Reading and replacing mica caps-Solved

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Reading and replacing mica caps-Solved

Post by Ben Delk on Sat Apr 07, 2012 10:44 am

Reading old mica can be very challenging as they get older, the color fades, gunk builds up in the divits and so on. I have a Howard 300 I just opened up. The cord was badly frayed so I started there, as I was removing the old cord I noticed a crack in the seam of the mica connected to the same terminal strip then going to ground. If I moved the mica a little I got a correct reading, moved it a bit the other direction the value doubled. So I have taken the mica out and I'm looking for a replacement. Question is "replace it with what? The schematic shows it to be .02, but that's all it shows. The color code appears to be 3 dot left to right red>black>yellow, with a blue dot in the upper right corner. Are there vintage replacement? If not what is the best replacement type? FYI....this is only my second radio to repair. The first is working like a champ. I was so excited I had to jump on another one q fast as I could.
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Re: Reading and replacing mica caps-Solved

Post by Resistance is Futile on Sat Apr 07, 2012 12:15 pm

.02 MMFD 4th dot = blue =700V
red = 2
black = 0
yellow = 4
-------

volts =Blue = 7= 700v
-----------
200,000 mmf

http://www.cde.com/catalogs/STD-DIPPED.pdf
http://www.vcomp.co.uk/components/cap_codes/cap_codes.htm


Last edited by Resistance is Futile on Sun Apr 08, 2012 4:08 am; edited 1 time in total
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Re: Reading and replacing mica caps-Solved

Post by Ben Delk on Sat Apr 07, 2012 5:01 pm

Ok....uncle....I give......i do not see a 200,000 mmf silver mica or a .02 mmfd mica cap on any vendors website. Sorry but I'm starting to fill like a brick, nothing is sinking in here. What am I missing here?
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Re: Reading and replacing mica caps-Solved

Post by Resistance is Futile on Sun Apr 08, 2012 1:54 am

OK lets try this again. my mistake it was late!
Only lets use an illustration and try to do it correctly Embarassed .


So reading from this chart
not from the bottom but from the top.
Left to right in a clockwise direction
"The color code appears to be 3 dot left to right red>black>yellow, with a blue dot in the upper right corner."

So reading from the top
dot A if blank I guess is considered Black. or no color if no coler I would consider it as 0 if it is the same color or no color as the body then if the body is brown then use the number 1
dot B is blue so it would be 6
disregard the the class color red
on to the next dot C = yellow is 4
dot D is black = 20% tolerance

So black = 0 or if brown 1
blue = 6
yellow = 4 = 0000

060000 now that is for mica and micas are always in picofarads
So either 60,000 pico farads
or 160,000 picofarads
.02 connected to oscillator coil and 30,000 ohm resistor

.00005 connected to grid of 6AB6T and 40,000 ohm resistor and to oscillator coil.

.25 connected to cathode and grid of 6SK7 and to 200? ohm and 40,000 ohm resistor

.05 connected to yellow antenna loop wire and secondary if 1st IF can and 22 megohm to phono switch

.0003 to ground and to secondary of 2nd IF can on the other side of cap

.006 to volume control center tab and 16 megohm resistor

.00005 or .00006 to 500Kohm and grid of 6V6GT

.0003 from 6Q7GT plate to ground

.006 to plate, grid and primary of audio (jumpered to both ends of primary)
---------------------------
so it looks like these are mmfd values so it has to be one of these by value

1-.02
2-.00005
1-.25
1-.05
2-.0003
2-.006

Read this : http://www.tpub.com/neets/book2/3g.htm
However it could be a paper.
I am going to ask bill to step in and help answer this or Anyone else have an answer?
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Re: Reading and replacing mica caps-Solved

Post by Ben Delk on Sun Apr 08, 2012 2:10 pm

Well that clears up a few things. You are a thorough man and I truly appreciate your guidance. At least it confirms I was reading the color code correctly red2>black0>yellowx4=200000mmfd. The link below is an img of the mica cap....maybe this will shed some more light on the matter.

http://www.servimg.com/image_preview.php?i=2&u=17421296][img]http://i42.servimg.com/u/f42/17/42/12/96/th/100_2711.jpg
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Re: Reading and replacing mica caps-Solved

Post by Resistance is Futile on Sun Apr 08, 2012 4:00 pm

Just a note: Alen Douglas on ARF says that sometimes they can be resistors. confused
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Re: Reading and replacing mica caps-Solved

Post by Ben Delk on Sun Apr 08, 2012 6:54 pm

Alen, that did cross my mind but the schematic shows a 0.02 cap symbol going to chassis grd. Thanks anyway.
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Re: Reading and replacing mica caps-Solved

Post by Ben Delk on Mon Apr 09, 2012 8:25 am

Nothing ventured nothing gained. Took the 3 dot mica rd>bk>yl, with blue extra=200000 pico = 0.2micro.
using a .22mf, 10%, 630v. None of the magic smoke has released yet.
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Re: Reading and replacing mica caps-Solved

Post by Ben Delk on Mon Apr 09, 2012 8:55 pm

Looks like the gamble paid off....this time....I used the .22mf cap and a 250 mf resistor that was burned so bad I could not tell the color so I eliminated everything else and decided that had to the be the one burned. Replaced it and music is sounding good. I guess now I'll go ahead and replace the rest of the paper caps and borderline resistors. Trying to decide about the cabinet. It is borderline strip or clean and polish. I'll try rubing it out with some rotten stone, apply some wax and decided from there. Thanks for the help guys.
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Re: Reading and replacing mica caps-Solved

Post by Resistance is Futile on Mon Apr 09, 2012 11:29 pm

I wll mark this topic as solved Smile
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