This Eico 950B was the first piece of vacuum tube test equipment that I recapped. Most of the capacitors look like they were original, so based on the date code, it was working for a solid 65 years until I got ahold of it - from 1957 to 2022. It does work again, and is much more accurate than when I started, so I’m pleased with the results so far. There are a few modern safety features as well, including a safety capacitor, fuse, and grounded line cord.

Repairs

The Eico 950B is a popular vacuum tube capacitor tester, with quite a few good articles written about restoring them:

• https://antiqueradio.org/EICO950BBridge.htm
• https://lazyelectrons.wordpress.com/2017/11/10/eico-950b-repair-restoration/
• https://people.ohio.edu/postr/bapix/CapChkr2.htm

The wealth of existing literature encouraged me to pick one up from eBay and fully recap it, saving all of the testing for the end. It was an interesting choice and not necessarily the method I would recommend. Testing each component after you replace it takes a while, but you can take before and after reading with an LCR meter - even if there are other components in parallel, if you replaced the original part with a similar value, the final reading should also be similar. Be careful of the testing frequency, that can really impact the readings for some capacitors.

Most of the linked articles write about their difficulty finding an 8uF, 525V electrolytic capacitor. I ended up staying within the Vishay family and used a pair of 20uF, 350V parts from the TVA ATOM range, with a 470K, 1W resistor across each. I picked up a few extras and selected the lowest values, both in the 18-19uF range, resulting in a 9.13uF capacitor that should be good for at least 600V.

After replacing all of the passives, even the single precision mica, the meter is pretty accurate. Some of these test components have drifted from their original values, but the reading looks to be within ±2-3% of what the DER DE-5000 reports. While taking pictures, I was interrupted by a creature that also stores energy in a static electric field.

This tester can find failures in some high value, high voltage capacitors, the kind you don’t want to touch. When a capacitor is on the verge of failing at lower voltage, like the 25 MFD wax shown in the photos, the eye will wobble and open slowly, then stop opening when you reach the failure voltage. Since this applies voltage to the capacitor, make sure to discharge them after testing and reduce the voltage dial slowly, not all at once.

Replacement Parts Installed

I measured all of the replacement components while installing them, but cannot find notes on those values, and had to remeasure the capacitors in-circuit after the rebuild was complete.

Original Parts Retained

The potentiometers all read within tolerance with the leads cut, so I left those in place and reattached them with some soft silicone wire. The tubes appear to be working and I don’t have a way to test them out of circuit, same with the transformer. There are no diodes or other components, this is a very simple circuit with very high voltage.