The AM Forum

The SX-28 is a nice and great sounding receiver but it drifts a lot. What can be done to reduce the drift? I guess the VFO is to blame. The resistors and wax paper capacitors in the VFO circuit have been replaced.

Component and tube checks are certainly in order for an old rig like that, but I'd start with checking the AC line voltage from the wall socket. I had a nice SP-600 that wandered all over the place, even after it was warmed up for hours. Turns out it had a multi-tap power transformer for different voltages in. Think it was set on 115 or 117. Les Locklear clued me in to check it out, said to move it up to the next tap, 130v.

Turns out my local line voltage had it right against the peg where it was set. Anytime the fridge, furnace, or anything of any size kicked on, away it went. Moved it up to 130v and it settled right down. Very stable after a few minutes warm up time.

SX-28 is actually a pre-WWII design, so I'd give it a good look there. I don't think it has a multi-tap transformer but if you've got issues, a Variac or bucking transformer set up should take care of it.

The line voltage varies a lot here so that will have to be looked into and see if it moves with the line voltage. Maybe I should get a 'Sola' type ferroresonant unit for the receivers and see if that helps.

I was thinking that someone had written an old article about using VR tubes or a regulated supply to hold the oscillator B+ steady. - but wouldn't a filament voltage variation also affect it?

The VFO in my HB xmtr drifted with line voltage.  B+ was already regulated with a VR tube.  It was the filament voltage causing the drift.  Finally added a regulated DC supply for the VFO tubes.   The drifting was solved.  I also, for some time, use a ferroresonant 6 volt xfmr for the VFO filaments.  This also worked.  The DC regulated supply was just a simple 50W 6.2 volt Zener doing the regulating.

I also tried a small 6 volt Lambda regulated supply, this did not work.  It was affected by the surrounding RF near the xmtr.

Fred