From the Rain City Audio Repair Blog:
This Sherwood FM Stereo tuner came into the shop recently. It’s owner successfully completed a re-cap of the unit, but the specialized tools for performing an accurate FM Stereo alignment are beyond the reach of most hobbyists, and so he sent it into my shop for final adjustments to make it perfect. The owner reported it worked well on mono, but the stereo light never illuminated, and the dial tracking was a bit off.
The owner had done a good job on the re-cap, with nice clean joints, replacing all but the two output electrolytic caps.
Initial measurements validated the alignment; in mono it was receiving at about 1.6% THD.
Up first was a mono FM alignment, adjusting the cascode RF amplifier, oscillator, and IF chain to bring all the tuned circuits into proper adjustment, improve dial tracking, and center the tuning meter.
After those, and several more adjustments on the bottom side for the lower cores, distortion dropped off to around 0.05% under ideal conditions.
Unfortunately, there was no action on the stereo circuit even when fed from the Sencore SG80 generator. I spent some time tracing out the circuit to understand what’s going on:
One common problem on this style of tuner is that if the stereo indicator lamp has failed, the entire rest of the stereo circuit won’t operate. In this case, the lamp is a NE-2H neon bulb, which can lose its neon with age and fail to strike.
Swapping in a new bulb was the first step.
Success – now the receiver responds to the 19 kHz pilot signal!
Time to finalize the alignment, including the 19 Kc, 38 Kc coils and transformers, the 19 Kc null, and separation adjustment.
The 19 Kc null serves to remove residual 19 KHz audio from the final output signals. It’s adjusted for lowest 19 Kc level which provides proper stereo response.
All set! The indicator light correctly responds to stations transmitting in stereo, and there’s about 10 dB of separation between channels. Much more often just isn’t possible on a tube MPX design. All in all, it sounds great!