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 Post subject: 12AV6 flickering--need troubleshooting help
PostPosted: Jun Wed 13, 2012 11:30 pm 
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Location: Iowa City, IA
I recently bought a GE Model 427 at a local thrift shop, and brought it home to test. I connected it to my isolation transformer and variac, set the variac for 75 volts and switched the radio on. The dial lamp came on brightly, then dipped in brightness and came back up, just as would be expected. When the tubes warmed up, there was no intelligible sound or static, just a loud crackling. It was then that I noticed that all the tube filaments were glowing steadily--except for the 12AV6. That tube's filament was fading in and out slowly. I freely admit that I could be full of beans, but my best guess at this point is that since the 12AV6 is the detector and automatic volume control, that whatever is causing the filament to flicker is also causing the tube not to function as a diode, and that what I'm hearing through the speaker is the modulated carrier, rather than audio and the sound I'm hearing is loud because the tube is also not powering the AVC circuit. My question is: what would cause the filament to waver in brightness as it is presently doing? Any advice or assistance will be gratefully received.


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 Post subject: Re: 12AV6 flickering--need troubleshooting help
PostPosted: Jun Thu 14, 2012 12:11 am 
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Location: Cleona, PA
Clean the 12AV6 pins with electronic cleaner (CRC brand, auto parts stores, or whatever you already have, even WD-40) and plug in and out of socket while pins are wet a few times. See if this cures the problem. Check if the wires to the tube heater are solidly soldered to the socket terminals. And of course, the tube may have an internal intermittent: subsitute another 12AV6. If all is OK the set should start to play at 80 or 90 volts.

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 Post subject: Re: 12AV6 flickering--need troubleshooting help
PostPosted: Jun Sat 16, 2012 3:49 am 
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Thank you for your suggestion, but it apparently wasn't the solution. I set my variac as closely as I could to 117VAC, and checked some of the tube voltages. On pin 1 of the 12BE6, I got -45VDC where the schematic says I should have -6VDC. On pin 7 of the 12BA6, I got 0.52VDC where the schematic says I should have 0.7VDC. On pin 7 of the 35W4, I read 138.2 VDC where the schematic says I should have 129VDC. On pin 7 of the 12AV6, I read 1.05VDC, where the schematic says I should have 45VDC. On pins 1 and 7 of the 50C5 I read negative voltages and a positive voltage on the control grid. Either these voltages are way off, or I made a big mistake somewhere.


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 Post subject: Re: 12AV6 flickering--need troubleshooting help
PostPosted: Jun Sat 16, 2012 4:44 am 
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Location: Livermore, CA
Hi

Some of those readings are ok.

Plate of 12AV6 has a cap going to grid of 50C5. This cap is leaky and needs replacing. Positive plate voltage of 12AV6 is on grid of the 50C5.

Have other caps been replaced? If not more will be leaky.

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 Post subject: Re: 12AV6 flickering--need troubleshooting help
PostPosted: Jun Sat 16, 2012 5:24 am 
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I don't have all the caps I need on hand. Those that I don't have on hand, I've ordered, but I haven't replaced any of them as of yet.


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 Post subject: Re: 12AV6 flickering--need troubleshooting help
PostPosted: Jun Sat 16, 2012 5:28 am 
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Replacing caps will help.

Are you sure the output tube is 50C5 and not 50B5? On 50B5 grid #1 is on pins #1 and #7 and will be negative.

Also check plate resistor on pin #7 of 12AV6. It may be high in value?

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 Post subject: Re: 12AV6 flickering--need troubleshooting help
PostPosted: Jun Sat 16, 2012 5:34 am 
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As soon as the caps arrive, I plan to replace them. The power amp tube is indeed a 50C5. The schematic calls for a 50C5, and that's what's in the radio. Thanks for the tip about the plate resistor; I'll check that, too.


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 Post subject: Re: 12AV6 flickering--need troubleshooting help
PostPosted: Jun Sun 24, 2012 7:38 pm 
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I checked the plate resistor; it read high, so I replaced it. I replaced all the caps except for four caps in a couplate. The 12AV6 is no longer flickering, but I still have no sound, only crackling. There is negative voltage on pin 1 of the 12BE6, so I assume that the problem is not in the oscillator. Bottom line: I still have no sound, and I'm still stumped.


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 Post subject: Re: 12AV6 flickering--need troubleshooting help
PostPosted: Jun Sun 24, 2012 8:11 pm 
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Capacitors C9A-D are contained in a couplate.


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 Post subject: Re: 12AV6 flickering--need troubleshooting help
PostPosted: Jun Sun 24, 2012 8:30 pm 
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C9-D (.005uf) is usually one of the first to go bad... and needs to be replaced.
So remove those pins on the couplet and replace just THAT cap.. or go ahead and remove the entire couplet and replace each cap & resistor in it with individual discrete units.

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 Post subject: Re: 12AV6 flickering--need troubleshooting help
PostPosted: Jun Mon 25, 2012 2:48 am 
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I can see C9A and C9D (I think), but I'm having trouble locating C9B and C.


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 Post subject: Re: 12AV6 flickering--need troubleshooting help
PostPosted: Jun Mon 25, 2012 4:35 am 
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All four caps should be inside that one copulate... right?
So the whole thing should be clipped off and use four individual caps... making sure you get all the new connections correct.

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 Post subject: Re: 12AV6 flickering--need troubleshooting help
PostPosted: Jun Mon 25, 2012 11:10 am 
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Pbpix wrote:
All four caps should be inside that one copulate... right?
So the whole thing should be clipped off and use four individual caps... making sure you get all the new connections correct.


That's clear enough, but my problem at this point is that I can see the connections for two of the caps, but I'm having trouble locating the connections for the remaining two. There are four caps in this couplate, but there are only six leads coming out of it, not eight. As nearly as I can tell, C9B and C9C share connections, but I can't tell where to separate them.


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 Post subject: Re: 12AV6 flickering--need troubleshooting help
PostPosted: Jun Mon 25, 2012 6:53 pm 
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(9b & 9c) as well as (9c & 9d) are tied together inside.

Image

Something like that

But since you are replacing all 4 caps anyway.... who cares!
....just snip off that whole module and put in the 4 new caps where they belong according to the print.

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Last edited by Pbpix on Jun Mon 25, 2012 11:39 pm, edited 1 time in total.

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 Post subject: Re: 12AV6 flickering--need troubleshooting help
PostPosted: Jun Mon 25, 2012 7:18 pm 
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I bet you have issues with IF cans - silver mica migration


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 Post subject: Re: 12AV6 flickering--need troubleshooting help
PostPosted: Jun Tue 26, 2012 3:40 am 
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Pbpix wrote:
(9b & 9c) as well as (9c & 9d) are tied together inside.

Image

Something like that

But since you are replacing all 4 caps anyway.... who cares!
....just snip off that whole module and put in the 4 new caps where they belong according to the print.



I may not have been very clear, but I am confused about where to make the connections for this set of caps. In other words, I'm having difficulty seeing where to connect the caps on the actual chassis.


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 Post subject: Re: 12AV6 flickering--need troubleshooting help
PostPosted: Jun Tue 26, 2012 3:45 am 
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Meatloaf Lover wrote:
I bet you have issues with IF cans - silver mica migration


I would not be at all surprised if that turned out to be the case. Please pardon my ignorance, but just what is "silver mica migration?" I have another dumb question as well: I have reason to believe that this radio has been mucked with at some point before I got it. The IF cans are not soldered into the chassis; they plug in and are secured with screw clamps. Assuming that someone has been fiddling around with this thing, would it make any difference if the cans had been switched around?


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 Post subject: Re: 12AV6 flickering--need troubleshooting help
PostPosted: Jun Tue 26, 2012 4:30 am 
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Exprof wrote:
Pbpix wrote:
(9b & 9c) as well as (9c & 9d) are tied together inside.

Image

Something like that

But since you are replacing all 4 caps anyway.... who cares!
....just snip off that whole module and put in the 4 new caps where they belong according to the print.



I may not have been very clear, but I am confused about where to make the connections for this set of caps. In other words, I'm having difficulty seeing where to connect the caps on the actual chassis.

Hi again:
I'll try to lay it out real simply.
Just connect to the RED dots...
locate the pins on the tubes as marked and solder there.
One of the wires coming out of the IF can goes to one end of C9B and the other end of C9B goes to pin-2 of V3
Also one end of C9-C goes to pin-2 of V3
The other end of C9-C goes to pin 7 of V3.
One end of R6 goes to V3-pin-7
Also ...One end of C9D goes to V3 pin-7 and the other end of C9D goes to pin-2 of V4
One end of R7 also goes to pin-2 of V4
One end of C9A goes from the CT of the vol control and the other end goes to pin-1 of V3
one end of R-5 also goes to pin-1 of V-3

It can't be much simpler than that.
Follow those red-dot connection points.

Image

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Last edited by Pbpix on Jun Tue 26, 2012 4:51 am, edited 3 times in total.

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 Post subject: Re: 12AV6 flickering--need troubleshooting help
PostPosted: Jun Tue 26, 2012 4:40 am 
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Location: Haledon, NJ, usa
Exprof wrote:
Meatloaf Lover wrote:
I bet you have issues with IF cans - silver mica migration


I would not be at all surprised if that turned out to be the case. Please pardon my ignorance, but just what is "silver mica migration?" I have another dumb question as well: I have reason to believe that this radio has been mucked with at some point before I got it. The IF cans are not soldered into the chassis; they plug in and are secured with screw clamps. Assuming that someone has been fiddling around with this thing, would it make any difference if the cans had been switched around?

Silver mica migration is also known as the "dreaded silver mica disease"
S.M.D. but ... it all means "Silver Mica Disease"

If your radio uses slug-tuned IF cans... you probably have the "dreaded silver-mica disease"

Those IF cans used in 1949-50s AA5 radios used a cheaply built IF transformer which uses slug-tuned tank circuits in both the primary and secondary sides of the IF transformer... but the culprit is the capacitor.

Unlike earlier (good) types.... that used fixed coils and variable capacitors... these lousy IF cans use a single common piece of silver-mica on one substrate to form both the primary and 2ndary capacitors.

Each 1/2 of the substrate has silver sandwiched between the mica sheets to form two separate and distinct capacitors each to operate separately with their respective coils on each side of the transformer.

HOWEVER.. over the years that silver oxidizes ... and as it does it migrates across the space between the two sides of the common substrate. When this happens it creates a partial electrical path across the primary to the 2ndary side of the transformer.

The primary side has high B+ voltage on it to feed the plate of the primary side tube. Some of that B+ leaks across that silver-mica leakage path over to the grid input of the next stage causing LOUD static crashing sounds.

If you hear loud static crash sounds.. then you might probably have this condition.
. and it can be repaired.

You can dis-assemble the transformers and pull out the old mica sheet completely... and simply replace it with a small 100-120pf cap on each side of the transformer across each coil... then re-align the set.

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 Post subject: Re: 12AV6 flickering--need troubleshooting help
PostPosted: Jun Tue 26, 2012 4:44 am 
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Location: Boston, MA USA
Before you jump into replacing the couplate, which is ceramic and rarely fails, let's dig into why you have crazy voltages. Some make no sense at all. Make sure your meter negative lead is connected to the B- bus, not to chassis. Pin 7 (plate) of the 50C5 should not be negative under any circumstance.

I have a GE 427 and I did have trouble with loose/dirty tube sockets. Try wiggling the tubes while the set is on -- if you get loud static as you are moving the tubes then that is your problem.

Not a lot of capacitors in these sets to replace.

Yes, the IF transformers do plug in, and to the best of my knowledge they must not be interchanged.

-David


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