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 Post subject: Capacitor question "Composition" vs Mica vs Ceramic....
PostPosted: Dec Thu 06, 2018 2:52 am 
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Location: Warminster, PA
Bill of materials from the RCA radio I'm working on calls for various types of capacitors, paper, ceramic, mica, etc..... All of which I have various values of.

A couple of them are listed as "Composition" capacitors 1pf, 500v; 4.7pF, 500V and others.

Safe to sub in a mica cap? It's used in the RF section.

Tom
W3TA


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 Post subject: Re: Capacitor question "Composition" vs Mica vs Ceramic....
PostPosted: Dec Thu 06, 2018 3:43 am 
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thoyer wrote:
A couple of them are listed as "Composition" capacitors 1pf, 500v; 4.7pF, 500V and others.
Safe to sub in a mica cap? It's used in the RF section.
Hi Bill,

I've never encountered "composition" capacitors.

From the low values, I would guess they're axial ceramics.

I do not recommend replacing them unless troubleshooting shows them to be bad.
Low-value axial and radial ceramic caps are the most reliable parts ever invented.

If you don need to replace any, try Surplus Sales of Nebraska in the Capacitor section.
http://www.surplussales.com

- Leigh

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 Post subject: Re: Capacitor question "Composition" vs Mica vs Ceramic....
PostPosted: Dec Thu 06, 2018 4:11 am 
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"Composition" or "compensation?" Some capacitors, usually low value, adjust themselves for temperature variation.

Norm

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 Post subject: Re: Capacitor question "Composition" vs Mica vs Ceramic....
PostPosted: Dec Thu 06, 2018 6:28 am 
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Tom, I think Norm has the right idea - someone probably made an error on the doc.

Is there an NTC (negative temp compensation) value associated with them? They are hard to find these days, but can be. Mica should be good as they are usually 'NTC 0' or NP0 (negative positive 0).

Regards, Larry


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 Post subject: Re: Capacitor question "Composition" vs Mica vs Ceramic....
PostPosted: Dec Thu 06, 2018 1:59 pm 
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Joined: Jan Fri 20, 2012 10:25 pm
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Location: Warminster, PA
Thanks for the responses.

They are Axial with the three color bands on them, maybe 1/4" in length. I don't believe they are typo's in the BOM as ceramic, paper and mica are listed throughout.

There is one area that was reworked some years back (poorly I might add) and one of them has a lead that looks like if you sneeze on it it will break off at the body. I need to move it to replace a shorted paper cap below it. In case I do damage the part beyond use I would like to order a replacement to have on hand. I have lots of parts in stock - but none of these.

Figures - the part you need is never one you have - like that 4.7 meg resistor I found last night that measure over 6 meg..... I hate having to order one part (I'll buy 10 or so but still....). I'm also not a fan of stringing parts together to get the value but in this case I might. A 3.3 meg and a 1.5 meg will put me well within the tolerance band.....

Tom


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 Post subject: Re: Capacitor question "Composition" vs Mica vs Ceramic....
PostPosted: Dec Thu 06, 2018 2:03 pm 
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What is the radio, and what schematic are you using?

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 Post subject: Re: Capacitor question "Composition" vs Mica vs Ceramic....
PostPosted: Dec Thu 06, 2018 3:38 pm 
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pixellany wrote:
What is the radio, and what schematic are you using?


RCA 8516 - have original schematic

viewtopic.php?f=5&t=348504

Tom


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 Post subject: Re: Capacitor question "Composition" vs Mica vs Ceramic....
PostPosted: Dec Thu 06, 2018 4:40 pm 
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thoyer wrote:
They are Axial with the three color bands on them, maybe 1/4" in length.
Hi Tom,

Yep, that's an axial ceramic cap.
Surplus Sales should have them.

That style of ceramic cap has very limited temperature coefficient options.
Most likely they were NP0, as previously mentioned.

- Leigh

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http://www.AtwaterKent.info
Click "Grebe Stuff" for Synchrophase info


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 Post subject: Re: Capacitor question "Composition" vs Mica vs Ceramic....
PostPosted: Dec Thu 06, 2018 4:50 pm 
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That type of cap very rarely fails so I wouldn't touch it unless testing/symptoms indicate a problem.

I replace all old wax paper type caps (including all of the encapsulated wax/paper types like black beauties) and electrolytic caps when going through a vintage radio but other types are replaced only when they are found to be defective.

When you get into small value caps like these you are often treading into territory where temperature characteristics, Q, and installation lead dress/placement are critical so with the low likelihood of component failure wholesale replacement is likely to create problems where previously none existed.

Rodger WQ9E


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 Post subject: Re: Capacitor question "Composition" vs Mica vs Ceramic....
PostPosted: Dec Thu 06, 2018 5:09 pm 
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rsingl wrote:
That type of cap very rarely fails so I wouldn't touch it unless testing/symptoms indicate a problem.

Rodger WQ9E


But he HAS to touch it to get underneath it.


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 Post subject: Re: Capacitor question "Composition" vs Mica vs Ceramic....
PostPosted: Dec Thu 06, 2018 5:58 pm 
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dtvmcdonald wrote:
rsingl wrote:
That type of cap very rarely fails so I wouldn't touch it unless testing/symptoms indicate a problem.

Rodger WQ9E


But he HAS to touch it to get underneath it.



Correct - a - mundo. As I said, a previous owner did some work in this area and the cap in question was not handled very well.... It looks like the minute I go to unsolder one end to gain access to the parts below, the wire is going to break off of the body. Happened to the 2.2k right next to it - but I have those......... :D

There is some exposed lead so I think I'll cut the lead near the solder joint and gently pivot it from the other side to save stress on the questionable side. That's tonight's plan of attack.

Tom


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 Post subject: Re: Capacitor question "Composition" vs Mica vs Ceramic....
PostPosted: Dec Thu 06, 2018 5:58 pm 
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Joined: Jan Fri 20, 2012 10:25 pm
Posts: 62
Location: Warminster, PA
Leigh wrote:
thoyer wrote:
They are Axial with the three color bands on them, maybe 1/4" in length.
Hi Tom,

Yep, that's an axial ceramic cap.
Surplus Sales should have them.

That style of ceramic cap has very limited temperature coefficient options.
Most likely they were NP0, as previously mentioned.

- Leigh



roger

Thanks


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 Post subject: Re: Capacitor question "Composition" vs Mica vs Ceramic....
PostPosted: Dec Thu 06, 2018 6:03 pm 
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Joined: Jan Fri 20, 2012 10:25 pm
Posts: 62
Location: Warminster, PA
rsingl wrote:
That type of cap very rarely fails so I wouldn't touch it unless testing/symptoms indicate a problem.

I replace all old wax paper type caps (including all of the encapsulated wax/paper types like black beauties) and electrolytic caps when going through a vintage radio but other types are replaced only when they are found to be defective.

When you get into small value caps like these you are often treading into territory where temperature characteristics, Q, and installation lead dress/placement are critical so with the low likelihood of component failure wholesale replacement is likely to create problems where previously none existed.

Rodger WQ9E



I was "trying" to get away with not going after all of the paper / wax decoupling caps but after getting the radio up and running then having it go silent only to find a decoupling cap shorted in the filament string (why they put all of the filaments in series is a good question) and spending 2 evenings trying to figure out what went wrong, I think I'll be spending some time replacing caps this weekend.

Not looking forward to is as it looks like during assembly they were some of the first parts installed and everything is built above them making access kind of a pain - thus the question that started this whole thread!

Thanks for the support (everybody!)

Tom
W3TA


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 Post subject: Re: Capacitor question "Composition" vs Mica vs Ceramic....
PostPosted: Dec Thu 06, 2018 6:21 pm 
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thoyer wrote:

I was "trying" to get away with not going after all of the paper / wax decoupling caps but after getting the radio up and running then having it go silent only to find a decoupling cap shorted in the filament string (why they put all of the filaments in series is a good question) and spending 2 evenings trying to figure out what went wrong, I think I'll be spending some time replacing caps this weekend.

Tom
W3TA


The AR-8516l receiver design was for use on either AC or DC shipboard power so a series string filament arrangement was used. I have read about AR-8516 units that came with a transformer and standard 6 volt tubes instead of the mix of filament voltages found in the typical AR-8516. I don't know if these were special order units or a depot modification.

The very similar RCA CRM-R6A was designed for shore use and was AC only. Although these receivers look very similar there are differences beyond just the tube type and power supply design.

Rodger WQ9E


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 Post subject: Re: Capacitor question "Composition" vs Mica vs Ceramic....
PostPosted: Dec Fri 07, 2018 1:33 am 
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Joined: Jan Fri 20, 2012 10:25 pm
Posts: 62
Location: Warminster, PA
rsingl wrote:
thoyer wrote:

I was "trying" to get away with not going after all of the paper / wax decoupling caps but after getting the radio up and running then having it go silent only to find a decoupling cap shorted in the filament string (why they put all of the filaments in series is a good question) and spending 2 evenings trying to figure out what went wrong, I think I'll be spending some time replacing caps this weekend.

Tom
W3TA


The AR-8516l receiver design was for use on either AC or DC shipboard power so a series string filament arrangement was used. I have read about AR-8516 units that came with a transformer and standard 6 volt tubes instead of the mix of filament voltages found in the typical AR-8516. I don't know if these were special order units or a depot modification.

The very similar RCA CRM-R6A was designed for shore use and was AC only. Although these receivers look very similar there are differences beyond just the tube type and power supply design.

Rodger WQ9E


Rodger,

I always am impressed by your wealth of knowledge about these old radios and how much time you spend helping others on this forum getting their radios up and running. Thank you.

Tom


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 Post subject: Re: Capacitor question "Composition" vs Mica vs Ceramic....
PostPosted: Dec Fri 07, 2018 2:19 pm 
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Tom,

You are welcome and it is enjoyable reading about your restoration of this great receiver. RCA is better known for their earlier ACR series ham receivers and the later AR-77 and AR-88 models but the AR-8516 and its sister CRM-6RA are also very good performers from the Radio Marine group.

Other offerings from that group are a bit less inspiring like the RCA branded variant of the general coverage version of the Harvey Wells R-9 amateur band receiver. It works about as well as the Harvey Wells R-9 but is a big step down from the sophisticated older RCA offerings and my copy was further degraded by having a reduction tuning bandspread knob added to the upper front panel, apparently a common mod because I saw another one for sale with the same obviously post-factory addition.

Rodger WQ9E


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