I am glad to be the owner of a 5A90 Radio-Gram

I need some mean assistance in working out the exact values of the sealed silver mica caps as getting their exact values is proving to be difficult.

I have a schematic from radiomuseum.org, however, the values are almost in-decipherable.

As the original values have long since rubbed off these caps, would anyone have a better idea of their real values?
Could this be a very similar schematic to a previous model?

Cheers,

Android

If those caps are near the audio amp I'd guess around 100pF.

That diagram at radiomuseum is far too small to be of use.

What are their locations within the circuit? I can make out most, if not all, of the values.

There are a number of Stromberg Carlson models starting with 5A on Kevin Chant's site.
You may find one that's a good match. Generally they are complete manuals and good quality scans too.

Are you planning to replace mica caps? May I ask why?
Unless they are in a circuit where they have had more than 50 or so volts across them for a very long time (silver migration), you don't need to change them. And silver migration in Oz-made caps (as these will be) is rare enough that I only replace them when they are proven to be leaky.

Now having said all that, if you have an intermittent crackle in an audio stage and there is (say) a 100pF mica from the anode to ground, yes, that can be the reason, but so can a carbon composition resistor.

Micas in tuned circuits are a case in point. Leave them, they will be OK.

Micas in tuned circuits can fail. A case in point was the fixed broadcast band padder capacitor of the STC 830 restored by David Tipton (see his website). This cap is "shielded" from HT by the oscillator anode mica and the mica on the oscillator grid. He did not mention that these had also failed or had been changed in the past, so that is a possible reason for the failure. If one of these hadn't also failed at some time that leaves the padder cap failing due to some inherant fault.

He picked up the fault because the tracking wasn't right and checked the value of the cap which was out - high or low I don't remember.

David's videos are well worth watching just to remind you of the necessity for a disciplined approach. He makes clear he is not trained serviceman - this is what he has picked up from experience which most with an analytical background of any sort can pick up. And the videos are a pleasure to watch anyway.

My point here is that silver migration doesn't happen unless there is a DC voltage. Other environmental issues such as corrosion of course are still possible, but rare.
You need a good reason to change them.

I agree, the only one I have ever changed, many years ago, had been hit by something which broke a lead off. No diagnosis needed, obvious on first general inspection.

Is the radio working? if so do not change them.

I believe that I have mentioned this several times before.

The majority of Australian Mica caps after 1938 rarely fail. Albeit that I will HV test them if its out of circuit. Quite often you will find unmarked ones on resonant coils. Woe betide they who interfere with them without cause.

Tuning gangs and their coils are a "Coil Set". That boils down to them being matched when built so that they "track" any unwarranted
interference with any of that coil set will throw them off frequency, or band spread. Mechanical padder caps can be damaged.

With sets & radio grams into the sixties beware of the silver clear plastic encapsulated axial Stryroseals these are not Mica and they are easily damaged by soldering.

Styroseal are generally very reliable and have great temperature stability.

But they have a rare but annoying habit of going O/C intermittently and coming good when "shocked" by a meter probe, for example.

If you have an oscillator that applies no voltage across a styro when it is not oscillating, and the oscillator is dependent on the cap to run (such as a capacitively tapped Hartley), you have a candidate for this issue. Certain model Kriesler TVs were notorious for this, oscillator would fail to start, 6CM5 would red-plate and fail. Replace the 6CM5 and the oscillator would start! Until next time.

Kriesler had some special extended-foil Styros made by AEE to solve this issue. They are yellow/beige coloured.

Kevinchant.com might have a clearer schematic.