Having trouble

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Having trouble

Post by Patrick on Sun Dec 09, 2012 5:05 pm

Working on a 1955 Emerson 823B AM radio. All caps replaced and tubes checked. When turned on I may get a station for about 1 second then nothing but erratic static that gets louder with the volume control. I don't know what to check or do next. Any help is appreciated.

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Re: Having trouble

Post by Wildcat445 on Sun Dec 09, 2012 6:18 pm

Sorry, but I don't have the schematic for your radio in front of me, so my comments will be of a general nature, rather than specific.

What kind of test/repair equipment do you have available to you? VTVM, signal generator, signal tracer? VOM? Do you have a schematic for the radio? I would start by getting a schematic if you have not already done so. Then I would wiggle the tubes to see if a bad tube socket contact is causing you the problem. My initial guess would be that the oscillator is quitting after a short time. Converter tubes can be a finicky lot sometimes. Might try subbing that tube and note any affect in performance. If the oscillator was running, you should get some sort of reception, however weak. To check whether the oscillator is running, measure the voltage at the grid of the oscillator tube. IT should show negative voltage, more negative at the high end of the dial. Something on the order of -5 to-15 volts. Then measure the voltage on the other tubes and find out if and where B+ is lost or the wrong value. Without a little more information, that is about the best that I can offer.

Best regards

WC

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Re: Having trouble

Post by Doug Burskey on Sun Dec 09, 2012 9:47 pm

Sounds like a bad IF transformer.

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Re: Having trouble

Post by Tony V on Sun Dec 09, 2012 10:40 pm

...


Last edited by Tony V on Tue Dec 11, 2012 11:33 pm; edited 1 time in total
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Re: Having trouble

Post by Patrick on Tue Dec 11, 2012 3:47 pm

What options are open to me to get this radio repaired? Do you think a novice could replace the capacitors in the IF transformer? How do I know which IF is bad? If I fail the radio has little value anyway. Right? I have the schematic but it is a little unsure as to what values are on which caps in the IF.
Wildcat445 - I have a VOM and a capacitor meter.

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Re: Having trouble

Post by Wildcat445 on Tue Dec 11, 2012 9:42 pm

The IF transformer "guess" is just that. A guess. You don't have enough evidence to point to an IF transformer being your problem. Did you check the voltage on the grid of the converter like I suggested? The test for your IF transformers is to measure the resistance on the primary and secondary windings of the transformer. Then check for positive grid voltage on the stage following the IF transformer. Positive voltage here suggests silver mica migration in the IF transformer. Do you have a schematic? Not having a schematic and trying to fix a radio is like driving someplace where you are not familiar without a road map. The schematic is the road map to your radio. It tells you how the signal and the voltage needed to amplify it are distributed in the radio. Without a schematic, you are lost, especially at the start when you are learning your way around.

The dreaded Silver Mica Disease is, IMHO, is massively over-diagnosed. While it is a real condition, some people would rather just yell "SMD!" than to perform the simple test to confirm it.

You certainly can change an IF transformer should the need arise. Your radio has value. IF it did not, why would you try to fix it in any case. We can talk you through this repair. There is lots of experience on here.

You need to develop and practice a disciplined diagnostic procedure so that you can make directed tests, and not spend time, and gain frustration, chasing your tail.

So that we are clear, try this. ( This step assumes that you have a schematic and have measured B+ and all is well there.) When the radio fails after playing a short time, put your finger on the center terminal of the volume control. If you hear a loud buzz, the audio is okay. If not, troubleshoot the audio. Doing this will essentially break the radio in half. You can then be certain that the problem is in the "front end" of the radio, ahead of the volume control. Then measure the voltage on the grid of the converter tube. It should be negative, like I said earlier. If that is okay, check for positive voltage on the grid of the tube following the IF transformers. And measure the resistance of the IF transformers, as stated above. This should give you enough evidence to help us guide you through the next step.

Good luck

WC


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Re: Having trouble

Post by Guest on Tue Dec 11, 2012 9:53 pm

When the radio fails after playing a short time, put your finger on the center terminal of the volume control. If you hear a loud buzz, the audio is okay. If not, troubleshoot the audio. Doing this will essentially break the radio in half.

I ageed completely, this is what I was taught by an old Navy Chief in the late sixties. He called it; "divide and conquer"! Smile

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Re: Having trouble

Post by N7ZAL on Tue Dec 11, 2012 10:14 pm

When the radio fails after playing a short time, put your finger on the center terminal of the volume control. If you hear a loud buzz, the audio is okay.
Yep, did that when I was about 10 with no test equipment. Wink Thought about mentioning it, but was reluctant to in case someone got electrocuted. Shocked Things are a lot different today.
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Re: Having trouble

Post by Resistance is Futile on Wed Dec 12, 2012 2:53 pm

Patrick wrote:What options are open to me to get this radio repaired? Do you think a novice could replace the capacitors in the IF transformer? How do I know which IF is bad? If I fail the radio has little value anyway. Right? I have the schematic but it is a little unsure as to what values are on which caps in the IF.
Wildcat445 - I have a VOM and a capacitor meter.

Just as a note: Most but not all I.F. transformers are all designed to filter for 455KHz. That means that the caps should all run the same value. I forget now what it is, but I can do some research on it.

http://www.angelfire.com/electronic/funwithtubes/IF_Can-1.html
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Re: Having trouble

Post by N7ZAL on Wed Dec 12, 2012 3:14 pm

Excellent link and article, Cliff. Thanks !
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Re: Having trouble

Post by Guest on Wed Dec 12, 2012 3:51 pm

Thanks Cliff that is something I intend to try!

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Re: Having trouble

Post by Patrick on Sun Dec 16, 2012 6:21 pm

OK. I finally got back to working on this radio. I am doing tests I have never done before but hopefully learning as I go by reading about radio repair. I checked the voltage on the filament string and it is close to being right. Meaning the voltage after each tube is within 3-6 volts of where I expected. My finger on the center volume terminal did no buzzing. Of course the volume was turned way down or else the static is too loud; if that makes any difference. The voltage at the grid of the converter tube was negative. Not sure which tube follows the IF transformers but I assumed it was beginning with the antenna and moving back. Two pins seem to connect to the grid of tube 12AT6. I measured pin 1 and got nothing. Pin 6 was 1.7 volts accompanied by buzzing when I measured. Resistance on all windings of the IF transformers was about 18 ohms.
Not sure what IMHO or SMD means but I hope someone can help with this information. Remember I am actually a retired horticulturalist trying to learn a new hobby so be kind.

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Re: Having trouble

Post by chrisc on Mon Dec 17, 2012 2:21 pm

The idea of the "buzz" test is that it checks the final stage of the radio, where the audio signal is prepared and passed to the loudspeaker. Unfortunately this does mean that the volume needs to be turned up somewhat. Hearing that buzz means that the second half of the radio is good and therefore the problem is in the first part - before the volume knob.

If you do get a buzz, you could check to see if the oscillator tube is working if you have another AM transistor radio that works. Set the 2nd radio to about 1100 kHz (somewhere close to that where there is no station), hold it as close to the dud radio as you can and then turn the dial of the problem radio between minimum and about 700 kHz - you should hear a sharp whistle from the good radio as you get to / go through about 640 - 650 kHz. This is quite distinctive and repeatable. This shows that the oscillator circuit is working, if indeed it is. You can do that with the volume of the problem radio turned down if you prefer.

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Re: Having trouble

Post by N7ZAL on Mon Dec 17, 2012 2:40 pm

My finger on the center volume terminal did no buzzing. Of course the volume was turned way down or else the static is too loud; if that makes any difference.

Shouldn't matter where the volume control is set, as long as you touch the middle terminal.
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Re: Having trouble

Post by chrisc on Mon Dec 17, 2012 3:11 pm

Sorry - Bill is of course correct about that - not thinking straight today.

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Re: Having trouble

Post by Wildcat445 on Mon Dec 17, 2012 4:39 pm

If you placed your finger on the center terminal of the VOLUME control, you should hear a loud buzz regardless of volume control setting, IF the audio section of the radio is working properly. The buzz would be loud enough that static or other noise in the speaker would be drowned out. If you were on the correct terminal of the volume control and got no buzz, you need to start troubleshooting the audio section, starting at the speaker. The volume control is the input side of the audio section and the speaker is the output of the audio section, to state the matter simply. You should measure B+ on all the elements of the tubes in the audio section, likely a 12AV6 (or equivalent) and the 50C5 output tube (or equivalent.) You have measured filament voltage, which is really redundant if the tubes are lighting. You have measured the voltage at the converter grid, and the grids of tubes following the IF transformers. Now measure B+. I am hoping that you have a schematic for this radio at this time, and can follow it sufficiently well to know what these voltages should be.

IMHO means "in my humble opinion." SMD means "silver mica disease", a condition where the silver mica capacitor sometimes found in an IF transformer, "migrates" or moves in such a way to create a short between the transformer windings, causing a thunder crashing sound in the speaker, sometimes kills the signal entirely and puts positive voltage on the grid of the tube following the IF transformer with the fault. Silver mica disease IMHO, is vastly over-diagnosed and is still relatively rare. There are other causes for the symptoms of silver mica disease. Checking for positive grid voltage is the surest way of getting an accurate diagnosis.

Please tell us that you are NOT trying to fix that radio WITHOUT a schematic!!

Regards

WC

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Re: Having trouble

Post by Bill Cahill on Mon Dec 17, 2012 5:05 pm

Actually, you are both wrong. If you turn down the control, you aren't shoting the high end. You are shorting the center tap. It will make a difference.
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Re: Having trouble

Post by N7ZAL on Mon Dec 17, 2012 5:30 pm

The center lug on the pot is the wiper? Hence that goes directly to the audio stage?
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Re: Having trouble

Post by Bill Cahill on Mon Dec 17, 2012 5:37 pm

Yes. And, when you turn down the pot, it shorts the wiper to ground.
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Re: Having trouble

Post by Wildcat445 on Mon Dec 17, 2012 5:37 pm

Bill, I think we are talking about the same thing, just saying it differently. The center terminal of the volume control goes to the grid of the first tube in the audio section. The ungrounded terminal of the volume control comes from the last stage in front of the volume control, in this case, an IF transformer. The volume control varies the amount of signal delivered to the audio tube grid. If you place your finger on the center terminal of the volume control, the input to the audio section, the position of the volume control will have no effect on how loud the buzz is. Where did I go wrong in my explanation? I may having been understanding this wrong for 40 years.

When I place my finger, (or, preferably the tip of a plugged in soldering iron) to introduce a buzz into the volume control, I am not "shorting" anything. I am introducing a 60 cycle note into the audio to test it. Whatever signal was there is still there. The buzz from the speaker will simply over-ride it. I read somewhere that an audio amplifier in good condition will take to having its grid tickled like a male lion will having his tail twisted. A loud growl.

Regards

WC

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Re: Having trouble

Post by Wildcat445 on Mon Dec 17, 2012 5:44 pm

Okay, Bill, I see how you are explaining this now. Please consider this: when I inject a 60 cycle note into the center terminal (wiper) of the volume control, am I not bypassing the volume control, thereby negating its effect on the audio? That IS the purpose of this test. To "break" the radio in half. To try to find whether the problem is before or after the control. Not in the control.

I fear that those of us who know what we are doing, but explaining theory in different manners will only serve to confuse the OP. My intention was the opposite. I apologize for any confusion.

Regards

WC

WC

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Re: Having trouble

Post by Bill Cahill on Mon Dec 17, 2012 5:53 pm

Wildcat445 wrote:Bill, I think we are talking about the same thing, just saying it differently. The center terminal of the volume control goes to the grid of the first tube in the audio section. The ungrounded terminal of the volume control comes from the last stage in front of the volume control, in this case, an IF transformer. The volume control varies the amount of signal delivered to the audio tube grid. If you place your finger on the center terminal of the volume control, the input to the audio section, the position of the volume control will have no effect on how loud the buzz is. Where did I go wrong in my explanation? I may having been understanding this wrong for 40 years.

When I place my finger, (or, preferably the tip of a plugged in soldering iron) to introduce a buzz into the volume control, I am not "shorting" anything. I am introducing a 60 cycle note into the audio to test it. Whatever signal was there is still there. The buzz from the speaker will simply over-ride it. I read somewhere that an audio amplifier in good condition will take to having its grid tickled like a male lion will having his tail twisted. A loud growl.

Regards

WC

No. I F doesn't go to grounded side. It goes to the high side.
That may be your problem.
Not so. If you short the grid, that cuts your signal path.
Turning vol. control down, that's counter clockwise, does this.
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Re: Having trouble

Post by Patrick on Wed Dec 19, 2012 5:37 pm

Back to the Emerson. I turned the volume up somewhat and still no buzzing when I touched the center volume control terminal with my finger. It did buzz when touched by a plugged in soldering iron though. I checked the voltage on all the tube pins in the audio part (12AT6 & 50C5) and everything was fairly close to what is listed except the #6 pin on the 12AT6. It was -2 volts DC and should be -.65. I don't know how much variance is allowed. I put in a new 12AT6 but the radio static continued.
Where do I go from here?

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Re: Having trouble

Post by Bill Cahill on Wed Dec 19, 2012 9:42 pm

You are getting radio static? Does the volume go down when you decrease it with the volume control?
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Re: Having trouble

Post by Patrick on Wed Dec 19, 2012 10:29 pm

Working on a 1955 Emerson 823B AM radio. All caps replaced and tubes checked. When turned on I may get a station for about 1 second then nothing but erratic static that gets louder with the volume control.

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Re: Having trouble

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