EH Scott Radio Enthusiasts

The Fine Things are Always Hand Made

Super XII Troubleshooting & Alignment

I've spent several days working on a particularly difficult chassis and learned something I thought I'd share. First, here is what I had to start with: A nice single chassis Super XII. The IF aligned beautifully, and the 3 SW bands also aligned "by the book". The set sounded great on any SW band, AM was basically dead. There are only 3 things that are AM only, so I verified each coil. While checking continuity on the oscillator coil, I happened to notice that the 1K resistor seemed to move a little too freely. (See diagram). 

The solder joint for the 1K resistor was cold, basically not connected. Resoldering it got the AM band back, but then I had another problem. The set aligned well at the high end (1400), but was <way> off at the bottom...my local 610 station was coming in about 640 on the dial. Too far for the padder to bring it in. Scott has a suggestion for that in the alignment instructions:

"Spread turns on the oscillator secondary". Interesting, this set apparently had issues before, because the coil had previously had turns moved on the lower half of the coil. I fiddled with the coil there but it made little difference. Then the word "secondary" came to mind...what about the other end of the osc coil? So I carefully separated a half dozen turns from the lacquer near the top of the coil...and bang - huge difference (red circle end of the coil). I recovered about 1/2 the frequency shift I needed. More coils moved and recheck the alignment several more times and I finally have the AM band working well. Even with the padder, it's still just a touch high (610 looks like its about 615) but the rest of the band is good and it tracks reasonably well. I could probably do a little better by removing a few turns, but that would be painfully difficult.

Moral of the story: know your primary and secondary and never trust the tech before you.

Kent

 

Views: 134

Comment by Jon Ketron on July 10, 2017 at 8:30am

Very interesting, thanks for sharing Kent.

Jon

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