Old B/W IF Alignments Almost Too Simplified

[jshorva65]

Tuner alignment is seldom required unless capacitors and/or resistors inside the tuner required replacement during the recapping of the chassis or some other "major surgery" on the tuner was required for other reasons (as correcting a previous repair which had been done poorly). A response check of individual channels can be set up in a manner similar to the IF setup described above, but with the Sweep center and Master marker settings changed to those of the channel to be checked or aligned and the injection point moved to the Antenna terminals. Service manuals generally include the basic procedure.

Oscillator alignment using the Sound Carrier frequencies of each channel as Sweep center and Master markers, turning off the Interval marker, and narrowing the Sweep Width to about 200 kHz can be accomplished with great accuracy on sets having Discriminator sound circuits by viewing the S-curve at the Sound Detector output and setting the marker at the center of the S-curve for each channel. This applies to switch-type tuners such as those used in RCA models 621TS, 630TS, 721TS, and similar which have provisions for Oscillator alignment on individual channels.

Also, the RCA Service Manual for the 630TS includes a procedure on Page 18 to diagnose and correct Oscillator Pulling. If the correct settings of individual Oscillator adjustments become progressively farther from center and/or entirely out of range of the trimmers with each of the channels below 13 or 6, Oscillator Pulling is the most likely culprit.

RCA tuners (and many others) of late-1940's vintage use push-pull circuitry in the RF and Oscillator stages, and a proper "balance" between the two triode sections of the 6J6 tube used in each stage is critical. A properly-balanced 6J6 is also important in the Mixer stage, but is absolutely critical for RF and Oscillator tubes, just as proper balance is essential in Driver and Output stages of push-pull circuitry in Hi-Fi amplifiers. Oscillator Pulling effects may be reduced by employing a Dynamic Mutual Conductance tube tester to identify properly-balanced 6J6 tubes for use in these critical stages.

If preliminary tests indicate that the tuner does not need alignment and all tubes are "Good" this indicates that any imbalance observed with the tubes' test results had been compensated adequately and has not changed since the tuner was last aligned. In other words, "If it isn't broken, ..."