[David G4EBT]

There is only one 820pF cap shown on the circuit and that is C47, shown with one end to ground, the other to R19 and to switch contact 19. Those pitch covered caps in Philips/Stella are notorious for become leaky in the electrical sense and what you do about that is a personal choice. They can only be tested for leakage on an insulation tester at their rated working voltage, and even if they aren't leaky now and their capacitance is within their stated range of +/- 10%, given that they're now at least sixty years old, personably, I wouldn't spend time testing them - I'd be swapping them. I say that based on my own experience of these Stella pitch encapsulated caps. As an example, below are pictures of a Stella ST105U I restored - the first is the underside of the chassis with the caps in place - the second shows them after removal and replacement. On some, the pitch had fallen off in places. I tested them out of curiosity at their rated Voltage on my Victor VC60B insulation tester and they all leaked like sieves. One or two had basically morphed into high value resistors.

It's always wise to change components one at a time and check as you go along rather than just yank them out in one go.

How we approach component replacement is a personal decision. Some people just want to 'mend it', 'do it up a bit', 'get in going', 'repair it' or whatever. I have no problem with that approach, but personally, I only ever fully restore a radio, replacing any out of spec components, and perished insulated wiring, because radio restoration is my hobby. I always make a simple 'cradle' on which to mount the chassis so I can work on it and carry out tests safely and if a set doesn't interest me enough to want to fully restore it and keep it, it would never come into my possession and I wouldn't even bother fishing it out of a skip. My view is that routinely replacing paper caps on the sets I restore - whether waxy or pitch coated - isn't a slapdash approach, it's systematic and methodical restoration based on my experience of how such capacitors behave after more than six decades since they came off the assembly line.

As to your Stella ST239U, perhaps the most critical capacitor - the audio coupling capacitor (.022uF).

First pic below shows the underside of my ST105U unrestored chassis.
Second pic is the pitch caps I removed and replaced.
Third pic shows just one cap passing several mA at 150V (that was on my capacitor reformer which I use to reform electrolytics).
Fourth pic is the chassis mounted on a cradle.
The last pic is after restoration. It was an unsafe non-working heap when I got it but it isn't now.

Every success with your set Alan, whatever approach you take.