[Eric Adams]

Hi Guys,

I've got a strange one. On AM, It works great until I tune 720 or below. It cuts out and crackles like the plates are rubbing, but they're not. I have cleaned and cleaned. I even ran some heavy paper(business card) between every plate-more than once. If I switch to SW or FM, I get no noise at all up and down the dial. I thought that the cap tunes all bands  
Any ideas where to look?

BTW, When I finished the chassis I had no problem. This came on after a few months of listening.
http://www.nostalgiaair.org/Resources/637/M0013637.htm
Thanks,

[morzh]

It is probably the oscilator problem, if your cap is not shorting and you are sure of that.
See if the tube is finicky.

[Radioroslyn]

What I've done is unsolder the lead to the stator then measure the resistance from the stator to the cap's frame. This way you can be sure. Sometime they can be hard to find.
Terry
ps use the highest scale on your meter.

[codefox1]

And if you fine a short, slip a piece of paper between frotor and stator plates to find it. Problem will usually will be the first or last (outer) leaf.

[Eric Adams]

I haven't checked the cap as Terry suggested yet, but I have already done what Codefox1 said to try with no change. That's why my question remains: Why only on AM and not SW or FM?

[Mondial]

Eric, there are separate sections on the tuning cap for FM and AM, SW. The FM section is entirely independent. Both AM and SW use the same section, so it is unusual that SW works and AM doesn't.

You can try localizing the problem and see if its the oscillator or antenna sections causing the problem. Connect your scope probe to pin 4 of the 7F8 converter tube. There should be a 2.2k resistor there to ground. You should see the oscillation at that point. Rotate the tuning cap and see if the signal drops out on the scope as you tune below 700 kHz.
If it does, then the osc is the problem. If the signal remains below 700 kHz, but the noise and reception problem is evident then its most likely the ant section causing the problem

Good luck!

[Eric Adams]

Thanks, Mike. I'll give that a try and report back.

[Eric Adams]

OK Mike, I'm not that great with a scope yet, but I'll tell you what I found. I get a solid pattern:
   

When I tune under 700, where it cuts out, the pattern just jumps but is still there. Does that tell you anything?

[Mondial]

Yes, it says that the oscillator continues to operate below 700 kHz, but doesn't tell you if it is at the right freq below the 700kHz dial setting.

Turn up the time/div knob on the scope so that you can see the individual osc cycles instead of the solid RF envelope. There should be a setting ariound the knob like .5 us/div, or 500 nsec/div which will allow you to see the individual cycles and estimate their frequency. Set the scope to auto trigger so it will display the sine wave cycle waveform.

When you have the scope set up this way, as you turn down the dial on the radio you will see the spacing between cycles increase, which is telling you that the freq is decreasing. As you drop below 700 kHz, the spacing should increase smoothly with no sudden jumps. If the is a sudden change, them there is probably something going on with the tuning cap, causing a large jump in freq which would lead to noise and no reception.

[Eric Adams]

Now I get it. I'll check it out. Thanks, Mike.

[Eric Adams]

Mike,
I got a nice sine wave (thanks) which got larger as I got to 700. I never lost the wave but it did bounce, very slightly when it made noise. Does that mean it is the tuning cap? If so, how can I determine which part of the cap is for AM?
Thanks,

[Mondial]

If the osc waveform only bounces slightly, but never drops out or changes drastically as you tune below 700, then the oscillator is probably not the problem. More likely the trouble is in the antenna tuning section of the cap, which is shorting out the signal at the low end of the band.

The AM sections of the tuning cap are the ones with many aluminum plates. There are three FM sections, but these have only a few plates each and are mounted between the AM sections. I am going from memory here, but I believe the AM ant section is the one closest to the front of the chassis and the osc is toward the back. You can confirm by touching each AM stator contact at the top of the tuning cap with your finger. The osc section will show a define reaction on the scope waveform when touched, the ant section little or nothing.

You can also try connecting an ohmmeter on high range to the top of the ant cap with the set off, as Terry mentioned previously. You don't have to disconnect any wiring. This cap section should measure several megohms to ground, as it is isolated by 4.7 megohm resistors to the AVC line. If the resistance changes suddenly as you tune below 700, then there is something touching between the plates and shorting out.