37-630 B+ Ripple - Printable Version
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37-630 B+ Ripple - AvoJim - 01-20-2011
Hi
New member here. I am not exactly a newbie but I don't have a lot of experience. I have done about 20 radios in the past 10 years.
My 37-630 is driving me nuts. Most of the time it doesn't work but sometimes it works great. The problem appears to be bad filter caps but I have checked them on a Heathkit condensor checker and they check good, both value and leakage. Most of the time the radio makes what sound like the 120HZ growl which would indicate bad filter caps but occasionally the problem totally goes away. It went away long enough for me to give the set a good alignment and it worked like a champ on all three bands. I was ready to put it back in the cabinet but checked it one last time and the problem was back. When the problem is present I measure 15 to 17 VAC on C62, 7 to 8 VAC on C60 and 3.5 VAC on C18. I would think these voltages should be a few tenths at most. Also the problem is not nearly as apparent on the high shortwave band. Last night when the BC was totally unuseable I got WWV clearly at 15 MHz with just five feet of antenna wire. The loudness of the growl is controlled by the volume control and it does vary somewhat over the band. I have reflowed the solder on the filter caps, twice. The value of the field coil is not specified on my schematic but I measure about 1150 ohms which I am guessing is about right. The noise goes away completely when I pull the grid cap off the 6Q7. Same thing on the 6A8 but I don't know whether that means the problem is in the input or the output of the 6A8. When the problem is present I can usually get a very faint station on the broadcast band which is at the right freq which makes me think the oscillaator is ok.
I have changed out all the caps I could get to without tearing into the RF sub-chassis which I really don't want to do. The resistors I can get to check out ok also
I have test equipment. O'scope, frequency counter, Heathkit signal generator, digital and analog cap checker and several multimeters both digital and analog.
Your thoughts and guidance will be greatly appreciated.
Jim
PS I just realized I have not measure the B+ ripple when the problem was not present. I guess I'll go do that now.
Re: 37-630 B+ Ripple - Radioroslyn - 01-20-2011
Two things come to mind. One check all ground connection particularity rivets. Two grid to filament leakage in one of the tubes. The fact that you can disconnect the grid lead on the 6Q7and in goes away should eliminate the power supply.
Terry
Re: 37-630 B+ Ripple - AvoJim - 01-22-2011
Hi Terry
Thanks for the reply, I have been away for a couple of days. How do you suggest I check for the grid leakage? I have a Jackson 648 tube tester but it didn't show anything. Maybe if I can pin down the stage the noise coming from I can just swap tubes. I'll try that.
What do you think of the amount of ripple I am getting on B+? The radio decided to work properly again, I am sure for just a short time, and the ripple voltages are about the same as when all the noise is present.
Jim
Re: 37-630 B+ Ripple - Radioroslyn - 01-23-2011
Don't think the ripple is the problem. You could strap another filter cap across the the input cap in the power supply and see if that lowers the hum level.
You could start pulling tubes out till the hum stops, barring the #80 and the 6F6. This will help you isolate the problem.
Terry