ZENITH L515G Loud Buzz Despite...

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ZENITH L515G Loud Buzz Despite...

Post by Hitachi on Sun Jun 18, 2017 8:32 pm

Hello, I'm a lowly parts subber, I guess (I do attempt to study, for example, the army tube theory course). But I've often gotten dead radios working, based on my very novice knowledge. In this radio, I've replaced all filter and wax / paper capacitors, as well as out of spec resistors. I see evidence of prior tampering, which of course presents a special challenge to one who is not well grounded in tube theory.

R7 and R6 and C5 were not even in the radio. Yet despite replacing them and all I've done, the radio still buzzes loudly (it's a buzz, not 60 Hz hum).  Also, I also tried replacing the tubes with known good ones.

For convenience, I have attached the schematic.

Thank you.

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Re: ZENITH L515G Loud Buzz Despite...

Post by Chas on Mon Jun 19, 2017 8:47 pm

Trace the power supply component by component, be aware of bias supply if any and incorrect negative connections of filter electrolytics.

GL

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Re: ZENITH L515G Loud Buzz Despite...

Post by Hitachi on Tue Jun 20, 2017 1:01 am

I sure will try that, Chas, thank you. I'm a bit limited reading schematics yet though.

Yes, I was wondering about correct grounding. I grounded the new electrolytics to chassis, because they were originally grounded to their can, which was riveted to chassis with no phenolic washer (which would make it a floating ground?). Hope I did that much right.

By the way, the radio had this loud buzz even before I replaced the electrolytics (silly me me thought replacing them would do the trick).

Thanks again.

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Re: ZENITH L515G Loud Buzz Despite...

Post by jukeboxman on Tue Jun 20, 2017 2:34 am

are you sure the buzz is not coming from a power supply in a dvd player or a florescent light?
i had a loud buzz in several radios --turn off all the breakers in your house except the one the radio is using -see if it goes away --if so -turn them on one by one see if it comes back --if it does one by one unplug all things on that circuit -like dvd players computers lamps --mine was a cheap dvd player that worked perfect but the cheap power supply was sending loud noise into the rf spectrum

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Re: ZENITH L515G Loud Buzz Despite...

Post by Dr. Radio on Tue Jun 20, 2017 10:57 pm

Other possibilities include a simple wiring mistake or a bad output tube (50C5) which could have a heater to cathode leakage.
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Re: ZENITH L515G Loud Buzz Despite...

Post by Hitachi on Thu Jun 22, 2017 7:23 am

jukeboxman wrote:are you sure the buzz is not coming from a power supply in a dvd player or a florescent light?
i had a loud buzz in several radios --turn off all the breakers in your house except the one the radio is using -see if it goes away --if so -turn them on one by one see if it comes back --if it does one by one unplug all things on that circuit -like dvd players computers lamps --mine was a cheap dvd player that worked perfect but the cheap power supply was sending loud noise into the rf spectrum

Thanks, jukeboxman. Well, I've had other radios on the bench. No buzz. And the radio buzzed at a friend's home as well. But I'll check for the possible culprits you point out.

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Re: ZENITH L515G Loud Buzz Despite...

Post by Paulieb880G Yesterday at 11:59 am

 If there are signs of tampering, the best thing would be to check each component and connection against the schematic.

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Re: ZENITH L515G Loud Buzz Despite...

Post by Hitachi Yesterday at 4:02 pm

Paulieb880G wrote:  If there are signs of tampering, the best thing would be to check each component and connection against the schematic.

Yep, paulie, I'm attempting that as best as my novice schematic reading skills will allow. Thank you.

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