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Stupid questions I always wanted to ask. Insulators and Caps.

Low_Boy

Low_Boy

Sr. Member
Jan 21, 2010
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Rochester N.Y.
Ceramic disc capacitors. Example PC122XL, C139 it is a 390PF, 50V, 10% U J. All the 390PF 50V disc caps. I see are a lot smaller in size. probably because they do not have the U, J rating. So with shipping I do not want to spend a fortune for a couple caps. how long will the non U,J cap last under the same use?

I am starying to wonder if I sound like Andy Rooney here.

I often wondered why in many older radios the mica insulator is layered. One piece mica, some heatsink compound, another mica insulator and compound on the transistor and heat sink. Does this work better? Were the mica insulators too thin and they thought they needed something thicker?
 
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Multi faceted question.

In some applications an Aluminium oxide insulator is used.
Replacing this with mica is generally not a good idea. I see people working on radios doing it and cringe.
As to why multiple mica insulators were used it was probably a cost saving measure.
It was probably cheaper than an Aluminium oxide insulator and they needed more isolation on that device.

As to the caps... knowing the application is important. You can't decide to replace it with something else without understanding what it's doing and expect things to work the same in most cases. You can often get away with it but these are bad habits to form.

C139 in a PC122 is a coupling/DC blocking capacitor on the output of the RF amplifier. It's value is important as is it's ratings. I would not substitute this cap with anything lesser since it will likely affect the match / frequency response between the amplifier and filter stage.

The tolerance is taken into account, yes. but if they used a 10% part and you use a 20% part you could alter the performance of the amplifier. It may not even matter or make a noticeable difference but it's bad practice to go down what is a slippery slope.
 
Martian said:
Multi faceted question.

In some applications an Aluminium oxide insulator is used.
Replacing this with mica is generally not a good idea. I see people working on radios doing it and cringe.
As to why multiple mica insulators were used it was probably a cost saving measure.
It was probably cheaper than an Aluminium oxide insulator and they needed more isolation on that device.

As to the caps... knowing the application is important. You can't decide to replace it with something else without understanding what it's doing and expect things to work the same in most cases. You can often get away with it but these are bad habits to form.

C139 in a PC122 is a coupling/DC blocking capacitor on the output of the RF amplifier. It's value is important as is it's ratings. I would not substitute this cap with anything lesser since it will likely affect the match / frequency response between the amplifier and filter stage.

The tolerance is taken into account, yes. but if they used a 10% part and you use a 20% part you could alter the performance of the amplifier. It may not even matter or make a noticeable difference but it's bad practice to go down what is a slippery slope.
Click to expand...
Martian it is next to impossible to find that exact capacitor. Any hints?
Thanks
 
390pF is a pretty common value.
Digikey, donor chassis, etc.

What's the problem with this cap?
Ceramics have a MTBF that can be in the hundreds, if not thousands of years.
Was there some catastrophic circuit failure or did the cap get physically damaged?

I get not wanting to go to a company like Digi-Key and buying a bunch to use one cap.
Just did a quick search on ebay and turned up a lot of results.

I think your fine just by finding a 390pF cap that's in the 10% tolerance range thats >= 50 volts.

Something like this:
https://www.ebay.com/itm/10pk-390pf-500V-Disc-Capacitors-10/390398987174

Is probably just fine.
 
The ceramic insulators are generally used in High Voltage applications. I have seen them in cb radio and wondered WTF? No high voltage in here so why are they doing this?
 
Welcome to the world of Network admittance.

In personal experience with Mica - that "translucent" insulator serves several events - one mostly being the dielectric insulator that prevents the Collector from shorting to ground. Why are some so much thicker than others?

Dielectric strength - ability to withstand high voltages and maintain integrity - as an insulator.
Perforation resistance - both a Physical and Electrical attribute to the performance of high pressure (to remove or transfer heat from one surface to another requires that a considerable amount of force to handle the pressure to maintain thermal contact throughout the surface area) and maintain both planar alignment and prevent the shifting of the part and possibly damage from torque as the mounting of the physical part is on one type of material but the part this is electrically attached to is different.

IT is attached as physically as well as electrically, keeping separate the mounting location - rear panel heat sink from the electrical board (like the main PCB) - as well as prevent the part and the material COMBINING - casing a short (perforation).

Without going into a lot of details...

The 390pF chosen was not exactly a "precise" science - for this C139 COUPLES the output of the 2312 to the NETWORK that makes the 50 ohms impedance.

Have you tried others values in there?

You'll see results in multiples of that value - PLUS the values used at C143.

Again, without going into a lot of details - many radios "miss out" on having a more idealized matching between the final and the network due to less than idealized conditions the transistor loads into the network due to the capacitive impedance mismatch.

You also need to have some idea that the wide DYNAMICS of SSB modes versus the typical AM or FM modes places upon the network and it's ability to capture and direct that energy - in this case to the network.

I have been taught to allow for the proper resonance or admittance for the transfer of energy from one stage to another (your C139 value) and to also provide a path of low-impedance for the potential higher harmonics and also the cyclic currents that form as standing waves within a set parameter (were are not taking about parasitic events) of conjugate match output region. Or what the network can readily accept in the transfer of energy from the device to either the next stage or the output network - and provide a means for any parasitic energy and unwanted multiple of the frequencie(s) of interest to be removed or reduced by providing a path for them to ground that prevents the signal you want to transfer - from being severely attenuated. - hence C143.

In simpler terms - the C139 helps the transistor to see a more idealized place to send energy from itself - and the C139 along with the Network and the use of C143 - allow the transistor an amount of places in which to appear as a given set - of nearly ideal reactance "points" (Conjugate) even though these regions are applied in parallel - the transistors own admittance is less affected by mis-matching further past the C139 (downstream) and the C139 also reduces the effects the mismatch in the unbalanced load the Network is utilizing.

IN regards to series - versus - parallel actions - Parallel circuits although appear as low-ohmic values the individual parts of the paralleled network can have large changes in there reactive C or L and yet have little impact on the overall circuit once you establish multiple venues to distribute the RF energy (C139 and C143 for instance)

C143 also - being seen a paralleled reactance - lessens any mismatch occurrence by also having an ability to absorb part of the energy that would otherwise would have to be dissipated in the part, or the network as heat. It also has (at) the frequency reactance product to provide lower impedance path for higher frequencies.

Again the two Suffixes' for C139 and it's tolerance requirement - also means the part has characteristics that make it the idealized part for that low-impedance transfer into a higher impedance. (IT doesn't matter to it - just it transfers energy at that frequency for a greater range of energy levels as well as maintain a designated Q or reactive impedance in the environmental conditions So the NPO (J) range as well as the U - temperature range - this combination of both allow the transformation that occurs between the two - Network and the Output "tank" circuit - with the part as it drifts - would not act too harshly in the transfer characteristics versus the transformation characteristics - or altering the energy transfer in a negative way.

I am well aware the others will chime in but this is how I look at your questions - they're not easy nor easy to explain - I hope this can cover much of what you needed to know.

:+> Andy <+:
 
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This radio was dead. I got it working, It is working well at that. One of the problems is that C139 must have smoked. They had the wrong value in it's place. I put in the correct size and voltage cap. I have that value here but it does not have the U or J rating. I can't find any at a decent price and was wondering just how bad it would be to leave what I have in the radio in. I could not find a one in any of my parts radios. Thanks for the help.
 
Handy Andy said:
Welcome to the world of Network admittance.

In personal experience with Mica - that "translucent" insulator serves several events - one mostly being the dielectric insulator that prevents the Collector from shorting to ground. Why are some so much thicker than others?

Dielectric strength - ability to withstand high voltages and maintain integrity - as an insulator.
Perforation resistance - both a Physical and Electrical attribute to the performance of high pressure (to remove or transfer heat from one surface to another requires that a considerable amount of force to handle the pressure to maintain thermal contact throughout the surface area) and maintain both planar alignment and prevent the shifting of the part and possibly damage from torque as the mounting of the physical part is on one type of material but the part this is electrically attached to is different.

:+> Andy <+:
Click to expand...

What? There is an old saying that if you can't dazzle them with brilliance.....
I have seen in other applications another product works better and does not require grease at all. I believe it was manufactured by a company named Chomerics.
https://krayden.com/our-lines/chomerics/

We routinely tested isolation at 5000 Vac and did not have any failures or flash over.
 
Last edited:
Low_Boy said:
This radio was dead. I got it working, It is working well at that. One of the problems is that C139 must have smoked. They had the wrong value in it's place. I put in the correct size and voltage cap. I have that value here but it does not have the U or J rating. I can't find any at a decent price and was wondering just how bad it would be to leave what I have in the radio in. I could not find a one in any of my parts radios. Thanks for the help.
Click to expand...
Have you tried Digikey??
 
Low_Boy said:
This radio was dead. I got it working, It is working well at that. One of the problems is that C139 must have smoked. They had the wrong value in it's place. I put in the correct size and voltage cap. I have that value here but it does not have the U or J rating. I can't find any at a decent price and was wondering just how bad it would be to leave what I have in the radio in. I could not find a one in any of my parts radios. Thanks for the help.
Click to expand...

Would like to know what you had to do?

If you have done a MOSFET swap, then 390 is a moot point - but if you've got a 2312 - then you need to scope that radios' output - you don't want flakey parts in the network screwing up an otherwise great little radio.

If you have anything 390 (391) with a "K" rating - would work.

The other part, C143 - is important - and they can smoke the Final real quick.

If you haven't touched it, and the radio seems to work fine, then leave it alone.

C143 was one of the parts that changed thru the years - for reasons unknown. They used anywhere from 221 (220pF) on down to 101 (100pF) - they too can use "K" for they did not always follow the part lists either.

But as for the fun and games thru the years the radio was made, they played with the matching pair of the C145 a anywhere from 561 (560pF) to 391 (390pF) and also played with C146 from 471 (470pF) on down to 391 (390pF) - so each time they retooled the line - the parts were modified - mostly in pairs.

Another region to check is C148 0.0022uF (222) on over to 0.0018uF (182) - and C149 0.01 Y/D to 0.01uF "K".

Both again IF IT WORKS now...don't keep fixing it.

:+> Andy <+:
 

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