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 RCA X551 Humming " issue SOLVED"

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frank1956

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Join date : 2014-08-17

PostSubject: RCA X551 Humming " issue SOLVED"   2014-09-08, 8:55 pm

I picked up an RCA Model X551, needless to says the filter cap and capacitors need to be replaced. I finally finished up tonight, but the loud humming is still present. I have gone back to ensure the filter cap's were in the right place and they are. I  inspected all solder joint, tube sockets, and insured nothing was touching underneath that was not suppose to and it is not. Any and all excess solder was cleaned up. Now the question is why the humming still present? The tube are all good as well. The humming can not be controlled by the volume control either.
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frank1956

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Join date : 2014-08-17

PostSubject: Re: RCA X551 Humming " issue SOLVED"   2014-09-09, 5:56 am

Here is the schematic's for that radio.

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FrankB
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Join date : 2010-11-22

PostSubject: Re: RCA X551 Humming " issue SOLVED"   2014-09-09, 11:49 am

That could be caused by several things.
A F-C short or lkg. in a tube, shorted tube, a bad ground, a wire moved during the repair to name a few.
I will assume you used new stock filter caps and tested them before installation too.
Not that I have ever had new filters bad right out of the box. Nope, only several times.
That taught me to test ALL parts before installing them. Just because they are new doesn't mean they are good.

You did try reversing the line cord in the outlet too? Sometimes that makes a difference if its a NP plug.

You can parallel the filter caps and see if that knocks down the hum. Be sure to turn the set off before you do that though. The surge can temporarily heal a bad cap. Found that out the hard way too......

Have you scoped the B+ for hum? If you don't have a scope, you can check for AC ripple with a voltmeter. Just set to "ACV" and look at the B+ line. Should show very little ripple.

Is the hum present with the vol. control all the way down? If so- filter caps are the suspect.

Does the vol. control work? I have seen them open and /or lose their ground path and cause hum too, as well as not being able to turn down the vol.
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frank1956

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Join date : 2014-08-17

PostSubject: Re: RCA X551 Humming " issue SOLVED"   2014-09-10, 9:17 am

Radio is working. cheers  It was a simple fix and something I over looked. Double checked the filter cap's and found the neg was incorrect. Fixed it and no hum, stations come in loud and clear. Thanks


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FrankB
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PostSubject: Re: RCA X551 Humming " issue SOLVED"   2014-09-10, 2:25 pm

Even us old timers can do an OOPS! once in a while Embarassed
Glad its up n going!
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