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 on Fri Jun 23, 2017 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 on Fri Jun 23, 2017 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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Re: ZENITH L515G Loud Buzz Despite...

Post by Paulieb880G on Sat Jun 24, 2017 9:04 pm

It would sure help if everything inside the radio was as neat as it looks on a schematic!

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Re: ZENITH L515G Loud Buzz Despite...

Post by Hitachi on Sun Jun 25, 2017 1:09 am

Paulieb880G wrote:It would sure help if everything inside the radio was as neat as it looks on a schematic!

Yep, I'm still at the stage where I tend to see a schematic as a wiring diagram, where it actually is an electrical path, I guess. The DOTS--
those intersections / junctions--really mess with me.

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Re: ZENITH L515G Loud Buzz Despite...

Post by Paulieb880G on Sun Jun 25, 2017 9:54 am

Howard Sams of Indianapolis published a paper back book in 1956 telling how to read schematics. The kids got me a copy for Christmas one time. It was used, so I presume they got it on ebay or somewhere like that. Written by a man named Middleton, as I recall. It should help if you can find a copy.

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Re: ZENITH L515G Loud Buzz Despite...

Post by Hitachi on Sun Jun 25, 2017 7:44 pm

Nice, I'll find that, thanks.

Funny thing happened this morning. I was taking measurments with the meter leads and inadvertently touched between the electrolytic can ground post and chassis. The radio stopped buzzing and started playing, but overmodulatingly. I see in the schematic, there are two types of grounds--chassis and "common ground B - ".

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Re: ZENITH L515G Loud Buzz Despite...

Post by Paulieb880G on Wed Jun 28, 2017 10:33 pm

You should check the schematic to see if you have the e-caps wired correctly. The negative likely goes to common negative, one terminal of the power switch. If you have it wired to chassis, that would be the same as not having a capacitor there at all, hence the buzz.

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Re: ZENITH L515G Loud Buzz Despite...

Post by Dan Walker on Thu Jun 29, 2017 11:17 am

You might want to install C10 as per instructions at the lower part of the schematics.
I don't know enough about every purpose of each component, but I have never seen instructions like that before.

Maybe Zenith did have a different kind on electrolytic cap.
Dan in Calgary

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Re: ZENITH L515G Loud Buzz Despite...

Post by Hitachi on Fri Jun 30, 2017 1:07 am

Paulieb880G wrote:You should check the schematic to see if you have the e-caps wired correctly.  The negative likely goes to common negative, one terminal of the power switch.  If you have it wired to chassis, that would be the same as not having a capacitor there at all, hence the buzz.

Thanks, Paulie, I'd been wondering about that. Yes, I wired them to chassis. I'm now studying an article on ground. They were originally wired to the e-can ground post.

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Re: ZENITH L515G Loud Buzz Despite...

Post by Hitachi on Fri Jun 30, 2017 1:13 am

Dan Walker wrote:   You might want to install  C10 as per instructions at the lower part of the schematics.
I don't know enough about every purpose of each component, but I have never seen instructions like that before.

Maybe Zenith did have a different kind on electrolytic cap.
Dan in Calgary

Thank you, Dan. Yes, I saw that note, but didn't fully understand. I tried the recommended .05 cap from B+ to B- of C11A. No change. Still the motor boating starts after about five minutes and increases till the audio is unintelligible. I have a feeling I have not grounded the electrolytics properly. I did also try other 50C5 and 35W4 tubes with no success.

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Re: ZENITH L515G Loud Buzz Despite...

Post by Dr. Radio on Fri Jun 30, 2017 3:00 pm

I briefly checked the schematic at Nostalgia Air. Unless I looked wrong, it appears on this model B- is tied to chassis directly.

Others input?
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Re: ZENITH L515G Loud Buzz Despite...

Post by Chas on Sat Jul 01, 2017 2:56 pm

B- runs around the chassis as a common wire circuit but is not connected directly to it. See legend in lower left then look at area of C7 & C8... Therefore, filter cap must go to B-, not chassis. also, note remark of "special" filter caps and addition of a bypass cap in dots C10...

Also, radio uses tapped primary audio output transformer. If the tap is open no B+ for other circuits, replacement will be either OEM transformer or no tap and dropping resistor.

GL

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Re: ZENITH L515G Loud Buzz Despite...

Post by Hitachi on Sun Jul 02, 2017 9:11 pm

Thanks, Chas, let me ponder that. I did originally incorrectly have E-cap B- going to chassis but I have corrected that. The current situation is, the radio is now motor boating as soon as powered on. I have tried C10 bypass. No change. I'm getting good audio but as I say, it's blending with the motor boating.

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Re: ZENITH L515G Loud Buzz Despite...

Post by Chas on Sun Jul 02, 2017 11:18 pm

Putt-putt, eenh?

Put the C-10 back, it reduces hum modulation caused by RF not getting bypassed by the electrolytics.

Easy stuff first...

Try substitutes for all the tubes.

IF the lead dress was compromised when replacing the capacitors or resistors, set the radio up to function with access to the chassis and the Putt-putt condition. Start probing wires that WERE moved and see if that corrects the problem. Use a Popsicle stick, get these from a kid or at an art store...

If any caps/resistors were replaced be sure they are all the right values and connected correctly.

Same for resistors. If any of the leads for the caps or resistors were left longer than they should be, shorten them.

Look for solder bridges, wire snips and shorts between tube socket terminals.

Look for any cold solder joints around the volume control.

Check all the tube voltages, record them if they are more the 10% from the schematic.

Be sure the tuning condenser frame is NOT connected to the chassis, the frame is part of the AVC circuit

http://www.nostalgiaair.org/pagesbymodel/604/M0025604.pdf

GL

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Re: ZENITH L515G Loud Buzz Despite...

Post by Hitachi on Tue Jul 04, 2017 7:28 am

Chas, thanks for all that info. I've actually done most of what you outlined. And the tuning condenser is insulated from the chassis by thick rubber washers. I'll get the C10 back in there. Attached is something interesting, a photo of a curious component that I had to replace because it cracked. And this component is not in the schematics. Just yesterday we finally found out what it is, a "triode couplate." Also attached is the data sheet on that couplate. Earlier in this thread, I said there were resistors and a cap missing from the radio. By golly they were all in this couplate!

EDIT: Well, it's not letting me attach files. It says I'm exceeding the file size, which it says is 0 kb. I'll try later, but here's that link: http://www.amwindow.org/tech/htm/couplate.htm  The one that was in the radio was PC-84.

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Re: ZENITH L515G Loud Buzz Despite...

Post by Bill Cahill on Tue Jul 04, 2017 4:12 pm

Do you have Avast virus protection?
I do, and, it is apparently causing both of us trouble.
We need to delete the Avast, and, put in Norton. Right now, I'm low on money..

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Re: ZENITH L515G Loud Buzz Despite...

Post by Hitachi on Tue Jul 04, 2017 6:44 pm

Hi, Bill, I have only Windows Defender and Malwarebytes and have never had one issue. Till now, I wonder? Thank you for the heads up.

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Re: ZENITH L515G Loud Buzz Despite...

Post by Tony V on Tue Jul 04, 2017 8:42 pm

Hitachi, at the top of each category in the forum is a tutorial I submitted on how to post pictures to your thread. The method you used wont allow you to post pictures. Give the thread a look and try it. It's fairly easy once you do it a time or two.
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Re: ZENITH L515G Loud Buzz Despite...

Post by Chas on Tue Jul 04, 2017 11:43 pm

Hitachi wrote:Chas, thanks for all that info. I've actually done most of what you outlined. And the tuning condenser is insulated from the chassis by thick rubber washers. I'll get the C10 back in there. Attached is something interesting, a photo of a curious component that I had to replace because it cracked. And this component is not in the schematics. Just yesterday we finally found out what it is, a "triode couplate." Also attached is the data sheet on that couplate. Earlier in this thread, I said there were resistors and a cap missing from the radio. By golly they were all in this couplate!

EDIT: Well, it's not letting me attach files. It says I'm exceeding the file size, which it says is 0 kb. I'll try later, but here's that link: http://www.amwindow.org/tech/htm/couplate.htm  The one that was in the radio was PC-84.
The couplate can be replaced with individual components.

To answer anti-virus protection...

I have been using Ubuntu Linux for the last 10 years. By default all ports except the one for Firefox are open. Never installed an anti-viral, or other...

Current O.S. is Ubuntu MATE

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