[EdHolland]

I did get to the electronics bench today, and took another look at IF alignment up to the discriminator. It turned out to be an adventure. First off, I warmed up the generator and set it to 10.7 MHz, using a portable shortwave set to check frequency was accurate to a few hundred Hz. I made a coil of 22 gauge wire, probably 5-8 turns, and large enough to slip over the RF amplifier (6HA5/EC900). This made for a good injection that made it's way through the IF train, and level adjustable to suit any requirements.

Before making adjustments to the receiver, I thought to sweep the generator (by hand) across the IF passband. Even the AM on my old Heathkit has quite a warble, making for a useable FM tone at 400 Hz. It was evident that the Fisher's response was quite unsymmetrical as the generator changed either side of best audible reception, and measuring discriminator voltage at the IF test point. On to a repeat performance of alignment

This time, I attended to both top and bottom slug adjustment of the IF cans - Why I had not realised they nearly all have two adjustments is another matter for discussion... This went well, peaking up the voltage at the discriminator until I got to Z3. All of a sudden, the adjustment felt odd, and the meter showed the test point voltage had dropped suddenly. More gentle tweaks, but there was still an odd feeling to adjustment and more drop in signal. I feared something awful, and there was nothing for it but to unsolder Z3 and investigate. Wiring and orientations were noted and marked, and the transformer pulled from its can.

The construction is of a small coil, perhaps 3/16" diameter, with a large core has a central rod, flat top with a slot, and a wide skirt with a fine exterior thread. This rides in a two sided plastic frame. What I suspect, is that the top slug had jumped it's fine threads, to become cross threaded. I removed it carefully, cleaned the threads and applied just a little graphite using a pencil point. All back together, both cores adjusted freely, and Z3 was returned to the chassis.

Alignment resumed, now, of course way out of whack, but Z3 was brought quickly in to resonance, top and bottom, in a couple of iterations. On through to Z5 and a sound check was in order.

Much better already. Cleaner mono and stereo, with much less distortion and intermod artifacts.. Tomorrow, I'll finish the discriminator and detector alignments, now that I understand how to proceed (thank you again Ron).

Cheers

Ed