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Stupid questions I always wanted to ask. Insulators and Caps.
- Thread starter Low_Boy
- Start date
No, I pressed the red button once. Took out the mainframe. Not doing that again.
Low_boy, if we still haven't "blown" your mind over this...
Wanted to give you an example of what goes thru my head when you ask a question like the use of 390pF caps...
C139 yes, it's used, but there is another - C143 too...
It's the Xc thing ...
Xc = 1 / (2PI F C) deal - not easy nor is it easy to show the calculation ... but in essence the value is
Xc = 15.114 Ohms (approx.) at 27MHz using 390pF and Coulombs at 50 V is about 0.00196 Q (power-charge rating)
But
Xc at 52MHz (2nd Harmonic is)
Xc = 7.8 ohms - even lower...
C143 uses at most 180pF
So at 27MHz Xc is...
Xc = 32.74 Ohms (approx.) at 27MHz using 180pF and Q = 0.0009 Coulombs (power-charge rating)
But at 52MHz or the 2nd Harmonic we get...
Xc = 17.0 Ohms (approximately)
Disclaimer: This is Ohmic (Xc) not impedance (Z) results (interchangeable - yes to a certain degree but this is all relative) - your mileage may differ
So when you ask this type of question - you also have to look at a power rating too...
And I'm not going to go to the L portion - considering that Inductor is variable...but in any case you still have to look at how well that cap will couple to the network - and add in using the L of this "tank" circuit to match into the Network used to make it 50 ohm unbalanced at the antenna jack...Why, when that slug is adjusted, power goes up and down and when you use a complex wave of carrier and audio envelope with it - the power and the range of reactive components changes even more to a greater range of ohmic values - very dynamic.
Its' why I mentioned the Paralleled circuit - when one value is different - the other value left alone - the overall ohmic value of the circuit changes, yes, but to a lesser degree than had the circuit used a single capacitor to couple the output to the network.
So when you look at C139 - look at C143 too - because both work together.
And s they approach the network C136 also works as a divider to this power...
Again this is a simplistic review, but hopefully you can see why "J" or U may matter, but more so the overall values used between C139 and C143 - make the difference in how the multiple harmonics are handled.
In the pages I have at CB tricks - this is the best way I can show how those values work....
And why many radios "miss out" on some of the more efficient levels of power transfers that can be done between the output tank and the input side of the network. Just because there is a "Generic" (for lack of a better term) tank circuit used in nearly all the AM only radios (using 2078) these days, that could benefit from reduced harmonics had the output tank been made more effective by improving and idealizing the power transfer and reducing the harmonics produced back at the final / driver pair.
:+> Andy <+:
Wanted to give you an example of what goes thru my head when you ask a question like the use of 390pF caps...
C139 yes, it's used, but there is another - C143 too...
It's the Xc thing ...
Xc = 1 / (2PI F C) deal - not easy nor is it easy to show the calculation ... but in essence the value is
Xc = 15.114 Ohms (approx.) at 27MHz using 390pF and Coulombs at 50 V is about 0.00196 Q (power-charge rating)
But
Xc at 52MHz (2nd Harmonic is)
Xc = 7.8 ohms - even lower...
C143 uses at most 180pF
So at 27MHz Xc is...
Xc = 32.74 Ohms (approx.) at 27MHz using 180pF and Q = 0.0009 Coulombs (power-charge rating)
But at 52MHz or the 2nd Harmonic we get...
Xc = 17.0 Ohms (approximately)
Disclaimer: This is Ohmic (Xc) not impedance (Z) results (interchangeable - yes to a certain degree but this is all relative) - your mileage may differ
So when you ask this type of question - you also have to look at a power rating too...
And I'm not going to go to the L portion - considering that Inductor is variable...but in any case you still have to look at how well that cap will couple to the network - and add in using the L of this "tank" circuit to match into the Network used to make it 50 ohm unbalanced at the antenna jack...Why, when that slug is adjusted, power goes up and down and when you use a complex wave of carrier and audio envelope with it - the power and the range of reactive components changes even more to a greater range of ohmic values - very dynamic.
Its' why I mentioned the Paralleled circuit - when one value is different - the other value left alone - the overall ohmic value of the circuit changes, yes, but to a lesser degree than had the circuit used a single capacitor to couple the output to the network.
So when you look at C139 - look at C143 too - because both work together.
And s they approach the network C136 also works as a divider to this power...
Again this is a simplistic review, but hopefully you can see why "J" or U may matter, but more so the overall values used between C139 and C143 - make the difference in how the multiple harmonics are handled.
In the pages I have at CB tricks - this is the best way I can show how those values work....
And why many radios "miss out" on some of the more efficient levels of power transfers that can be done between the output tank and the input side of the network. Just because there is a "Generic" (for lack of a better term) tank circuit used in nearly all the AM only radios (using 2078) these days, that could benefit from reduced harmonics had the output tank been made more effective by improving and idealizing the power transfer and reducing the harmonics produced back at the final / driver pair.
:+> Andy <+:
Last edited:
He wanted to know about the tolerance of the cap and where he might be able to obtain a replacement.
He did not ask if we would redesign the radio.
The guy just wants to find something that will work.
It's a great answer, but offtopic.
CB radios are built to a price.
Does it pass? Yep. Build it, sell it.
The manufacturers of CB radios are not looking for optimal.
They are looking for "will it work and pass regulatory requirements"
Edit: Typo.
He did not ask if we would redesign the radio.
The guy just wants to find something that will work.
It's a great answer, but offtopic.
CB radios are built to a price.
Does it pass? Yep. Build it, sell it.
The manufacturers of CB radios are not looking for optimal.
They are looking for "will it work and pass regulatory requirements"
Edit: Typo.
Don't be so sure...
The real question was "size"
So that also includes - power capability translates to ability to transfer power...
Look at the ohmic range and he's thinking 20% of 15 ohms - not a big deal - question answered. (The post above is how I got to it)
However, Size and ability to dissipate power depends upon some other factors - but we started with size and ohmic - so would you expect a 1/4 watt rated resistor for 15 ohms to handle 40 volts with an expected power wattage output of up to 12 watts and those are from the Owners Manual Specifications?
Come on people - ohms law here - he's asked a legit question that demands a straight provable answer.
But...that answers his question...
Do you, Martian, have any?
While you are at it - look up Legacy as an Adjective - it may help all of you understand - it's not about you or me. It's what is left behind for others that may need to see these answers.
It's not just "HOW" a radio works, it's also the "WHY" they went about doing what they did in making it - that translates back to LEGACY - for this radio to be repaired - I leave a solution for others to review.
:+> Andy <+:
The real question was "size"
So that also includes - power capability translates to ability to transfer power...
Look at the ohmic range and he's thinking 20% of 15 ohms - not a big deal - question answered. (The post above is how I got to it)
However, Size and ability to dissipate power depends upon some other factors - but we started with size and ohmic - so would you expect a 1/4 watt rated resistor for 15 ohms to handle 40 volts with an expected power wattage output of up to 12 watts and those are from the Owners Manual Specifications?
Come on people - ohms law here - he's asked a legit question that demands a straight provable answer.
But...that answers his question...
Do you, Martian, have any?
While you are at it - look up Legacy as an Adjective - it may help all of you understand - it's not about you or me. It's what is left behind for others that may need to see these answers.
It's not just "HOW" a radio works, it's also the "WHY" they went about doing what they did in making it - that translates back to LEGACY - for this radio to be repaired - I leave a solution for others to review.
:+> Andy <+:
