RF Front End Tuning

Hi All,

This is my first post here so bare with me!

I have a Zenith 5G405 that I'm nearly done restoring and have started tuning it up. I adjust the trimmer on the tuning capacitor and it only reduces the signal as I turn it in (tighten). There should be a sweet spot where turning one way or the other reduces the signal, correct?

I decided to check the resonat frequency of the RF stage and it is off relative to the local oscillator. For instance, when I tune the radio to 1500 kHz received signal the front end RF tuning resonates at 1200 kHz. Shouldn't the RF stage track the local oscilator?

BTW, the antenna coil and the tuning capacitor forms the front end stage.

Thanks,

John

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John,

We are mostly the same restorers as on ARF where you asked a similar question.

From your description of the problem, it seems like the radio is receiving signals at the correct frequencies on the dial but the antenna circuit cannot be tuned to get a signal peak.  The signal is highest with the trimmer fully open which indicates there is too much capacitance or inductance in the loop circuit.

Your data on ARF indicated that the tuning capacitance was in the range to tune from 1700KHz to 540KHz.  That frequency range is 3.15 to 1 so the tuning capacitance range would need to be 9.2 to 1.  That would be something like 460pF to 50pF including the trimmer and all stray capacitances in the loop circuit.

Since the tuning capacitance appears to be near correct, the loop inductance may be too high.  Is there a possibility that the loop is not the original one for the radio?

Jay

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Jay,

Yes, that was me. As far as I can tell the antenna and the tuning capacitor are original to this radio.

Maybe I'm being a bit picky? 

I have made a loop antenna using magnet wire that I plan to do more experimenting with, namely, adjust it's inductance (number of turns) to bring the RF tuning inline with the local oscillator. If I can accomplish that then I will consider rewinding the original antenna to match.

I have resisted taking completely apart the antenna as it is held togeter with staples on the inside and sewn edge banding on the outside along with rivets holding the hinges on. I might be able to remove and replace the coil by just removing the staples. At 80 years old the antenna covers are getting a bit raggety...

Thanks,

John

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Normally you should be able to obtain a signal peak using the antenna trimmer when receiving a signal near 1400KHz.  If the trimmer will not produce a peak at that frequency, that would point to a problem in the loop circuit.  Often the tracking is off at the lower end of the dial but it is usually in the direction that requires more capacitance or inductance. 

One thing you should check for is a second trimmer on the opposite side of the tuning capacitor.  Sometimes the second trimmer is present but is neither on the schematic nor mentioned in the alignment instructions.

Jay

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Jay,

I did some measuring and ploted the data. I set the dial to 1270 where there is a station. I then adjusted the LO trimmer to that station. Then I adjusted the RF trimmer and antenna coil for resonance. I then set out to collect the data. I would set the dial to each setting and determined the LO setting and also the RF resonance. Here are the results:

John

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John,

Sorry if I am being dense but I don't understand what you mean by the local oscillator setting.

The local oscillator is supposed to be 455KHz (the intermediate frequency) higher than the dial setting.  With the dial set at 1270KHz, the local oscillator frequency should be at 1725KHz.

Can you explain what you mean by the 1270KHz for the local oscillator in the chart?

Jay

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Jay,

I appologise for the unconventual testing. The LO frequency defined is the RF frequency of the signal generator setting that the radio is tuned to.

I set the dial to the specific place on the dial (1000, 1100...) then set the signal generator to where resonance is found. That is the "RESONANCE" frequency indicated on the signal generator.

Then I turn the radio on and change the signal generator for maximum modulation tone from the speaker. That is "LOCAL OSC" frequency.

This makes it easy to see the delta of the three curves. The "DIAL SETTING" ends up being a perfectly straight line because it is the setting the other two are compared to.

After looking at the curves it is obvious I could adjust the dial pointer on the shaft and LO trimmer to converge all three curves at say 1000 KHz and that would put them nearly all on track for dial setting from 550 KHz to 1200 KHz or so. More tinkering is needed.

Is it customary to tweak the shape of the outter stator plates on the tuning capacitor to adjust tracking?

John

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Jay,

Maybe today I can do some more tweaking. Here is a delta plot:

John

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John,

If I understand your numbers, you need more tuning capacitance at the high end of the dial to bring the curves together.  That is where the trimmer cap has the largest influence so it should allow you to do some correction at the higher frequencies.  You may also need to adjust the dial pointer to shift the whole curve into alignment with the dial.

Usually the tuning capacitor plates are bent as needed during the initial alignment and don't need adjustment.  Most restorers leave them alone because a bending error can be almost impossible to correct.

Tracking is almost always a compromise between the high and low ends of the band.  If you can flatten the curve at the high end, I think you are close to being as good as it gets.  This radio is at a disadvantage because of the two ways to connect the loop.  The cable should be designed to introduce compensating amounts of inductance and capacitance but I doubt that the tracking could ever be the same with and without the cable connection.

Jay

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