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41-81 AVC Problem? - Printable Version

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41-81 AVC Problem? - Raleigh - 06-07-2010

Last week I finished work on a 41-81 portable. I think I may have a problem with the AVC, unless maybe it just isn't supposed to work too well on this simple radio. Far away stations come in quite weakly, though I can listen to them easily by turning up the volume. Strong, nearby stations are so strong that I can hear them easily even with the volume all the way down.

I replaced all of the tubular caps, and I'm sure I replaced the resistors in the AVC circuit (a 4.7 meg and a 10 meg.) There is a 250 mmf mica in the AVC that I did not replace. I think this one bypasses what's left of the RF to ground after the detector diode and before both the volume control and the AVC.

Before I tear into it again, does it seem to anyone that the 250 mmf mica is suspect? Or should I just not expect too much from this simple, 4 tube radio? I have other old portables that don't have great AVC either, but on a modern Walkman-type, the distant and strong stations come in at about the same volume. The noise level is higher on the more distant station, as one would expect.

Any other suggestions? I can't find any other components in the AVC that I did not replace, unless it is the IF transformers.

Re: 41-81 AVC Problem? - codefox1 - 06-07-2010

There's only one easy under a dollar way to find out, replace the cap.

Re: 41-81 AVC Problem? - Radioroslyn - 06-07-2010

You may want to test your tubes to see how much emission they have. A weak diode in the detector might get that problem. Also check with a vtvm your avc voltage to see if how much it makes and how much there is between a weak and a strong signal.

Re: 41-81 AVC Problem? - Raleigh - 06-18-2010

I've been out of town, so I'm just getting back to this today.

The 250 mmf AVC filter capacitor tests good, so I didn't replace it.

As 7estatdef suggested, I measured the AVC voltage from the IF side of the 10 Meg AVC resistor (#13) to ground. With the radio tuned to the strongest local station, the voltage measured about 750 mv DC. On a weak station, it was about 330 mv DC. Tuned away from any station, it was still about 330 mv.

The measurements were similar with my VTVM and with my modern digital multimeter, which is rated at 10 Meg input resistance. The VTVM measured about 30 mv higher. It is rated 11 Meg/volt (1 Meg in probe), and I'm not too confident of the calibration.

I don't know how to interpret these measurements. Does this amount of difference seem reasonable?

Edited to add: I moved the cabinet around to reorient the loop antenna with respect to various radio stations. The AVC voltage when tuned to a strong station went up to 950 mv DC, but it was still around 330 mv on weak stations or between stations. The chassis was outside the cabinet, connected to the loop and ground with test leads. My workbench is in my basement, so it is likely that I'm getting weaker reception on all stations than if I were upstairs. I imagine the AVC voltage on strong stations would increase even more if I were upstairs.

Edited again: I tried an NOS 1H5GT detector tube without any change.

Re: 41-81 AVC Problem? - rghines1 - 06-19-2010

Hi, An alignment may be needed but will need a signal generator. Would expect at least -5 voltage of AVC on strong local stations. Would not suspect the IF transformers unless during alignment a trimmer gives no response when detuned and peaked again. Richard

Re: 41-81 AVC Problem? - Chuck Schwark - 06-19-2010

Looking at the service bulletin, it sounds like you are not developing enough AVC voltage due to a possible short or open on the AVC bus, or you have an open (very common) second IF can secondary winding that's only passing a weak signal due to capacitive coupling between the IF transformer windings, but provides no DC path for the detector. Approx. -5 volts is usually an avarage AVC voltage with a good local station. Your -0.3 volts indicates AVC buss problem or a DC current path problem as above. Also check the first IF transformer's tertiary coil which passes the AVC voltage to the 1A7G converter tube through the loop antenna coil. Check the loop also for a possible open there. If you have replaced cap #8 and #16 and resistors #10, #15, #14 and #13 check ok for values, and you know the converter, IF and DET tubes are ok, then the suggestions above are all that's left that can effect the AVC bus.

Re: 41-81 AVC Problem? - Raleigh - 06-21-2010

Thanks, guys, for the tips. I'll probably have time this week to look at the things you suggested.

I replaced the detector (1H5G) with an NOS tube, and it made no difference. I don't have any other way to test the tube, but I am reasonably sure that the original detector is fine, or at least as good as the NOS tube.

More later, and thanks again for the help.