Sprague Model 16 Tel-Ohmike Capacitor Tester (1940)

This Sprague model 16 Tel-Ohmike is a handy instrument. Although its sensitive leakage tester can be used to check many components, its main attraction for me its ability to test capacitors at their rated working voltage.

I rescued it from a pile of scruffy testers at a swap meet, all marked at $5. I had heard about Tel-Ohmikes before and I was curious to find out how one worked. Although the initial appearance wasn't encouraging, it cleaned up well, as you can see in the photo.

If you are working on vintage electronics, you'll find many failed capacitors, especially the paper and electrolytic types. My recapping article has lots of information about capacitor replacement. In many cases, testing the old caps is a waste of time: when the failure rate is 100%, you'll save a lot of time by replacing them without testing.

Mica capacitors are another matter. They are considerably more reliable than the paper or electrolytic types, but they can still fail. To test them accurately, you need a checker like the Tel-Ohmike that can apply the full rated voltage to the capacitor.

Modern cap checkers can test capacitance values, but they only apply a fraction of a volt rather than the hundreds of volts used in vintage equipment. When old caps fail, it's usually not by changing capacitance but by leaking when voltage is applied. What you need is a Tel-Ohmike or similar vintage unit.

Here is the Model 16 operator's manual, provided by a fellow member of the Antique Radios Forum (thanks, Mike!).

Here's an interesting article with more information about this tester and other early Tel-Ohmikes.

I own two other cap checkers of this type, a Solar CB-1-60 and an EICO 950B:

All of them operate on the same basic principle. This Sprague can test up to 1000 volts, however, higher than the other two. I look forward to restoring it and giving it a whirl. Like any other electronic device more than 70 years old, this will need routine maintenance before going back into service.