Step by Step Restoration: GE T-106C, Let's go for it!

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Re: Step by Step Restoration: GE T-106C, Let's go for it!

Post by chrisc on Fri Jan 31, 2014 2:55 pm

Thanks for a really interesting and well explained thread - looking forward to the next installment !

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Re: Step by Step Restoration: GE T-106C, Let's go for it!

Post by jerryhawthorne on Fri Jan 31, 2014 7:31 pm

Hey Doc, enjoying the trip, great job. Can't wait tell you get to the caps. I do think I have seen some threads that did not have to remove the whole coil assembly (microsurgery), could this be specific to that type of transformer?
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Re: Step by Step Restoration: GE T-106C, Let's go for it!

Post by 75X11 on Sat Feb 01, 2014 12:03 pm

Excellent work once again. Definitely worth the wait!
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Re: Step by Step Restoration: GE T-106C, Let's go for it!

Post by Dr. Radio on Sat Feb 01, 2014 10:10 pm

Thanks for the feedback guys, it's definitely welcome. I welcome the questions and suggestions. This is meant to be "interactive". ...Don't be shy Embarassed 

I'm pleased at how the pictures are coming out. It can be tricky to get good quality images when dealing with pieces that are small and awkward.

jerryhawthorne wrote:Hey Doc, enjoying the trip, great job.  Can't wait tell you get to the caps.  I do think I have seen some threads that did not have to remove the whole coil assembly (microsurgery), could this be specific to that type of transformer?
Jerry

Jerry,

Glad you're enjoying the thread. You are correct, the "build" of the assembly pretty much dictates what is "possible" or not. But, it's my personal preference to rebuild the transformers so everything is self-contained like when they left the factory. Some restorers choose to disconnect (that's putting it pretty nicely) the original internals in the base and "hang" replacements on the connections exiting the transformer. I don't really like the looks or quality of building a "tinker toy structure" on the bottom externals. I've got plenty of pictures coming up.... Laughing
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Re: Step by Step Restoration: GE T-106C, Let's go for it!

Post by Dr. Radio on Sat Feb 01, 2014 10:44 pm

Okay, continuing on with the (okay, I'll say it..."tedious") IF transformer rebuild, I'll pick-up where we left off.  One thing I failed to mention earlier, before I removed the coil assembly leads, was I took a resistance reading of each coil. The schematic shows a value of 19 ohms for each. With my digital meter I read a resistance of approximately 21 ohms for each. Certainly close enough!

Earlier I also had posted about "reading time" as I referred to it, not be confused with book reading. I'm talking about taking measurements or "reading" the capacitor values inside the base of the disassembled transformer. Because we will be replacing the existing capacitors with modern replacements, we need to know what value(s) to use. Unfortunately our schematic does not indicate the values of these internal capacitors. This is par for the course. Remember, during the time these types of radios were being serviced, no one was concerned with doing "surgery" like we are doing now. It was simply a matter of removing the whole transformer and dropping in a direct replacement, bill the customer and move on to the next project. Surprised 

Luckily I have my trusty multi-function digital meter which has the ability to read capacitance--even very small values. Precisely what we will need for this type of project as we will be working in the pF (picofarard) range.

With this model, it's simply a matter of setting the range/function selector to the capacitor position and then utilizing the component "slots" where the leads insert. No need to use the actual test probes.



Now, with this meter, there are a couple of things to take into consideration. Even with no probes connected, there is still some stray or inherent capacitance read. Since there is no true "zero" with nothing connected, we'll remember what the baseline reading is when we calculate.



With nothing externally connected we have a reading of .113 nF. nF ???  That stands for nanofarads, another unit of measurements. This is quite often used in the European Union for example, but since I'm use to common US values, I'll have to convert it. This is the other consideration. After the calculations, we'll convert from nF to pF which involves simply moving the decimal point.

Now, before we can read values, we need a way to connect the internal capacitors found in the base to the meter. By not using the traditional probes we avoid extra stray capacitance or even inducing noise into the test equipment which could skew our reading. The meter has the test slots, but guess what? Those stubby connection on the transformer won't insert far enough to make contact. So, it's a matter of quickly tack soldering on some scrap leads just long enough to get the job done. These "scraps" are normally the throw-aways from capacitor or resistors legs being cut to size. They come in useful now.



The "legs" are added to one set at a time. To clarify the arrangement, I've drawn how the internal tabs link to the capacitors. Time to find those mystery values!

 Wink
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Re: Step by Step Restoration: GE T-106C, Let's go for it!

Post by Dr. Radio on Sat Feb 01, 2014 11:20 pm

I do one set at a time. Once a value has been established, then it's time to move on to the other set, or other side.



Sometimes this can be a bit tricky. Because many times the capacitor connections have already gone intermittent, there may be quite a bit of fluctuation in the readings (if any  Crying or Very sad ). Pressing on the base assembly may yield even more variance. Luckily for us, readings "leveled" out only jumping by a few pF's intermittently during the testing.



.225 nF read.

Time for math!

.225 - .113  =  .112

Now to convert from nF to pF involves moving the decimal 3 places to the right.

.112 nF = 112 pF

After repeating the same process for the other "side", the same approximate value was discovered.

Now, I'll let you in on a little "secret". 112 pF wasn't a standard value you'd run across in the design of GE IF transformers. 110 pF was. Our readings were within 2 pF, that pretty much confirms the dual 110 pF design by being that close. We'll need some 110 pF replacement capacitors of the silver mica variety. cheers 

Here's another little "secret" for you. The capacitor values found in these styles of IF Transformers are quite typically "odd" values.  Very common values in mid 1950s IF Transformers made by Automatic Manufacturing Corporation (which supplied TONS of "cans" to the biggest names in radio) were 106 pF and 131 pF. No, I'm not making this stuff up! rabbit

Now that we've confirmed values, we can go about removing the troublesome originals. The test "legs" have been removed and discarded. Carefully straightening the connection points to which the fine wires were soldered will allow for a more easy removal of the phenolic "top" covering the capacitor assemblies. With the aid of a dental pick style probe, the top is lifted up. What will be found under there?



Time to "pop the top" and find out!  Very Happy 

Stay tuned....
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Re: Step by Step Restoration: GE T-106C, Let's go for it!

Post by Guest on Sun Feb 02, 2014 10:24 am

Dr. Radio, this is an excellent thread and I plan to use it when the time comes. I have been fortunate that none of my radios have been effected by this yet. But we all know it's a matter of time.

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Re: Step by Step Restoration: GE T-106C, Let's go for it!

Post by oldgoaly on Sun Feb 02, 2014 3:09 pm

Nicely done Doc! Thanks for taking the time and sharing your knowledge!
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Re: Step by Step Restoration: GE T-106C, Let's go for it!

Post by Bill Cahill on Mon Feb 03, 2014 10:53 am

Indeed, and, you've certainly taught me a thing, or, two. Thank you very much. I have always wondered what na meant.

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Re: Step by Step Restoration: GE T-106C, Let's go for it!

Post by Dr. Radio on Mon Feb 03, 2014 6:57 pm

I know the suspense must be building... Smile

Now we'll see what those supposed capacitors really look like and do some in-depth review about the construction and failure points of them.

With the phenolic cover removed, that mysterious "assembly" is revealed. Remember this hasn't seen the light of day in over 50 years. Despite it being covered, Mother Nature and Father Time have taken their toll. GE's cheap assembly practices didn't help much either. Keep reading for more on this.



As I mentioned earlier, there aren't any capacitors as we typically know them found within. No sealed or dipped construction.  No completely independent units.  Something quite different.  The first thing you'll notice is the contact "tabs" that link connection pins to the mica wafer assembly. On this particular unit, 2 independent capacitors were formed on a single sheet of mica insulation. The capacitor is formed by applying a silver substrate on each side of the mica. These would represent the "plates" of the capacitor. The tabs hold the assembly in place and act as the "legs" found on typically built capacitors--electrically linking the formed plates to the outside world. The mica is the dielectric portion of the capacitor.  Silver mica capacitors are ideal for "tuned" circuits as they are well known for their stability and tight tolerance over time and temperature ranges.

By using the thin wafer design, manufacturers were able to accomplish building an assembly that was quite compact and easy to mass produce.
 study 
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Re: Step by Step Restoration: GE T-106C, Let's go for it!

Post by Dr. Radio on Mon Feb 03, 2014 7:11 pm

We need to completely remove the old capacitor assembly from the base. There isn't too much to this thanks to the design.  Using the dental pick tool again, it's just a matter of lifting up on the top contact tabs and moving them far enough out of the way to lift out the fragile wafer.



Lifting the edges back and forth until the wafer cleared the base yielded satisfactory results.  Once again for a size comparison, the wafer easily fits on the tip of an index finger! Laughing 



Looking back at the transformer's base assembly, you can see the bottom tabs that link to the other side of the capacitors.
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Re: Step by Step Restoration: GE T-106C, Let's go for it!

Post by Dr. Radio on Mon Feb 03, 2014 8:53 pm

Let's do some investigation and review.  Here's an "electronic autopsy" of sorts.  Shocked 

Using the "super zoomerific" imaging, we can magnify what is really going on with this troublesome design.  Rolling Eyes 



Again, what the IF transformer manufacturers were able to accomplish with the "wafer design" was compact size, cost-effectiveness, and easy mass production.  Cheaper doesn't always equal "better", a lesson that isn't new by any means.

With this design, the capacitors were literally just "parts". Nothing was sealed. Nothing was bonded (other than just mechanically compressed) and oxidation and migration took their tolls.

Just like your relatives' prized silverware, tarnishing was something that is very real and that started at the very beginning (with nothing to stop the air from reacting with the materials used).  In addition to just tarnishing, the oxidation or even mechanical stress on the connection tabs can cause high resistance to form between them and the silver substrate. Nasty intermittent "opens" can occur between the tabs and the silver.

In addition to these issues, electromigration can occur. This is apparent on our unit as well.  In simplest terms, this means "deposits" can occur from electrical and chemical reactions.  The blackened area on this unit pointed out is actually a tiny trail that is "growing". These trails are quite often the culprits in intermittent shorts as they are unfortunately unwanted conductive paths. These unwanted paths cause the tell-tale "pops" and "crackles" heard in the radio's output.  Under the worst case scenarios, the restorer might even seen a tiny lightning storm inside the base of the can with the lights turned out in the repair room!  affraid  This is where the "affectionate"  Evil or Very Mad  term "SMD" or "Silver Mica Disease" comes from--experienced restorers who recognize these symptoms.  

Based on my own experiences, I can say I've run into more of the "intermittent" operation due to the oxidation (or lack of good connection bond between the tabs and capacitor elements then I have with actual migration getting to the point of no return. Both easily occur though, due to this design.

Assuming you have already ruled out other suspects in the radio's circuitry, tell-tale symptoms can include:

-Pops, hissing, crackling noises in the output. "A lightning storm on a sunny day".

-Complete loss of any intelligible output (except maybe very faint rushing sounds from the speaker).

-Having a station tuned-in with ample volume, only to have the volume drastically lowered with no apparent cause. I've also likened this to having a strong radio station "magically becomes 10,000 miles away!"

-Intermittent, drastic changes in the receiver's ability to receive (massive loss of sensitivity).

The most unfortunate thing about this IF transformer design is, it's not a matter of "IF", but "WHEN".

Despite the work involved, I practice a proactive approach. I feel it's just better to get it over with. For the newbies out there, I would recommend you don't take on this "task" until you absolutely have to--with ample practice of course--junkers may great subjects. Embarassed
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Re: Step by Step Restoration: GE T-106C, Let's go for it!

Post by Dr. Radio on Mon Feb 03, 2014 9:08 pm

Now that everyone is reeling from the crash-course on electrochemical reactions Razz , maybe it's time for a bit of a breather.

We haven't done a quiz for a while....How about now?

Okay, during "reading time" it was determined that each capacitor inside the IF "can" was approximately 110 pF. In other words, there were two, formed capacitors in the base, of equal capacity. While there is no way to "eyeball" the amount of pF capacity, is there a way to "eye" that each capacitor is roughly equal in capacity to verify our findings? If so, how?


Post your answers/guesses/etc.... Razz
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Re: Step by Step Restoration: GE T-106C, Let's go for it!

Post by Bill Cahill on Mon Feb 03, 2014 9:42 pm

I'd say NO. And, by the way... I've seen them get so bad that the caps actually started on fire.

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Re: Step by Step Restoration: GE T-106C, Let's go for it!

Post by Bill Cahill on Mon Feb 03, 2014 9:45 pm

Hmmm. Just remembered a five tube min. Emerson I own. I was getting a nasty crackling sound after rebuilding. I turned the volume control, and, saw sparks in it. affraid I put my meter on the secondary of the second I F transformer, and, it had 35 volts of B plus.
Yes, I got the radio playing, again.

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Re: Step by Step Restoration: GE T-106C, Let's go for it!

Post by Resistance is Futile on Mon Feb 03, 2014 11:32 pm

I would do what all talented technician's should know. Measure the capacitive surface area and find and note the dielectric value of the mica wafer and then measure its thickness. I would use a dial caliper to measure surface area of silver (substrate) and thickness of dielectric.

Get out a calculator and do the math.
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Re: Step by Step Restoration: GE T-106C, Let's go for it!

Post by Resistance is Futile on Tue Feb 04, 2014 12:14 am

mica dielectric constant (permittivity) = 5.4

a= electrode area = 38 cm squared

d = electrode seperation = 1/16 inch

k= dielectric constant =  5.4
------------------------
c = capacitance = 114.449 pf

http://www.endmemo.com/physics/capacitor.php



http://www.ajdesigner.com/phpcapacitor/parallel_plate_capacitor_equation.php

Question: What would happen rather than replace the offending capacitors,( I would think) that you can take the disassembled cap and use a liquid silver cleaner to remove the silver migration and tarnish. (Of course using it as a bath) Rinse with distilled water Then use alcohol to remove any water, and air dry and reassemble?

That's what we used to do for cleaning Military RF connectors and silver migration on circuit cards.
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Re: Step by Step Restoration: GE T-106C, Let's go for it!

Post by chrisc on Tue Feb 04, 2014 1:33 pm

The question is how do we know that the 2 caps are roughly equal in rating by looking at them and I suppose if we can see pretty easily that if the surface areas are the same or very close to each other we'd know the 2 values will be the same. If you knew the value for one, it would be a fair assumption that the other one would be the same or close.

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Re: Step by Step Restoration: GE T-106C, Let's go for it!

Post by Resistance is Futile on Tue Feb 04, 2014 3:22 pm

I would assume that Both I.F. cans are the same and therefore the same value or very close to the same, However I have seen on some schematics, where they are different in resistance readings.
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Re: Step by Step Restoration: GE T-106C, Let's go for it!

Post by Bill Cahill on Tue Feb 04, 2014 10:05 pm

I rarely find both cans to be exactly the same...

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Re: Step by Step Restoration: GE T-106C, Let's go for it!

Post by Dr. Radio on Wed Feb 05, 2014 12:23 pm

Thank you for your participation Smile

Again, The original quiz question was:

Okay, during "reading time" it was determined that each capacitor inside the IF "can" was approximately 110 pF. In other words, there were two, formed capacitors in the base, of equal capacity. While there is no way to "eyeball" the amount of pF capacity, is there a way to "eye" that each capacitor is roughly equal in capacity to verify our findings? If so, how?

I appologize if I didn't make it come across that clear, I was simply asking about both capacitors inside just the -one- unit we are looking at.

The answer is "yes"....

Chris nailed it:

chrisc wrote:The question is how do we know that the 2 caps are roughly equal in rating  by looking at them and I suppose if we can see pretty easily that if the surface areas are the same or very close to each other we'd know the 2 values will be the same.  If you knew the value for one,  it would be a fair assumption that the other one would be the same or close.

That is what I was looking for. Even just 20 pF difference becomes "visually distinguishable ". Had one side of the silver substrate been larger then the other, there is no way they should read equally in capacitance. This quick visual is just used to reinforce our basic findings, not actually verify exact capacity.

Cliff, you went way above and beyond! Smile

Many sets use identical IF transformers (both in winding resistance and capacitor values). Many do not Very Happy




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Re: Step by Step Restoration: GE T-106C, Let's go for it!

Post by Dr. Radio on Wed Feb 05, 2014 12:32 pm

Resistance is Futile wrote:

Question: What would happen rather than replace the offending capacitors,( I would think) that you can take the disassembled cap and use a liquid silver cleaner to remove the silver migration and tarnish. (Of course using it as a bath) Rinse with distilled water Then use alcohol to remove any water, and air dry and reassemble?


Cliff,

Excellent question. This has been brought up by others based on there experiences as well.

Personally, I am against this type of rebuild, as ultimately it really doesn't solve anything. You may get the radio working again, but the oxidation and migration will rear their ugly heads again. With replacing with sealed modern capacitors, you pretty much "never" have to worry about this again. The other issue is getting the tight mechanical compression back. You may not be able to reproduce the factory pressure exerted on the tabs to make tight enough contact. After a few "thermal cycles" of the radio in use, you might be right back down the same road. Why practice the same "flawed" design and procedures when we can use modern technology for long-term reliability?

Those are just my thoughts
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Re: Step by Step Restoration: GE T-106C, Let's go for it!

Post by Bill Cahill on Wed Feb 05, 2014 12:38 pm

I agree with doc. No, I didn't get your question correctly. I agree in the same can they are, or, at least, tend to be the same value. Thanks for a great thread, doc.

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Re: Step by Step Restoration: GE T-106C, Let's go for it!

Post by chrisc on Fri Feb 07, 2014 11:08 am

If it's difficult to get the capacitor value could you somehow measure the coil characteristics and work out the capacitor value from that ? Or would that be a bit like the old joke of 'counting the number of legs the cows have and dividing by 4 ' i.e. going the long way round ?

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Re: Step by Step Restoration: GE T-106C, Let's go for it!

Post by Dr. Radio on Sat Feb 08, 2014 8:45 pm

chrisc wrote:If it's difficult to get the capacitor value could you somehow measure the coil characteristics and work out the capacitor value from that ?  Or would that be a bit like the old joke of  'counting the number of legs the cows have and dividing by 4  ' i.e. going the long way round ?

Chris,

You can. There are some catches though. First off, you have to remember the individual "inductor portion" of the transformer assembly is variable. Depending on the position of the "slug", will affect your measurements. You could measure it as-is (assuming it hasn't been messed with too far from it's original setting) using a bridge circuit (which would be more accurate than a simple LCR meter--do a search for Maxwell's Bridge and Hay's Bridge for determining unknown inductances) or you could assemble test circuitry to find the "peak" during a sweep of the position of the ferrite core (slug). Some people claim using a GDO (grid dip oscillator) would work, but I'd like to hear how well that would work with some IF transformer designs, specially the "cupped" core style made by Automatic Mfg. in which the ferrite part actually covers the coil and would interfere with you taking measurements.

I guess I'm stubborn, or I just dislike doing math lol!  I've worked a while on trying to find the *exact* cap values. I've been fortunate enough to find some rare examples of factory literature and service info that contains the "mystery" values and I've built on that using some detective skills and for the lack of better terms, "cross reference" using known designs and part numbers.

Lastly, what many, many restorers due is just pick a "general value" like 120 pF and install that capacitor on each "side" and thanks to the variable inductance, there is enough room to "play" and get a peak during alignment. Even the manufacturers realized there is no need for exact precision as they will have to be peaked anyway in-circuit to achieve proper resonance.

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Re: Step by Step Restoration: GE T-106C, Let's go for it!

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