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Old 24th January 2006, 04:07 AM   #1
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Default stupid question and strange testing method ,da pinch test

Ok , long day , its late and the mind just ain't working right either so bear with me on this . Still having trouble with the amp from long ago and the terible humm / static .Started back looking at it and found a cap , green shapedlike a big chicklet gum square with rounded edges , specs say 225k 100v . pluged up turned on I can take and pinch it with my fingers and the static /hum increases and decreases with the amount of pressure applied . I'm assuming that this may be the cause of al the problems .I have not found an exact match to replace it with , what are my options of caps to replace it with other than the original specs one .
any help greatly appreciated.
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Old 24th January 2006, 04:19 AM   #2
lineup is offline lineup  Sweden
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In older amplifiers, aluminium electrolytic capacitors may need to be replaced.
Like car battery they have a liquid inside.
By time they lose liquid and dry out.
This is why they should often be replaced.

How big is that cap?
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Old 24th January 2006, 04:30 AM   #3
clem_o is offline clem_o  Philippines
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Default Re: stupid question and strange testing method ,da pinch test

Quote:
Originally posted by fortuosity
Started back looking at it and found a cap , green shapedlike a big chicklet gum square with rounded edges , specs say 225k 100v . pluged up turned on I can
That's a 2.2uF 100V unit. Where is it on the amp? From the description it would seem to be a mylar unit, but I've not seen such a high capacitance rating on a mylar.
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Old 6th February 2006, 05:43 PM   #4
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Ok , back from an illness and rather than try to tell what and where here is a pic of what is in question .
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File Type: jpg pic1.jpg (29.3 KB, 92 views)
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Old 6th February 2006, 05:45 PM   #5
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better pic of the actual item.
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File Type: jpg pic2.jpg (15.7 KB, 88 views)
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Old 6th February 2006, 05:53 PM   #6
anatech is offline anatech  Canada
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Hi fortuosity,
It looks like a 0.22uF mylar cap., a 2.2uF I haven't seen that small. Replace it with a poly-anything-you-can-get, 100V or better cap. It does sound like that cap has a fault, manufacturing probably.

A visit to your local electronics suppliers should do the trick, or have a "walk" through a catalogue.

-Chris
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Old 6th February 2006, 05:54 PM   #7
anatech is offline anatech  Canada
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Hi fortuosity,
The second picture solves this. 224 = 0.22 uF.

-Chris
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Old 6th February 2006, 06:15 PM   #8
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Default Re: stupid question and strange testing method ,da pinch test

Quote:
Originally posted by fortuosity
.Started back looking at it and found a cap , green shapedlike a big chicklet gum square with rounded edges , specs say 225k 100v . pluged up turned on I can take and pinch it with my fingers and the static /hum increases and decreases with the amount of pressure applied . I'
I think that you have a cold joint or any other problem in your amp, and the fact that when you take and pinch it with your fingers , the static /hum increases and decreases with the amount of pressure applied , is because maybe this cap is in a high impedance , high gain part of the circuit and your fingers and your body are acting like antennas .

I say so because this greens mylar capacitors are very ragged and I haven't see no one fail to date...( but it can happen... ).
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Old 6th February 2006, 06:22 PM   #9
anatech is offline anatech  Canada
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Hi Tube_Dude,
Quote:
I say so because this greens mylar capacitors are very ragged and I haven't see no one fail to date
I have. I found them the same way, buy squeezing them with my fingers.

-Chris
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Old 6th February 2006, 06:32 PM   #10
jcx is offline jcx  United States
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you may be a big antenna, higher pressure just gives you more contact area and better capacitive coupling

perhaps you can apply pressure witout excess electrical coupling by using pencil erasers, plastic or wood

sometimes touching a good ground on the board with the same hand helps cut coupled electrical noise
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