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RCA X551 Humming

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Post by frank1956 on Tue Sep 09, 2014 12:52 am

Hi All,
Told you I had another project  cheers . 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 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.

Thanks
Frank
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Post by 75X11 on Tue Sep 09, 2014 1:33 am

Sleep on it, and tomorrow go over your wiring carefully with your schematic a couple of times.
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Post by jerryhawthorne on Tue Sep 09, 2014 2:07 am

Frank, it is always helpful to post the schematic which I will do. As 75 indicated review wiring. Hum not controlled by the volume control indicates a problem in the AF section after the volume control or a "dirty" power supply (filtering not correct). Do you have an oscilloscope? Often handy for doing a little looking?
Jerry

http://www.nostalgiaair.org/PagesByModel/948/M0015948.pdf

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Post by frank1956 on Tue Sep 09, 2014 9:13 am

Jerry, Thanks for the schematic's. No I do not have an oscilloscope.
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Post by Wildcat445 on Tue Sep 09, 2014 10:31 am

Post a picture of the underside of the chassis. Perhaps we can make a suggestion based on what we see that will help you. My only suggestion at this point would be to check to see that the polarity of the filter capacitors are correct. Good luck.

WC

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Post by Guest on Tue Sep 09, 2014 10:39 am

I hate to admit it but......I had to check my filter caps 3 times on one radio before I found I had wired them in wrong. Sometimes something so simple can blur our vision.

Very Happy I would recheck them very carefully!

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Post by willy3486 on Tue Sep 09, 2014 11:02 am

Don't forget to check the values as well. About 30 years ago I had a small motorola that I couldn't get it to work. I set it aside and got got back to it this last winter. I found out that instead of a say .05 cap I put in a .005 cap. I replaced with a correct one and it worked.

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Post by frank1956 on Tue Sep 09, 2014 12:04 pm

Here are some pic's of the underside. Also took voltage reading listed below. I did notice as you will see in the last two pic's someone improvised on a capacitor and a resistor  confused  

(1) 12BE6 pin 5 99-100 v
(2) 12BA6 pin 5 99-100 v
(3) 12VA6 pin 7 62-63 v
(4) 5OL6GT pin 3 98-99 v
(5) 35W4 pin 7 114-116v
RCA X551 Humming 100_4064_zpsedc9a665
RCA X551 Humming 100_4066_zps1f52b3c1
RCA X551 Humming 100_4067_zpsb93fb036
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Post by 75X11 on Tue Sep 09, 2014 12:11 pm

On the overall picture, what is that brown component that crosses the tube socket in the upper right hand corner of the chassis?
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Post by Wildcat445 on Tue Sep 09, 2014 12:50 pm

The schematic for this radio is in 1951 Beitman's, Page 110. The guess I was going to make is not your problem. You have the 33uf cap right after the rectifier cathode and the 47 uf after the choke. This looked backwards, but that is correct according to the diagram. ( I need to print the diagram, as I hate jumping back and forth.) Are the filter caps connected negatives to chassis? Is that correct? I can't see well enough to tell for sure.

WC

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Post by frank1956 on Tue Sep 09, 2014 12:53 pm

Wildcat445 wrote:The schematic for this radio is in 1951 Beitman's, Page 110.  The guess I was going to make is not your problem.  You have the 33uf cap right after the rectifier cathode and the 47 uf after the choke.  This looked backwards, but that is correct according to the diagram. ( I need to print the diagram, as I hate jumping back and forth.)  Are the filter caps connected negatives to chassis?  Is that correct?  I can't see well enough to tell for sure.

WC

WC, Yes the filter cap's are connected neg to the chassis.
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Post by frank1956 on Tue Sep 09, 2014 12:54 pm

75X11 wrote:On the overall picture, what is that brown component that crosses the tube socket in the upper right hand corner of the chassis?

That is the power cord connected to the volume control.
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Post by frank1956 on Tue Sep 09, 2014 1:08 pm

I just notice on the 35W4 tube pins 5 & 6 there is a 33 ohm resistor, per the schematic there is not suppose to be one there. There is no 33 ohm resistor listed on the parts list. I have removed it and the hum went away, but now there is nothing at all???
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Post by 75X11 on Tue Sep 09, 2014 1:44 pm

frank1956 wrote:
75X11 wrote:On the overall picture, what is that brown component that crosses the tube socket in the upper right hand corner of the chassis?

That is the power cord connected to the volume control.

Not the wire above V3's socket, the component that looks like a paper capacitor across the socket.
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Post by Wildcat445 on Tue Sep 09, 2014 2:01 pm

I printed the diagram, so here we go.  Very Happy

Frank, if you will look at the diagram, you will see that the negatives of the filter caps are"common wiring insulated from chassis."  So, if your filter caps are connected to chassis, that would be a no-no.  It shows common negative as being one side of the power switch.  As for the 33 ohm resistor, pin 5 is the plate of the rectifier and pin 6 is the center tap of the rectifier filament.  Removing it would kill B+.  Perhaps there is a revision on this chassis.  You need to replace that cap until we can figure that out.  I don't see it on the diagram, either.  Did the dial bulb work when the set hummed?  Good luck.

75, I see what looks like an old cap still in there as well. Good catch!

WC

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Post by frank1956 on Tue Sep 09, 2014 2:16 pm

It is an old type 470K resistor.
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Post by frank1956 on Tue Sep 09, 2014 2:17 pm

WC, Yes the dial light worked when the set hummed.
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Post by Wildcat445 on Tue Sep 09, 2014 2:23 pm

Referring to the first picture you posted, you will notice a black wire coming off the power switch.  It runs over the top of the 50L6 socket, over the 12BA6 socket, bends down under the 47 uf filter cap, goes toward the back edge of the chassis, winds up under the output transformer, then over to the rectifier socket, perhaps to pin 1, used as a junction point for B-.  Pin 1 may be the tie point you are looking for.  I can't see definitely how your e-caps are wired, but check those again to make sure they are correct.  Negatives to the chassis is definitely incorrect, and can cause a continued hum.  Good luck.

WC

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Post by Dr. Radio on Tue Sep 09, 2014 3:04 pm

Hard to tell from pictures, caps don't look right.

Both + ends of the electrolytics typically share a resistor like a large 1.2k 1 watt, this makes up the r-c Pi filter circuit. I haven't checked the schematic (I'm at lunch), but your caps each go to far reaching areas of the chassis, and I'm not seeing the typical rc-Pi tie together.
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Post by Bill Cahill on Tue Sep 09, 2014 3:05 pm

Me also. And, put the 33 ohm resistor back in. It feeds ac to the plate of the 35W4. Without that the radio can't play. There should be a .05 600 volt capacitor going from B- to chassis.
Filters do not go to chassis.

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Post by frank1956 on Tue Sep 09, 2014 3:44 pm

Here is a pic one that has the original filter cap's still in.


RCA X551 Humming 100_4068_zpse94e1ffb
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Post by frank1956 on Tue Sep 09, 2014 3:51 pm

WC,
The black wire you are referring to goes to the middle terminal of a 3 prong terminal strip,which intern is connected to a .005 capacitor.
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Post by jerryhawthorne on Tue Sep 09, 2014 4:04 pm

Frank your getting there.  There is only one connection to the chassis as indicated by the "rake" like symbol coming off the A/C line and it is only connected via a capacitor, isolating the chassis from the B- connections which are somewhat a buss running through out the schematic.  The B- is indicated by the inverted antenna like symbol.  Each of these symbols indicate an attachment on the buss, not the chassis.  Connecting any of the "antenna" connections to the chassis is not proper and could well result in some HUMMM.  The purpose of this set up is that it keeps the chassis from feeling the direct A/C hot side on the chassis, something dangerous.
Best,  Jerry

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Post by frank1956 on Tue Sep 09, 2014 5:10 pm

Well like the old saying goes if it wasn't for bad luck I have no luck at all. While replacing the 33 ohm resistor on the 35W4 tube, you guessed it the pin broke. I have removed the broken pin, but need to figure out how to replace the pin. I have some old tube sockets laying around. Pic attached.

RCA X551 Humming 100_4069_zps38948d96


Last edited by frank1956 on Tue Sep 09, 2014 5:11 pm; edited 1 time in total (Reason for editing : Incorrect word)
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Post by jerryhawthorne on Tue Sep 09, 2014 5:51 pm

Frank, it is called learning. Are you having fun yet? Yes, some time ago I found it less destructive to cut the leads off a component I'm replacing, leaving a perhaps 1/4" tag sticking out. Then, using my favorite size jewelers screwdriver tip, do a couple wraps around it giving a pigs tail. Slip it over the tag and solder. Not as neat as cleaning the old wire off the connection but less prone to damage. I'm like a bull in a china shop, I can break anything and try to minimize to possibility..
Not certain if you can replace that pin but if you have a replacement socket, go for it.
Enjoying you struggles, I have certainly been there.
Best, Jerry

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