[EdHolland]

I started through the FM alignment again today, focusing on what can be done on the multiplex decoder with limited equipment:

Signal generator, capable of audio through RF.
Fluke DMM
Communications receiver with 30 kHz lower end reception.

I do lack an oscilloscope, which is called for in the tests.

The first job is to measure bleed through of an 80 kHz signal, and set the coil, L100, to null it. I connected a wire to the test point, and wrapped this to an antenna wire (gimmick type coupling) connected to the receiver. 80 kHz was fed to the appropriate conection. It was easy to hear the rise and fall in signal, and set this filter per the instructions.

Next, 19 kHz, simulated FM stereo pilot tone is fed, and the transformer Z100 peaked up, measuring a DC voltage at the first section grid of V101. Straight forward and easy.

The next part is where a scope is called for. The 19 kHz is used to sync an oscillator running at 38 kHz, which operates the synchronous demodulation at V102. Normally this is done by establishing a stable 2:1 Lissajous figure on the scope. While tuning stations, I could use the receiver, tuned to the 38 kHz oscillator (actually a little off, to get a beat tone) and hear it lock in. No adjustment seemed necessary to Z101.

Returning to reception, things were improved a lot on all stations. The MPX has a high threshhold before auto switching to stereo (something I learned in an Audio Karma thread) so not all stations trigger the switch on my garage dipole. Those that do, are now crystal clear

Waiting for new valves now, and I'll call this one done

Cheers

Ed