Bumble Bee capacitors

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"Bumblebee" capacitors

A variety of paper-in-oil capacitors known as "Black Beauty" or "Bumble Bee" are found in much of the classic Tek gear. Some, not all, of these capacitors age very poorly, including shorting out, which can harm the surrounding components in a circuit. Many experts recommend replacing all Black Beauties with modern capacitors. Modern equivalents are not a problem to find.

If one does not wish to replace the old capacitors, testing them is an option. There are three main ways that capacitors go bad.

  • The capacitor can leak DC (or short completely).
  • The capacitor can drift significantly in capacitance.
  • The capacitor's equivalent series resistance (ESR) can increase significantly.
Original "Black Beauty" capacitors from the HV circuit in a 585 non-A scope (1960). These specimen leaked ~180 μA at 2 kV.
Color code

Each of these problems can be checked for using basic test equipment. It is time-consuming, but if, for example, one is determined to keep an instrument as original as possible, the following sequence of steps will find most of problems with black beauty capacitors. If, at any step, the measurements are bad, the capacitor is bad and it should be replaced.

  1. Lift one end of the capacitor from the surrounding circuit. Follow the soldering instructions in the manual.
  2. Check for DC leakage: To do this, you will need a DC power supply that has current limiting and goes up to the operating voltage of the capacitor. Disconnect one end of the capacitor. Attach a sensitive ammeter in series with the power supply and the capacitor, and bring the power supply up to operating voltage of the capacitor. The DC leakage should be less than 3 μA.
  3. Check for drift in capacitance: Discharge the capacitor (through a resistor, allowing sufficient time to account for dielectric absorption). Measure it with a capacitance meter. If it is outside the original tolerance range, it should be replaced.
  4. Measure the ESR: Discharge the capacitor. Calculate the frequency at which the reactance of the capacitor should be 0.1 Ω. Set a 50 Ω signal generator to that frequency and an amplitude of 1 V. Connect the signal generator to the capacitor. Probe the voltage across the capacitor with a good oscilloscope. There should be less than 100 mV of AC voltage across the capacitor.
  5. If the capacitor passes these three tests, it is probably OK, and the lifted terminal can be re-soldered. It is a good idea to somehow mark the capacitor as having been checked.

An easier way to test these capacitors for value, leakage and ESR is with an instrument that performs all three functions, such as a Sencore LC75. A benefit of using the Sencore is that leakage can be checked at the operating voltage of the capacitor, up to 600 V with the Sencore LC75, and up to 1000 V with the Sencore LC101.

Some vintage RC bridge testers offer capacitor leak testing as well, like EICO 950B, Heathkit IT-11, Heathkit IT-28, Heathkit C-3 Condenser Checker etc. These instruments supply up to 600 V DC (IT-11) and indicate the leakage current in a Magic Eye tube, usually with a closed eye indicating excessive leakage current.