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Thread: Heathkit SB-200 Component Question

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  1. #1

    Default Heathkit SB-200 Component Question

    Hi everyone. I want to replace the Grid Caps in my Heathkit SB-200. In the drawing these caps are labeled C14 and C15. The existing caps are Silver Mica 200uuf with voltage unknown; I am not able to determine the voltage from the markings on the caps. Also, the voltage is not listed in the parts list. Does anyone know what the voltage rating is on these two caps? And if the voltage of these caps is unknown does anyone know what the maximum voltage is in the grid circuit.

    Thanks

    Dan KI4AX

  2. #2

    Post

    Dan,

    Silver Mica capacitors are very reliable, are you having a problem?
    I never shot-gun replace these!

    Most silver mica capacitors are either 300 VDC or 500 VDC working voltage,
    there are versions for 1000 VDC
    http://www.justradios.com/mica.html
    Last edited by W9GB; 05-01-2012 at 06:15 PM.

  3. #3

    Default

    George,

    Thanks for the response to my post. Well, under normal circumstances I agree with you. I don't normally like to "shotgun" replace things. For one I don't like to waist money and I'm sure you can identify with that. Anyway, I only recently obtained the amp and I do not know much history on it. I believe that it was owned by a Cigarette Smoking CBer from the looks of the inside of the cabinet. The Cigarette Smoke was literally oozing down the inside of the amp. The reason I believe that it was owned by a CBer is because the big Tank Coil was real dark and the smoke was baked on to the 10 meter portion of the coil really bad. From the tap on the bottom of the coil forward the coil was almost black. I have it all cleaned up now.

    Anyway, one of the Grid Resistors is open and the other one has climbed in value and I believe the amp has been used and abused. As a result I would like to replace these two Capacitors, C14 and C15, to update and refurbish this 35 year old amp. I'm sure the one that is in the circuit, with the open resistor, has taken its share of abuse. Also, i read somewhere, on the internet, that changing the value of these caps from 200uuf to .01uf will help increase stability. I guess that remains to be seen.

    I appreciate your posts and any input that you may have. I am certainly no expert and always like to hear opposing viewpoints.

    Thanks
    Dan

  4. #4

    Default

    Quote Originally Posted by KI4AX View Post
    Hi everyone. I want to replace the Grid Caps in my Heathkit SB-200. In the drawing these caps are labeled C14 and C15. The existing caps are Silver Mica 200uuf with voltage unknown; I am not able to determine the voltage from the markings on the caps. Also, the voltage is not listed in the parts list. Does anyone know what the voltage rating is on these two caps? And if the voltage of these caps is unknown does anyone know what the maximum voltage is in the grid circuit.

    Thanks

    Dan KI4AX

     The rating is 500V Dan. The grid-V is much less than this but the caps have to be able to handle the rather substantial current they see on 28MHz. In our parasitic suppressor retrofit kits for 572B amplifiers we replace the 500v-rated caps with 2000v-rated caps. E-mail me ur adr and we will send you some grid bypass caps gratis. We buy 2000 at a time so the price/unit is low. - note - if you are using a grid-I fusing device, C should be increased 2- 3 x to prevent blowing the fusing device on 10m.
    Rich, AG6K, [url]www.somis.org[/url] 805-386-3734

  5. #5

    Default

    I use .01/2000V discs in place of those micas which generally worked OK with USA tubes but the squirrely Chinese tubes need a lot of help.

    Carl
    Amplifier Service and 6M conversions since 1964

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