40-155 Unusual buzzing - Printable Version

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40-155 Unusual buzzing - Guest - 03-14-2006

40-155 unusual buzzing
First, as a first time correspondent with this forum, let me introduce myself. My name is Mike and I have been interested in radio since my Ground Radio Repair days in the USAF during the 50's.
I recently recapped and aligned both my 40-180 and 40-155, which are nearly the same radio electrically. Both have VG sensitivity, selectivity and volume. However, with the -155, when the volume control is set for low volume, there is a distictive 60cy buzz--not hum. As the volume is increased to a more moderate level the buzz disappears--not masked. Looking at the schematic, this seems to coincide when the slider is about at the area of the tap. In both sets the 7C6 1st audio grid wire is positioned the same so I doubt that though it is unshielded it is the cause. There is likely a simple explanation for this but it eludes me. Any help given is appreciated. Thanks, Mike

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- Chuck Schwark - 03-14-2006

Hi and welcome Mike.

Have you replaced all the wax-paper caps and the electrolytics in the set yet? This is paramount - they will be leaky.

Also check for any resistors outside of +/-20% of nominal values and replace as needed.

SOP for old radios of this vintage. :-)

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- Guest - 03-14-2006

Thanks Chuck. I replaced all paper and electrolitic caps. I didn't measure most resistors, but will do so. I believe this is an audio section problem because the same symptom occurs when switched to phono.
Mike

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- Chuck Schwark - 03-14-2006

Look for loose/rusted rivet ground points, heater wires not dressed/routed near grids, etc.

Could also be some heater-to-cathode leakage in a tube too.

Be sure the NEG side of the first electrolytic off the rectifier goes to the HV center tap and not ground.

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- gary rabbitt - 03-23-2006

Hi Mike,
Welcome.
Does this buzz disappear when you turn the AC plug around 180 degrees?

One set I had I did not change one cap out because it looked like a large mica. It was a bypass cap , which was still a paper/foil in a bakelite casing. CHanged it out and the buzz went away.

Have you checked the volume control for any 'opens' along it's travel. Another set I had that buzzed, not hum, and turned ot to be a break or wore down carbon inside the volume control.
Good luck.
Gary

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