[Chad Hauris]

One problem that can occur is slight leakage in old electrolytics that is not enough to cause a noticeable overload but will cause trouble...this may not show up as an abnormal reading on an ESR meter. We use a "Silicon Chip" brand unit.

We have an early 60's VM tube type reel to reel machine we use for dubbing old reels...checked ESR on all electrolytics and the ESR was good. The audio was very weak and we found a cap section on a can had leakage and was loading down the supply to a 12AX7 preamp tube. Since the normal current draw was very low in that part of the circuit, the leakage didn't cause any noticeable damage to resistors but just dropped the voltage very low. All new caps cured the problem.

Never again! We always replace all electrolytics now in tube type equipment and pre-1970's solid state units and don't bother testing. We have always done it in customers equipment and now do it in all my own.
I actually have had surprise failures of old caps several times in tube equipment which had been working OK for years up to that point so that drove the point home for me that old caps only seem to get worse.