TV analyst/cap replacement philosophies

[jshorva65]

ESR testing and the inclusion of AC line, B+, and sweep circuit overload protection goes a long way and will prevent an unforeseen capacitor failure from resulting in chain-reaction damage such as power transformer meltdown.

Moving the B+ lead from its original connection on the rectifier tube socket (pin 8 on the 5U4 in RCA 721TS, for example) to an unused pin which a tube manual does not warn against using as a tie point (pin 1 in the 721TS example), I then join the "old" and "new" tie points with an appropriately sized PicoFuse. I use 500mA fast-acting in sets with a single 5U4 and 1A fast-acting in dual 5U4 sets. The design rule is a fast-acting fuse should be rated at 200% of normal operating current or a slo-blo fuse rated 125%. The 721TS transformer is rated to deliver 240mA, and 500mA is the closest available standard size. A 300mA slo-blo will also work.

Sweep circuts, I install a 250mA fast-acting PicoFuse in the damper cathode circuit by the same method as with a 5U4.

For AC line protection, I use a depth-limited drill to carefully remove one of the rivets holding the interlock connector in place. Using a Pop rivet gun, I re-attach the interlock with an AES p/n P-0100H terminal strip fastened under the new rivet, move one AC input lead to the new terminal, and calculate the appropriate fuse ratings based on the set's AC input current specification.

Electrolytics with unacceptable ESR, leakage, etc. are of course replaced, but properly-functioning units are left in place.

My ESR meter (Tenma) uses a 50kHz test frequency. I also have a Sprague TO-4 (original 12uF filters show low ESR and are still in place) with all paper capacitors replaced. The two critical bridge capacitors consist of matched pairs selected for +/-2% accuracy. A thoroughly-tested electrolytic stll reading low ESR, low leakage, and capacitance well within tolerance after 50+ years does not need to be replaced. If it does fail eventually, in a properly-inspected and serviced set, fuses will protect the major components and a subsequent ESR test will detect the failure.