suggest me a capacitor/resistor pair

Discussion in 'General Electronics Chat' started by mik3ca, Mar 4, 2008.

  1. mik3ca

    Thread Starter Active Member

    Feb 11, 2007
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    It seems that with my radio circuit, I have capacitors with about 20% tolerance to them and resistors with 5% tolerance. The problem with the tolerance is that it is causing me to insert random capacitors just to achieve exact values.

    Rather than buying standard 1/4 watt carbon resistors, and titanium caps with 20% tolerance, what kind of capacitors and resistors should I buy (at least for the quench oscillator found in typical receivers)?

    I don't want to spend a ridiculous amount on each capacitor, and I don't want to buy too many of them to make a certain value, and my only stores nearby are Sayal Electronics (which is the store I usually shop from), and Nu-tech electronics (which can become expensive).

    There is also the source by circuit city, but they are expensive and they have a limited supply.

    I'm hoping for a 2% or better tolerance.
    I could also order online if the site I order from supports paypal.
     
  2. nomurphy

    AAC Fanatic!

    Aug 8, 2005
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    12
    With lower values (say <1000pF), you should use NP0 (C0G) type ceramics which are very stable and accurate. Above that, up to about 1uF (although they can go higher) use X7R ceramics. Stay away from the older Y5V or such ceramics.

    Various types of polys (polypropolene, polystyrene) which are also fairly stable are available as well.

    A 10% tantalum (not titanium) is very procurable over the 20% version, and 1% resistors are now common (Radio Shack had 2% resistors decades ago).
     
  3. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    Radio Shack actually does still carry a (very limited) stock of 1% metal film resistors. Nothing like the 60's and 70's though. :(

    Actually, at the moment I'm working up a circuit to test caps and inductors, that might be quite helpful for you. It will be a modification of a circuit that AARL published years ago that someone else modified, and that I'm making mods to as I'm not satisfied with the mods that were made.

    Once I get it working to my satisfaction, and construction is repeatable using commonly available materials/components, I'll post it in the Projects Collection section so that others may benefit.
     
  4. Audioguru

    New Member

    Dec 20, 2007
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    Ceramic disc capacitors rated for NPO are stable and work well at VHF frequencies if their leads are short and their value is less than about 0.01uF.

    Metalized plastic film capacitors are good for audio frequencies, are available in 5% tolerance and are fairly inexpensive.

    Electrolytic capacitors are inductive above about 100kHz and therefore are not suitable for VHF frequencies.

    Typical receivers don't have a quench oscillator. They are super-heterodyne. Only super-regen receivers have a quench oscillator that stops the circuit over and over when it starts oscillating.

    Your quench oscillator must operate above about 25kHz so that you don't hear it and don't hear it beating with the 19kHz stereo pilot tone. You might hear it beating with the 38kHz stereo sidebands.
     
  5. mik3ca

    Thread Starter Active Member

    Feb 11, 2007
    189
    0
    Thanks.

    I guess that means I will have to either order them from somewhere special, or I will walk into nu-tech electronics, hope and pray they have them and pay an arm and a leg.

    Is there any special marking on the capacitor that makes it NPO? or do I look for the letters "NPO"?
     
  6. nomurphy

    AAC Fanatic!

    Aug 8, 2005
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  7. Audioguru

    New Member

    Dec 20, 2007
    9,411
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    Ceramic capacitors with a stable NPO rating are very inexpensive. I bought some from Digikey and Newark.

    A friend of mine worked at (now gone from Canada) Radio Shack and gave me many radio controlled toys' electronic circuit boards. They are full of low value NPO ceramic disc capacitors. He also gave me a bag full of portable radios.

    My son found a guy who worked for Best Buy Electronics. He came home with a truckload of "unrepairable" electronic products. I fixed a few by using parts from duplicates.

    I have the parts from many telephones and their answering machines.

    I have a stash of new parts that were added to orders to beat the minimum order amount.

    I have a few new-old printers that I havent opened yet.

    I have the computer from a car.

    Now I have millions of good parts for free.
     
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