Conventional wisdom on just about any vintage electronics is that if there's a problem it's nearly always capacitors. That's almost always true, since electrolytics and paper/wax capacitors degrade to junk very rapidly, and electrolytics age especially fast if the device is dormant for a few years or more. By the time a radio device is 50, 60, 70 years old, it's generally a given that it will have a lot of defective capacitors causing a myriad of troubles.
So assuming good tubes (there's more to know about tube testing as well; it isn't as simple as
good or bad), capacitors are the most common source of trouble in most vintage electronic devices. This accounts for many a radio repair tech's
default mode: First check the tubes, and then replace every capacitor, and the device will start working. A quick tweaking of the trimmers (or a proper circuit alignment), and you're back in business. Replacing every capacitor is also common practice when the goal is also to avoid future problems caused by continued capacitor degradation and failures. It can take less time to completely
re-cap a device than to trouble shoot multiple issues caused by several simultaneously defective components. This is why technicians so readily embrace this practice.
This is not true for 1929 RCA Theremins! I repeat: This is not true for RCA Theremins, or for that matter most any 1928–1930 RCA product.
For one thing, these early RCA's don't use ANY electrolytic capacitors, and the jellyroll (pitch, paper & foil) capacitors that RCA did use at this time are tremendously better than the paper/wax jobs that came out just a couple of years later.
The early RCA capacitors are ultimately pretty reliable. They may get somewhat leaky (degraded insulation resistance), but they almost never open or short, and the leakage (if there's any at all when operated at their intended voltages) remains substantially less than what we typically see in the later style paper/ wax capacitors that were factory installed in nearly every electronic device from the early '30s through the '50s. Further, our many years of radio servicing experience has shown that most 1928–30 RCA
Radiolas will not only perform, but perform impressively well with no excessive current drain, even when they have (original) capacitors that measure some leakage. The same has been found to be true with the contemporary RCA Theremin production.
Since every surviving RCA Theremin is a rarity, and to verify originality by comparing its details to known, unaltered original instruments, components should remain unmodified to the extent possible.
To illustrate the point, unrestored but intact, original vintage and special-interest automobiles have been bringing more money than show-quality restorations lately, because unaltered examples have become so much scarcer than restored ones. The mantra in the car collecting world has become
It's only original once! Some evidence of aging is perfectly acceptable, and is preferred over eliminating every trace of it. This authenticates the history of the piece. The same thing applies, if not more so, to RCA Theremins.
The last thing you want is to assume defective capacitors, and get rid of the originals (which almost never need replacing) or make questionable modifications in an attempt to get an RCA Theremin working properly. This would be fine on a commonplace $25 flea market radio find, but it is best avoided on rare and valuable artifacts. By replacing components in the RCA Theremin unnecessarily, you not only compromise elements of originality but if it didn't work before replacing capacitors, you likely still won't have a working instrument after replacing them. You've just made it potentially less valuable and less attractive to a discriminating buyer. It's only original once! When spending thousands of dollars to acquire an RCA Theremin, the last thing the astute collector wants is to deal with someone else's modifications and attempts at repairs. With unnecessary changes, reliability may also be impaired. It's surprising how few people know how to solder properly; even if they've been doing for years it doesn't mean they've been doing it well. Every solder joint that's been altered becomes another possible point of trouble if it isn't done right.
So, if you take capacitors out of the mix, and assuming that no one has attempted repairs that caused additional problems, there are other common things that afflict RCA Theremins which we have written about.
In the realm of manufactured goods, RCA Theremins are extraordinarily rare with only 500 produced to begin with, which amounts to less than 1% of a single-week production of other goods at RCA alone, not to mention industry-wide contemporaneous production by Atwater Kent, Crosley and the other big manufacturers of the era. Due to the embryonic nature of the production run, economy of scale was never achieved with the RCA Theremin, and many of its individual components had to be hand-selected, individually tested in circuit, and even modified by the factory in order to achieve proper functionality. The total run of 500 units would more aptly be called a
pilot production run, rather than
The RCA Theremin was engineered by Lev Termen, and is as close as most people will ever get to his genius, and to the manufacturing origins of the entire electronic music industry. Keep those RCA Theremins as original as possible, and when troubleshooting or replacing components, treat the instrument as the precious and rare artifact that it is. Be minimally invasive, maximally informed and keenly observant when working on one of these historic instruments.