Treating SMD

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Treating SMD Empty Treating SMD

Post by Wildcat445 on Thu Aug 02, 2012 10:10 am

How critical is the value of the silver mica caps in IF transformers? I have been using 100-120pf caps to repair SMD in AM radios with good results. I have done three radios, 2 Admirals and a Zenith. They have aligned well and they operate well. Usually, the value of the mica caps in IF cans is not specified in service literature, hence the "guess" value that I use. I recently came across a schematic just thumbing through Riders that actually gave the value for the mica cap in an IF transformer as 330pf. I am not working on that specific radio, but it caused me to stop and think about whether the method I was using was going to work on almost every radio I enountered with SMD. What say you?

Thanks,

WC

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Post by willy3486 on Thu Aug 02, 2012 10:35 am

I think I have used around that value myself. One thing I do is to put a cap meter across the old cap to see if I get a reading. Then I replace it with that. I think for the most part it has run between 100-120 pf on the ones I have changed.

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Post by Bill Cahill on Thu Aug 02, 2012 11:04 am

The problem with guessing is that even though some am radios do use that value, I've seen some that use 48pf, and, 56pf.
The problem, as you know, is alot of these radios were considered to be a throw away item.
Plus, on many sets, they were still making correct replacements for alot of them.
Bill Cahill

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Post by Wildcat445 on Thu Aug 02, 2012 11:33 am

I don't have a cap meter. Maybe that needs to be on my bucket list? Anyway, Bill, let's assume that I replace a cap with a 120pf and the value should be 48 or 56pf. What problem may I have? Alignment? Poor operation?

Thanks,

WC

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Post by Bill Cahill on Thu Aug 02, 2012 1:23 pm

Alignment would definately be one probability. You'd see a major change in positon of tunning slugs on I F transformers.
Reception would certainly be affected due to having to completely realign set.
It may still work out fine, but, no guarantees.
Even in the early 50's there was some minor experimentation with I F frequencies. Most were at 455kc.
But, I've seen a couple of early 50's sets at 462KC.
If you are having to do alot of realigning, chances are you are making a change from the original I F frequency.
This can sometimes have a major affect on how well the radio plays.
First, pay attention to the rated frequency of the set as listed on tube chart, or, service schematc.
That would at least be a guide.
Most test equipment for radios will cover different ranges of I F frequency. You can align it by that.
Note. I said most. Not all.
I have one cheapie Conar made for NRI institutes that doesn't appear to cover those frequencies at all.
Hope this helps.
Bill Cahill


Last edited by Bill Cahill on Thu Aug 02, 2012 1:27 pm; edited 1 time in total (Reason for editing : Spelling. Isn't it wonderful???)

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Post by willy3486 on Thu Aug 02, 2012 1:51 pm

Not sure if it would be similar but I had a aa5 tube radio from late 40s/early 50s. One of the IF transformers was shot. The cap part was bad and one side of the coil was open. I wasn't able to connect to the wire for a signal so it was probably open in the coil. The closest coil I could find was a little off on the cap reading. I couldn't measure one side of the coil but the other I could. The new coils side I could read and it was off. The resistance was different but not much off.

Anyway it was a pull from a junk one. I used it in the radio and got it to working. The signal was a little weak but I could align it. It probably wasn't at peak performance like if I had a exact one but it did pull in decent.

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Post by Resistance is Futile on Thu Aug 02, 2012 3:22 pm

Sorry, but What is SMD?

OH wait! duh! Silver Migration Disease?
Surface Mounted device?
I have a brain pause saying what the heck is dat. After I started posting I figured it out. But it would help to state the obvious to us old folk and newbies too. Sleep Embarassed

Answering your question though, is yes it is critical to use the exact values as a replacement, because the I.F. and R.F. coils are tuned and balanced to use the capacitor as a bypass or bandpass depending on the intended frequency. (I.F. = Intermediate Frequency and R.F. = Radio Frequency)

This is the nature of determining the resonance of the two types of coils. Since almost all radios use 455Khz for the oscillator and I.F. coils the values are consistent between manufacturers.
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Post by Wildcat445 on Thu Aug 02, 2012 7:23 pm

Thank you all for your input.

To see if I understand correctly........The reason I have had success "guessing" about cap values thus far is the fact that all the radios I have replaced the silver mica caps on have been on IF transformers that were 455kc. Had I done a set with IF transformers of a different frequency, then I could have possibly experienced some of the symptoms you have cited above.

What I need to start doing is this: first acquire a cap tester. Then measure the caps that I am about to replace and get as close as possible to that value. Then, of course, pay closer attention to service literature and find the correct value of these caps if they are listed. Since most of the radios I have are 455kc, all I may need to do is to stock up on 120pf silver mica caps. I had hoped to not have to stock a whole drawer full of different value caps to correct SMD.

Best regards,

WC

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Post by willy3486 on Thu Aug 02, 2012 10:59 pm

Years ago when I worked on tube radios the first time I really didn't notice it. Then I got into them again in the last few years and I have had to catch up on it. This link really helped me. Check the other posts as it has some interesting stuff.
http://ppinyot.com/if_transformers.htm

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Post by terrydec on Mon Aug 20, 2012 11:43 am

I actually repaired a case of silver mica disease. I drilled out the rivet, cut out the old caps and put in mica caps. luckily I had the correct value in stock, 144mmf if I remember correctly. Then the radio didn't work. It took hours to finally figure out I'd reversed the osc. coil leads. Geez!
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Post by Bill Cahill on Mon Aug 20, 2012 12:40 pm

.... Yep. That 'll do it............

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