Yes, that non-hermetic C3 is a wet-slug Ta cap. These are quite special
(and expensive), and can be amazingly good, or cause lots of grief if
they leak. The key is in the seal around the Ta positive lead, and how
well it holds up over time. Ultimately, the cap either loses its
necessary electrolyte hydration water (by diffusion around and through
the seal) and capacitance, or in bad cases, the sulfuric acid solution
leaks past the seal, and flows down the lead by capillary action. If it
gets far enough, it can corrode and ruin adjacent circuitry. This
process may take twenty to fifty years or more though - they are quite
long lasting. There are true hermetically sealed types too, but these
are in a class by themselves, in terms of cost and lifetime.
I happen to have a complete junker A7, which I'm currently looking at
gutting out in order to build a 10 MHz power amplifier into the box, as
part of the "Z3801A in a 5065A carcass" project that I've discussed
recently. The other day, I pulled that cap and checked it. It's pristine
in appearance, and measures within one percent of its 100 uF - not bad
considering its 1968 vintage. This one's a keeper - I always save these
kinds of caps for special uses, and have quite a large collection.
Anyway, C3 is in the front end amplifier, and as Corby said, if the
leads rots open, then you lose the AC gain. If the proper 137 Hz signal
is present though, then it's likely OK and working. If it's not right,
then C3 could be the cause, even if it looks fine physically. It could
have lost its water, as mentioned above. Measuring it will tell for sure.
If it is bad, and needs replacement, it may be an expensive proposition
to get the same part type new. Fortunately, in this application, the
circuitry impedance is low enough that you can probably get by with a
regular dry Ta cap, especially if the unit does not have to operate at
high temperature extremes. The main issue is DC leakage of the
capacitor, and the wet-slugs excel in this, especially at high temperature.
Ed
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