Zenith 12-S-265 chassis #1204, questions on the LO

Zenith 12-S-265 chassis #1204, questions on the LO - Printable Version

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Zenith 12-S-265 chassis #1204, questions on the LO - BDM - 09-13-2008

Ok, here's where I get confused sometimes. While aligning the upper SW band around 18/mc, you of course get two peaks from the sig gen(or any sig for that matter. I know it's the LO + the IF. Some sets you align below the LO, some you align it above the LO(so I've been told). The second peak is probably double the strength of the first peak. I assume I should align it to the second stronger peak? I know it may be a noob question, but it's something I've had trouble grasping...... Icon_confused

Re: Zenith 12-S-265 chassis #1204, questions on the LO - Ron Ramirez - 09-13-2008

Hi Brian

I can't speak with authority on Zenith...it's my least favorite brand... Icon_lol

On Philco sets, when aligning the shortwave bands, it is helpful to check for an image of the signal generator frequency below the desired frequency. Let's assume a 37-650, IF of 470 kc. You begin to align the upper SW band at 18 mc. If you get the signal at or near 18.0 on the dial, then check to see if you can hear a weaker signal at 17.06 mc (940 kc or 470 x 2 below the desired signal). If you can't hear the weaker image, try increasing the signal strength from the signal generator. If that doesn't work...the set is, most likely, badly misaligned.

In other words, the stronger peak is the one you want.

The same probably holds true for Zenith. YMMV.

Re: Zenith 12-S-265 chassis #1204, questions on the LO - exray - 09-13-2008

Some sets go with the LO below the fundamental on the top band. There's no rule-of-thumb and no easy way to tell in advance. Plus I don't think I've ever seen any alignment instructions that would give a specific clue. Some will advise that there may be two peaks on the trimmer and to use one or the other.

Most radios will readily peak on either side AT the upper end alignment frequency on the top band. Image rejection on most consumer radios really stinks up there. But it will generally be obvious in tracking towards the low end if you have hit the incorrect peak.

That RCA that I recently did, Brian, was a classic case. It didn't align particularly well either way.

EDIT - an additional thought. I suppose a guy could sit down with a counter (or digital radio) and measure the LO at various points across the band compared with the dial - adding or subtracting the IF accordingly. One set of measurements *should* line up better than the other against the dial readout. I've never taken that approach and quite frankly some radios might be too poorly calibrated to tell but it might give a clue as to which injection side shows the most promise. -

Re: Zenith 12-S-265 chassis #1204, questions on the LO - BDM - 09-14-2008

Well, it definitely prefers the high side, or the stronger of the two. The BCB and lower SW band(or MW if you will), aligned up nicely. But the upper SW band is a joke. Above 10/mc, it aligns fairly well, but below that, it goes to H**l in a hand basket real quick! Example, 7/mc is at 7.25/mc, 6/mc is at 6.3/mc. No adjustment for the lower portion of the frequency's Icon_confused

Instead, Zenith used fixed padders. God I hate those things. Oh, I did try loosening them, made sure they're grounded properly, etc. etc. Icon_rolleyes

Re: Zenith 12-S-265 chassis #1204, questions on the LO - Doug Houston - 09-15-2008

I once had a 12 tube Zen-itch chassis, which Ire-capped to take to Elgin and peddle. It shaped up OK, and I proceeded to align it. I have the Zenith books, so I followed the alignment procedure. All went well untol the high band. For Pete's sake, No alignment trimmers. There was no way to bring in the alignment of that stupid thing on that band. No wonder Ron says what he does. The worst was that this chassis was about a 1937 12 tube set with P-P 6L6 output. Wot crap!

I have a couple of Zenith prewar sets that I like, but I feel that most of the Zenith afficianados go for them because it's just the thing to do. The "Walton" cabinet reminds me of a couple of big nostrils.

Re: Zenith 12-S-265 chassis #1204, questions on the LO - BDM - 09-16-2008

Man, the BCB and MW bands aligned up nicely. But the SW band has me pulling my hair out!! Icon_mad

Re: Zenith 12-S-265 chassis #1204, questions on the LO - Doug Houston - 09-17-2008

Lucky you! I wish I had some to pull out.

Re: Zenith 12-S-265 chassis #1204, questions on the LO - BDM - 09-17-2008

lol Icon_mrgreen