Capacitor archaeology

[Rhodesplyr]

I've got a 1958 Hammond B-3 preamp on the bench at the moment, and it has some coupling capacitors in the .033uf to .33uf range that are sealed in large, hard brown plastic tubes. According to the EIA codes, they were made for Hammond by the John E. Fast Company in 1956.

Just out of curiosity, does anyone know what sort of dielectric these might be? I've heard various speculations over the years. I do know that '56 is too early for Mylar/polyester film. Paper is certainly likely, but there are claims that they were polystyrene (which was around at the time). The packages are very large considering the stated values, and Hammond was known for spending a bit extra on parts where they wanted reliability.

In any case, a rough in-circuit measurement with my ESR meter shows that they're spot-on what you'd expect the meter to show for the nominal values, and they don't leak.

By the time my 1959 C-3 rolled out, these capacitor types had been replaced by Z5U ceramics.

[Enzo]

They were probably paper. Nice molded plastic ones took over for those god-awful wax things. It was just an organ, there was no need for any fancy esoteric stuff. Especially if they were replaceed by ceramics later. If they are still good, then they are still good, y'know?

[Rhodesplyr]

Since the power supply is completely disconnected, I ran some in-circuit tests with my bridge, and the dissipation factor readings point to paper--the same readings I'd get from high quality paper caps like the ones hermetically sealed in metal tubes.

On the topic of "if they are still good," the preamp has two electrolytic cans, both of which were disconnected by a previous tech. Though I won't put it back in use, according to my test equipment, the three-section higher voltage can is still pretty good. On the other hand, the other can with two slightly lower voltage sections and two cathode bypasses is completely shot--dissipation factor so high my bridge can't read it. They measure open on my ESR meter.

The point I'm making here, with regard to discussions I've read on here about replacing old electrolytics, is that it seems problematic to me to make any generalizations about them. Here, we have two cans, same manufacturer, that have been close together in the same preamp for >50 years. One can looks OK to my diagnostic gear; one looks totally shot. And the one that measures OK is located closer to hot tubes.

[jrfrond]

I go the other way on the electrolytic theory. In my eyes, ANY e-lytic that's as old as me is a potential trouble source and should be replaced. You've got one bad filter can. The other one is good. I call that a lucky shot, for now, and an accident waiting to happen.

Caps were definitely of a hardier breed back then, but electrolyte does dry out, no matter who made the cap or when it was manufactured.

[Enzo]

I differentiate between electrolytics and paper or film caps. The lytics dry out and go bad, the film or paper caps CAN last for decades. Many don;t but some do. SO if the old paper caps seem to be OK, they probably are. If 50 year old lytics seem OK, they may be at the moment, but they could also wake up tomorrow puking. Pre-emptive change there makes good sense.

[Rhodesplyr]

John,

All electrolytics were replaced in this repair with Panasonic EE/ED series for the high voltage (rated long-life and 105C) and Nichicon KZ for cathode bypasses. I haven't reinstalled it in the organ yet, but with inputs grounded, the output is close to dead quiet on a scope, even without a shield on the bottom.

I only mentioned the electrolytics because I recalled reading a thread where there were competing claims about whether or not one should replace them simply on the basis of age. I've replaced more aged electrolytic capacitors than I can remember. Interestingly, in this repair, two 1995 date code (I think) NTE 33uF/450V electrolytics from a previous cap replacement measured completely open--not shorted, but no measureable capacitance whatsoever.

In particular, in consumer Hi-Fi tube gear, there was always a premium on small chassis size. This places the caps close to other hot parts and spells death. In pro audio gear, I notice that there's often a little bit more breathing room for them.