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 Post subject: Touchy Tube
PostPosted: Jun Tue 11, 2019 3:35 pm 
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Joined: Jan Sat 12, 2019 5:24 pm
Posts: 44
While troubleshooting my RCA 56X3, I noticed when I touch the 35L6 output tube, it causes a noticeable hum.........Anyone ever experience this? Maybe this is normal... Thanks........


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 Post subject: Re: Touchy Tube
PostPosted: Jun Tue 11, 2019 4:08 pm 
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Joined: Feb Thu 28, 2019 7:48 pm
Posts: 253
Location: Lawrenceville, Illinois 62439
Check/clean your tube pins. Also look over the tube socket carefully for minute cracks. And, as crowded as some of those under bellies are, look over all the connections for cracked/cold solder joints.
Dan


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 Post subject: Re: Touchy Tube
PostPosted: Jun Tue 11, 2019 4:35 pm 
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Joined: Jan Sat 12, 2019 5:24 pm
Posts: 44
Thanks Dan, I did clean all of my tube pins with a little brass brush. But, I will will use my magnifying light to check the socket again. I've changed all paper caps, two resistors, filter caps and most tubes except my converter (12SG7). Still no reception or volume response......I'll keep at it...still learn'n.......................


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 Post subject: Re: Touchy Tube
PostPosted: Jun Tue 11, 2019 5:37 pm 
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Joined: May Sat 06, 2006 4:03 am
Posts: 3382
Location: ZIP 23831 South of Richmond, VA 25 miles down the pike.
Is that all that's wrong, the hum, otherwise working? You are aware the resistors with "M" indicate thousand, e. g., 220M is 220,000 ohms.

Bill J.


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 Post subject: Re: Touchy Tube
PostPosted: Jun Tue 11, 2019 10:59 pm 
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Joined: Jan Sat 12, 2019 5:24 pm
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Hey Bill, I am not getting reception But, I'm waiting for a replacement for a converter tube that test really weak...And yes, I learned early on about the "M" for thousands....I love working on these radios and can't wait until I can properly troubleshoot...I really appreciate all of the help I'm getting from the forum...Thanks.............


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 Post subject: Re: Touchy Tube
PostPosted: Jun Wed 12, 2019 3:34 am 
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Location: Tucson, Arizona U.S.A.
I don't have a schematic for your set, but, in general, a 12SG7 wouldn't be used as a converter tube (it could be but usually isn't). A 12SA7 is by far more common. Do you have one or more wrong tubes in your radio?

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 Post subject: Re: Touchy Tube
PostPosted: Jun Wed 12, 2019 3:47 am 
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Joined: Jul Sun 09, 2006 3:11 am
Posts: 6230
Location: Aurora, CO 80013
Could easily be a noisy or shorted 35L6. Easy to test with a swap. A 50L6 will work in a pinch.


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 Post subject: Re: Touchy Tube
PostPosted: Jun Wed 12, 2019 3:52 am 
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Joined: Jan Sat 12, 2019 5:24 pm
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Hey Jim, I wish it would be that easy....But the schematic and the label on the radio calls for 12SG7.


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 Post subject: Re: Touchy Tube
PostPosted: Jun Wed 12, 2019 4:11 am 
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The 35L6 has been replaced with NOS, along with all other tubes except the 12SG7, that I'm still waiting on delivery....All new tubes are tested on my Seco tube tester before I install them in my radios. I've replaced paper caps, filter caps and dull solder joints......


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 Post subject: Re: Touchy Tube
PostPosted: Jun Wed 12, 2019 5:56 am 
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Location: Vancouver Island, Canada
At this point. I would inject a signal from an external source to the volume control. If it can be heard thru the speaker, then you have narrowed the problem to the front end.
Next do the second radio oscillator test. Do a search at the top of this page for instructions. That is the Local Oscillator (LO).

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Burl Ives, RIP, oldtimer.
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 Post subject: Re: Touchy Tube
PostPosted: Jun Wed 12, 2019 3:08 pm 
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Joined: May Sat 06, 2006 4:03 am
Posts: 3382
Location: ZIP 23831 South of Richmond, VA 25 miles down the pike.
Can't tell how that bunched up electrolytics are wired. But what is your B+ readings? Here is schematic, best I could come up with.

Bill J.


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 Post subject: Re: Touchy Tube
PostPosted: Jun Mon 17, 2019 7:16 am 
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Joined: Jan Sat 12, 2019 5:24 pm
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My B+ is 113V, but should be 135v.....


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 Post subject: Re: Touchy Tube
PostPosted: Jun Mon 17, 2019 3:43 pm 
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Location: Vancouver Island, Canada
Sandmann55 wrote:
My B+ is 113V, but should be 135v.....
Voltages on the old schematics were measured with a vacuum tube voltmeter (VTVM), which tend to give a higher reading because they did not put a load on the circuit. A good quality digital meter will not put a significant load on the circuit, but it will read somewhat lower as a rule.

Do you get a noise from the speaker when you scratch the volume control terminals with a screwdriver, power on?

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Watch the doughnut, not the hole.
Burl Ives, RIP, oldtimer.
[:l>)


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 Post subject: Re: Touchy Tube
PostPosted: Jun Mon 17, 2019 10:39 pm 
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Location: Thornhill, Ontario, Canada
AuroraOldRadios wrote:
... Easy to test with a swap. A 50L6 will work in a pinch.
Actually, better than "in a pinch"! A 50L6 will make the radio "more comfortable" on our typical 125 VAC line voltage. My bedside Stromberg-Carlson 761, an AA6, has had a 50L6 in it for years... it warms up a bit slower, but otherwise is fine.
Cheers,
Roger

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Roger Jones,
Thornhill, Ontario
Ontario Vintage Radio Assoc. http://www.ovra.ca


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 Post subject: Re: Touchy Tube
PostPosted: Jun Mon 17, 2019 10:52 pm 
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Joined: Jan Sat 12, 2019 5:24 pm
Posts: 44
No, I don't get anything from the volume pot, as you would normally. But I do get something on pin 5 on my 35L6, which connects to the tone switch...I have put a signal to my volume pot and get the tone out of my speaker.....So my reception problem must be in the front end..


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 Post subject: Re: Touchy Tube
PostPosted: Jun Mon 17, 2019 11:04 pm 
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Joined: Jun Fri 19, 2009 6:34 pm
Posts: 8544
Location: Long Island
In answer to the original question, yes it is possible that touching an output tube (or the detector/1st AF tube) will cause hum. It's really not much different than the buzz you get when you touch your finger or the tip of a screwdriver to the high side of a volume control. It's random electrical noise picked up by the body, getting coupled into sensitive amplifier grid circuits. Should not be a problem as long as it goes away when you take your finger or hand away.

As for why the radio is not working, yes a bad 12SG7 will cause the symptom but so will a lot of other things. For starters, the loop antenna needs to be properly connected. Try holding a small transistor radio near the tuning cap of the RCA. With the "test radio" set about 455 kHz higher than the dial of the RCA, you should be able to pick up a whistle if the oscillator tube in the RCA is working. Check the voltages on the plates and screen grids of the tubes ... carefully of course.

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 Post subject: Re: Touchy Tube
PostPosted: Jun Wed 19, 2019 1:54 am 
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Joined: Jan Sat 12, 2019 5:24 pm
Posts: 44
I traced out the loop antenna (OK), the SG tube is NOS and is good (all tubes test great), did not get a whistle when using a transistor radio at 910...Voltage readings are: 12SQ7..plate-(53v)(should be 60v)....grid...(-.53v), 12SK7..plate-(84v)(should be 90v)...grid-(-1.2v), 12SG7...plate-(80.6v)(should be 90v)....grid-(.011v), 12J5...plate-(84.7v)(should be 100v)...grid-(-3.33v), 35L6...plate-(101.8v)(should be 120v)...grid-(84v)..................


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 Post subject: Re: Touchy Tube
PostPosted: Jun Wed 19, 2019 8:17 pm 
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Joined: Jan Sat 12, 2019 5:24 pm
Posts: 44
Correction: Oscillator is working...I had my volume on the two radios "bassackwards"...Turned my transistor radio volume up and my RCA down....I could hear the oscillation fine...


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 Post subject: Re: Touchy Tube
PostPosted: Jun Mon 24, 2019 4:01 am 
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Joined: Jan Sat 12, 2019 5:24 pm
Posts: 44
Finally, I have this radio working...Out of suspicion that this one had been molested, I started tracing every circuit and highlighting the schematic as I traced. First I found a few wires that the jacket just crumbled when touched. Replaced them with new. Then I found a capacitor can "missing"...C-8 & C-9......Bam, that was it....The IF cans were WAAAAY OUT of adjustment...Someone had also replace the speaker with an alien with a giant permanent magnet that was sitting on the chassis (lots of flux)..I had it sitting on the bench with jumpers,,,I'll have to hunt a good replacement.......Thanks for everyone's help....


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