac power-up.


PS. There are people in this world who believe in Oxygen-free copper, normalized impedance transmission line audio cables, breaking in these cables, and, amongst other things - in leaving the paper caps in until proven guilty. They are likely good people and I respect them, as people. I also think their understanding of the process (they might have tons of experience, but I am talking about understanding) is garbage and you would do yourself a big favor if you politely listen to them (no reason to make enemies) and then forget what they told you.

Amen.