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 Post subject: IF construction changes
PostPosted: Dec Sun 01, 2019 12:42 am 
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Location: Nipomo, CA 93444
I've been doing some research on coil winding, especially for IF transformers, and I realized that the physical sizes of IFs had changed quite a bit from the 30's to the 50's. Some late of my late 30's Zeniths and Philcos had cans that were easily 2" square, while 50's chassis had cans(SMD no extra charge) that were closer to half that size. Coil diameters and spacing were obviously quite different.

Was it capacitive tuning vs. slug/inductive tuning? Better materials?

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 Post subject: Re: IF construction changes
PostPosted: Dec Sun 01, 2019 2:56 am 
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Location: Austin, Texas
Almost totally cost driven. The later 3/4" cans are built on an automated line. Less material and less hand labor for lower cost.

Lifetime was also sacrificed. Most 3/4" have failed or will fail soon. The 2" units will probably last a 100 years or more even with the radio in a barn.

Jay


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 Post subject: Re: IF construction changes
PostPosted: Dec Sun 01, 2019 3:43 am 
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JnTX wrote:
Almost totally cost driven. The later 3/4" cans are built on an automated line. Less material and less hand labor for lower cost.

Lifetime was also sacrificed. Most 3/4" have failed or will fail soon.

News to me; none of mine have.

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 Post subject: Re: IF construction changes
PostPosted: Dec Sun 01, 2019 1:38 pm 
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Location: Pennsville, NJ 08070
I suspect the better way to compare it is they sacrificed long term stability for short term cost and performance in a smaller package. The whole issue as I understand it is the Achilles Heel if the 3/4” small cans of the 1950s was the silver mica capacitors used in them were not sealed but exposed to the atmosphere and whatever contaminants and humidity were around where the radio was used. So where the older types would perform reliably under even less than ideal conditions, the newer type were more vulnerable to humidity or contaminants.

Having said that, I have close to 100 working radios, most having the post war 3/4” mica cap IF fans. In all that collection, I have encountered precisely 4 which had confirmed silver mica disease. I would say it is a real problem with that design but the problem seems to be less than prevalent than people make it out to be. I suspect people shy away because of a set comes down with it, the repairs are not easy unless one is lucky enough to source an NOS replacement that itself is still good. I have repaired one by replacing the mica cap with a modern silver mica capacitor and the work with hair thin wires coming off the coil is definitely not for the faint of heart...


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 Post subject: Re: IF construction changes
PostPosted: Dec Sun 01, 2019 10:07 pm 
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jdivito wrote:
The whole issue as I understand it is the Achilles Heel if the 3/4” small cans of the 1950s was the silver mica capacitors used in them were not sealed but exposed to the atmosphere and whatever contaminants and humidity were around where the radio was used. So where the older types would perform reliably under even less than ideal conditions, the newer type were more vulnerable to humidity or contaminants.

Good point, and maybe the best answer. In general, I've found that radios I've bought on eBay over the years coming from dryer climates, such as states in the Southwest, overall perform better than those from humid and damp climes, such as from Florida. The latter environments seem to negatively affect coil inductance, and resistor and capacitor values.

jdivito wrote:
Having said that, I have close to 100 working radios, most having the post war 3/4” mica cap IF fans. In all that collection, I have encountered precisely 4 which had confirmed silver mica disease. I would say it is a real problem with that design but the problem seems to be less than prevalent than people make it out to be. I suspect people shy away because of a set comes down with it, the repairs are not easy unless one is lucky enough to source an NOS replacement that itself is still good. I have repaired one by replacing the mica cap with a modern silver mica capacitor and the work with hair thin wires coming off the coil is definitely not for the faint of heart...

I agree that the problem is well overblown. While not having as many tube sets as you do, I believe I have come across a total of two instances of SMD over the years. In garden variety AA5/6's, the simplest method for cure is to simply remove the damaged IF can, and replace it with a generic, which in most cases will operate adequately, although perhaps not to peak performance.

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 Post subject: Re: IF construction changes
PostPosted: Dec Wed 04, 2019 1:37 am 
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Coils wholly dependant on their construction can over time change. That can be anything from the plastic former decomposing, to the insulation and the mechanical stability of the coils. The construction of the caps is also a factor. With Mica caps there is more than one design here. After around 1935 the open type of compressed leaves clamped together were reliable as were the encapsulated version of the same thing.

We do not seem to have the same issues with Silver Mica here. In several hundred sets getting close to 50 year period, in IF's there were possibly three, all mechanical fails, & very few others that have failed. Despite that I will insulation test any out of circuit. If it has "Micamold" on it irrespective of what it is, I have a thing about them & it will be treated with similar contempt to a wax paper & oil filled caps.

Marc


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