Hi Guys,
I finally completed the cabinet on my 48-482 and thought I'd go back and give it one last alignment following the FM alignment procedure to the letter.
There is one point that I don't understand.
It says: It is essential that the output from the generator be kept below the point where the detector-oscillator locks in, otherwise an erroneous zero-beat will be obtained. When a single very sharp zero-beat point is obtained, the adjustment is correct.
Can anyone explain this to me?
Thanks.
'Zero-beat' traditionally refers to the audio product of two RF carriers together, with the difference between them showing up as a tone or whistle through some sort of detector and reproducer (speaker or headphones).
The closer they are together, frequency-wise, the lower the tone. You will find there is a null in between the lowering and raising tone.
When the tone finally gets past slow to no sound, it is said to be at 'zero-beat'.
Your FM radio probably has some sort of AFC, or automatic frequency control. It will tend to make nulling a bit more difficult, hence the instruction to keep an eye on levels and not let that function affect your test.
Dennis
Thanks, Dennis. That makes sense. I'll give it a try.
Eric, I was just doing the alignment on my 48-482 today, so maybe I can give a little guidance. You need to input an IF signal at exactly 9.1 MHz at a low level, just enough that it can be heard. You place a jumper wire between the plate of the FM1000 and the 47K load resistor, effectively shorting out the secondary winding of the FM detector transformer. Then carefully adjust the trimmer capacitor (not the slug) until you hear a squeal. You then need to turn the trimmer back and forth until you find the silent spot in the center of the squeals. This is the zero beat. The frequency of the squeal will decrease as you approach zero beat, if you continue to turn the trimmer it will increase again. Use the minimum signal which will still produce the sound for the most accurate adjustment.
The procedure then says to remove the jumper and then adjust the detector transformer slug to achieve zero beat again. Here I was unable to get a real zero beat, rather I adjusted the slug for best audio with a FM modulated signal. Perhaps you will have better luck. Finally I got out my sweep generator and adjusted the slug for the most linear slope on the detection curve, which was a pretty critical adjustment.
After this alignment the FM reception sounded great and the stations tuned in without distortion, which was not possible previously. The FM 1000 detector can perform really well, but the alignment is very critical.
Here is a link to an explanation of how the FM 1000 detector works, with drawings of the response curves obtained with proper and improper alignment.
http://www.philcorepairbench.com/tube/fm1000.htm
Let me know how you make out!
Thanks, I'll get on it tomorrow and see how it goes.
Mondial, I got the same results that you did. Like you, I was unable to get a squeal from the slug, but I adjusted for the best volume/sound. I think that will do it.
One weird thing that is happening: Only on FM, when I turn the volume control I get a sound like it is dirty. On AM and SW there is no noise. Any thoughts on that one?
Thanks,
Sounds like you might have a leaky coupling cap C304, (.03 uF) which passes the audio from the FM detector to the volume control. If this cap is leaking it will put DC voltage from the plate circuit of the detector into the volume control pot, making it noisy. Since you don't get the noise on AM or SW when C304 is disconnected, my guess is that the cap is bad.
You can confirm by connecting a DVM to the test jack J200 on the back right of the chassis. This test jack connects directly to the top of the volume control, so you can monitor for DC voltage as you switch from FM to AM. There should be no DC voltage at that point on FM, SW or AM.
If there is no DC when in FM mode but the pot is still scratchy, then there might be a possibility that here is an ultrasonic oscillation or RF on the FM detector out. You could check this by connecting an oscilloscope to J200 and looking for any unusual signal there.
In any case, the volume control should not be noisy in FM position, as mine is equally quiet on all bands.
Great, I'll check it out.
Thanks,
Wow Mondial, You were right on the money! I had 160 VDC at J200. I checked C304 and found that it failed. It looks like I touched it with my soldering iron. When or how I'm not sure. All I know is that I replaced it and the last of my issues with this 48-482 is corrected.
This is the reason why this forum is the best!
Thanks again,
I'm watching this thread and am glad you guys have some 48-482 experience...there is one on my shelf waiting for attention and it looks like it will need a lot of work. Eric..thanks again for the lead on getting the clear button covers from Dave Frush; he had them!
So now that I'm finally finished with the first one: [
attachment=2157]
Now to start on the 2nd one. With all the issues I had with the first one, the next one should be a piece of cake!
Thanks, it was my most extensive project to date.
Very nice looking radio! Good thing you didn't ask me what "zero beat" meant. I thought that was what you had when you upset the drummer and he quit the band!!! Take care, Gary.
