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 Post subject: Phenolic capacitors
PostPosted: Nov Sun 03, 2013 11:07 pm 
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Location: Fort White, FL
I am getting ready to replace some caps and resistors in a heathkit rf-1. It has in it a small molded phenolic capacitor with a 2.2 pf or µµf capacitance. It has a 20% tolerance so it may be good but if it isn't what is a good replacement? I never dealt with and know nothing about this type of capacitor. How reliable are these?


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 Post subject: Re: Phenolic capacitors
PostPosted: Nov Mon 04, 2013 1:48 am 
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Probably still good. Use a dipped mica or NP0 ceramic disc to replace it; but I doubt it is bad.


Pete


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 Post subject: Re: Phenolic capacitors
PostPosted: Nov Mon 04, 2013 2:02 am 
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What is special about this type of capacitor? What are its characteristics? I can't find any info on them.


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 Post subject: Re: Phenolic capacitors
PostPosted: Nov Mon 04, 2013 2:12 am 
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Anson wrote:
What is special about this type of capacitor? What are its characteristics? I can't find any info on them.
I expect it's a mica capacitor, often called "postage stamp" caps.
These have a thin rectangular molded case with colored dots.

Those are very reliable and stable. Failures have been seen, but that's a rare event.

I would never replace one unless it was proven to be bad, or physically damaged.

Note that mica caps have characteristics like temperature coefficient that may be critical to proper operation
of the circuit. Modern replacement parts may not duplicate those characteristics.

The value and other characteristics are normally denoted by colored dots.
There are many different "standard" formats, plus proprietary formats, using from three to nine dots.
Versions with more than six typically have dots both front and back.

A word of caution:
These can be damaged by moving them, particularly by rotating them around the leads to read the dots.

Note that other larger parts of the same general appearance exist, but they're long and skinny.
Such products made by Micamold are bad, period. They may be capacitors or resistors.

- Leigh

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 Post subject: Re: Phenolic capacitors
PostPosted: Nov Mon 04, 2013 3:09 am 
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This one is a small tubular shaped cap that looks nearly identical to a half of a 1/2 watt carbon film resistor.

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 Post subject: Re: Phenolic capacitors
PostPosted: Nov Mon 04, 2013 3:44 am 
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That's an unusual type of tubular ceramic capacitor. Relatively rare animal.
It should be quite reliable.

Its value appears to be 2.2pfd. WHT as a multiplier = a factor of 0.1.

Motorola used these by the zillions to couple the IF transformers in their early FM two-way radios.
The values were typically less than 1pfd, although the product was available in much higher values, perhaps 100pfd max.

- Leigh

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 Post subject: Re: Phenolic capacitors
PostPosted: Nov Mon 04, 2013 4:06 am 
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It's the first I have seen. Couldn't find any info on them.


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 Post subject: Re: Phenolic capacitors
PostPosted: Nov Mon 04, 2013 4:12 am 
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They very rarely fail. Being ceramic I wouldn't even waste time testing it for leakage.

If you have operational problems, that's the only time to be concerned with testing it.

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 Post subject: Re: Phenolic capacitors
PostPosted: Nov Mon 04, 2013 4:17 am 
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Anson wrote:
It's the first I have seen. Couldn't find any info on them.
They're no longer manufactured.

- Leigh

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 Post subject: Re: Phenolic capacitors
PostPosted: Nov Mon 04, 2013 6:24 am 
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That is a molded ceramic capacitor. As others noted, they rarely go bad, especially in that low of of a value.

If you do need an exact replacement, they are available here:

http://www.surplussales.com/Capacitors/RF-Ceramic.html

Pete


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 Post subject: Re: Phenolic capacitors
PostPosted: Nov Mon 04, 2013 4:06 pm 
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Location: Fort White, FL
The unit works pretty well. The only issue I see which is not a huge deal is that the audio output is around 215hz instead of up around 400hz. Not sure what is causing it yet. Voltages seem to be pretty close to spot on. I am going to try it after replacing the electrolytics. There are only a few caps that are not ceramics in this and most of those are the better quality sealed types. I will check them all though.


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 Post subject: Re: Phenolic capacitors
PostPosted: Nov Mon 04, 2013 4:44 pm 
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Location: Long Island
Like most AM/CW signal generators, the RF-1 actually contains two oscillator circuits. One makes the RF carrier, the other generates the audio tone for modulation.

The 2.2-pF ceramic cap is in the RF section and has no influence on the frequency of the audio section.

As for the audio section, it's what's known as a phase shift oscillator. The phase shift--and the operating frequency--are determined by tapped inductor (T2) between the grid and cathode of V2A, and the capacitors across the inductor, (C12 and C14). C13 is a blocking capacitor which keeps the grid bias from getting shorted out by T2. There are only a couple of ways this could be off frequency. One is if a capacitor is bad, or somebody installed the wrong value. Yon mentioned that these are sealed caps, but some of those are merely plastic-encased paper, or metalized paper jobbies which could still be leaky or bad. The other thing is if T2 was miswired, or replaced with a different component. (The chances of it going bad are practically nil, but mentioned for the sake of completeness).

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 Post subject: Re: Phenolic capacitors
PostPosted: Nov Mon 04, 2013 6:45 pm 
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Joined: Aug Sat 17, 2013 12:37 am
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Thanks! Now I know where to look.


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