[time-nuts] Re: Fw: HP 5065A, no 2nd harmonic.

[ed breya]

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