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 Post subject: Bad IF transformers
PostPosted: May Thu 13, 2021 7:14 pm 
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Location: 3717 NW 66th pl gainesville FL 32653
i am working on a small GE AA5, the caps have just been replaced but it only hums, I started poking around and got no continuity on one or both of the if transformers, how likely is there to be an open coil on one of these, should there be resistance across the coils? i am not sure how to test one.


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 Post subject: Re: Bad IF transformers
PostPosted: May Thu 13, 2021 7:53 pm 
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Location: Mission Viejo, southern California
Are they disconnected from other items? If so, just use an ohm meter. If they are still connected, first look at what else is connected to check for shorts and shorted components.

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 Post subject: Re: Bad IF transformers
PostPosted: May Thu 13, 2021 8:29 pm 
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Location: 3717 NW 66th pl gainesville FL 32653
one of the coils is open, I am having a hard time finding a replacement, it is the first stage transformer and it has a couple of trimmer caps in the top instead of adjustable coils. most sellers do not specify and the newer ones seem to just have adjustable coils.


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 Post subject: Re: Bad IF transformers
PostPosted: May Thu 13, 2021 9:02 pm 
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What do you mean "most sellers?" Are you only looking on Ebay? Please post a photo(s), and indicate the schematic and IF frequency. After that, we can all work together to determine what you need. You can then place a wanted ad here, and/or check with you local radio club. I have a variety of IF cans with the trimmer capacitors in the top, and am certain that many others here have such IF cans in their parts supply.

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 Post subject: Re: Bad IF transformers
PostPosted: May Thu 13, 2021 9:17 pm 
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GE made hundreds of millions of consumer radios.

It would be very helpful if you told us exactly what you are working on. (Model number)


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 Post subject: Re: Bad IF transformers
PostPosted: May Thu 13, 2021 9:25 pm 
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Here's a work around;


Attachments:
home made If Xformer.jpg
home made If Xformer.jpg [ 180.39 KiB | Viewed 655 times ]

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 Post subject: Re: Bad IF transformers
PostPosted: May Fri 14, 2021 12:18 am 
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Location: Austin, Texas
As you can see from this Arvin diagram, there were radios that used only a single tuned circuit for one IF. If it is the second transformer that has an open winding, you can copy this circuit to get the radio going.
Attachment:
Arvin356T.jpg
Arvin356T.jpg [ 69.48 KiB | Viewed 588 times ]

Jay


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 Post subject: Re: Bad IF transformers
PostPosted: May Fri 14, 2021 3:09 am 
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Why don't you simply open up the transformer and fix the broken wire? It would probably be faster than building the little circuit and work better too.


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 Post subject: Re: Bad IF transformers
PostPosted: May Fri 14, 2021 3:30 am 
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Joined: Feb Wed 08, 2012 3:09 am
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Location: Detroit, MI suburbs
Does your radio have the type of IF transformers with adjustable capacitors or the later tunable inductors? I have encountered a few of the later type with coil leads which were probably broken when someone tried to align the IF. Some of those IF cans are very delicate.

Larry


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 Post subject: Re: Bad IF transformers
PostPosted: May Fri 14, 2021 12:30 pm 
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What model is it? Take a pic of it, I might have a similar one you can use.


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 Post subject: Re: Bad IF transformers
PostPosted: May Fri 14, 2021 12:58 pm 
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Location: Long Island NY
There could be a lot of reasons why a radio just has a bit of hum but no audio. It's not necessarily one or both IF transformers.

To test an IF for continuity, you simply measure each winding. I've seen and worked on hundreds of GE AA5 radios and they all had plain two-winding IF transformers. While that is not conclusive, I'd suggest the odds are pretty good yours have two windings each. If they are older style transformers with wires, you should find a few ohms from the red wire which connects to a filter capacitor somewhere, to the blue wire which connects to the plate of the converter or IF tube. There should also be a few ohms from the green wire connected to a tube grid or diode section, to the black wire which connects to the AVC circuit. There should be infinite resistance (no connection) from the red/blue wires to the black/green wires.

In newer sets they went to miniature IF transformers which have solder lugs or are mounted on PC boards. It may be a little harder to identify which pin is which, but they should measure the same way; you'll find the primary pins or lugs on one side and the secondary pins on the other side.

Another way to do this is to measure the B+ voltages on the plates of the tubes. If an IF transformer has an open primary the B+ on the plate of the converter or IF tube will be missing. The plate voltage is normally almost the same as the B+ voltage. Grid circuits are a little harder to test this way since the grid voltage can vary for a number of reasons, and you need a very sensitive meter to get a reading. But if you identify the plate and B+ wires or terminals with a voltmeter, the other two have to be the grid or diode connections. You can then disconnect the power and test those with an ohmmeter.

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 Post subject: Re: Bad IF transformers
PostPosted: May Sat 15, 2021 12:21 am 
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Location: Montreal (QC) Canada
acbaker123 wrote:
one of the coils is open, I am having a hard time finding a replacement, it is the first stage transformer and it has a couple of trimmer caps in the top instead of adjustable coils. most sellers do not specify and the newer ones seem to just have adjustable coils.


As you stay, older IF transformers had trimmer caps, newer have adjustable coils.
In newer ones, we often find coils that have been wrenched by a butcher.
But in old ones with trimmer caps I have often found that the coil wire was cut outside the coil itself, therefore fixable with a delicate solder. Example pictured.

You definitely need to make some surgery and look where your coil is cut...

Attachment:
20210127_130540.jpg
20210127_130540.jpg [ 489.53 KiB | Viewed 407 times ]

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 Post subject: Re: Bad IF transformers
PostPosted: May Sat 15, 2021 1:09 am 
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It is not unusual with the effluxion of time for coils to change often the bobbin will slip as the glue fails, or wax melts. That along with corrosion, can result in wire breaks which in the main are an easy fix (some are not). That means that you explore the fix option first.

Of concern are plastic formers. I am seeing many early plastics, going brittle & decomposing: They are a real problem. There are various insulation compounds that can be used as a Mar to seal them & hold them in place. Any copper to dissimilar metal joint should be coated to reduce the risks of corrosion breaks.

I have added a photo of a patched coil where a rodent broke the top coil into three sections and I prised the ends out with a scalpel.

Marc


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DSC_2372.JPG [ 805.88 KiB | Viewed 402 times ]
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 Post subject: Re: Bad IF transformers
PostPosted: May Sat 15, 2021 10:48 am 
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Joined: Jan Thu 16, 2020 12:29 am
Posts: 1269
Marcc wrote:

I have added a photo of a patched coil where a rodent broke the top coil onto three sections and I prised the ends out with a scalpel.

Marc


Nice Job.

It is more difficult to beat the Rodent at its own game than most people would imagine.

Their breeding potential is nothing short of astounding.

Their urine is extremely corrosive.

Their intelligence, memory and cunning is also grossly underestimated.

Borne out by the recent behavioral science experiments where Rats were trained to drive small electric cars for rewards. And the most interesting thing was, by studying their serum cortisol levels, if a Rat was a passenger in the car that another Rat was driving, it suffered more stress than it would if it was the driver. What species does that remind you of ?

So If I am ever compared to a rodent, I take it as a form of a compliment, because, despite their pest like and maligned nature, I really admire them, what they can achieve, with a Rat where its brain is only the physical size and appearance of half a Walnut. Plus, they make wonderful pets. And of course, no Scientist's laboratory is complete without a pet Rat as a mascot.

Here is a video of a Rat driving a car:

https://www.youtube.com/watch?v=ZO11r_8_Xe4

Mind you if I was the Scientist conducting this experiment I would have put the Rat in a more Jazzy car, like a red sports car that Stuart Little had. And trained them to obey traffic lights and created a small Sim City with the Rats driving sports cars and obeying all the rules. Then, the next logical step in the experiment would be to ply them with alcohol and observe an increase in road traffic accidents and running red lights.


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 Post subject: Re: Bad IF transformers
PostPosted: May Sat 15, 2021 12:29 pm 
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Posts: 12261
Location: Victoria, Australia
The best thing from my point of view is I got away with the repair. No hope of getting replacements. Phasing the three sections was fun (sic). One knew the origin of two bits and their ends; but with the middle bit all one had was continuity.

As a handy hint for all; The phasing was done with "needle pulses" from a Function Generator and the watcher, an Oscilloscope. Wire joining by splice: All wire pre tinned. Luckily the breaks were near the surface and identified in dim light with an insulation tester @1KV.

Consider it a weird method: But provenance is in the result. Below NSW Australia is having a Mouse plague: Incredible viewing.

https://www.abc.net.au/news/2021-05-10/ ... 0of%20mice

Marc


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