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Help with capacitor value.


goingdownslow
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I have hopefully tracked a crackling/noise fault in my Hartke Kickback combo to a capacitor, when I touch/tap it with a plastic rod it gives me the fault. I have reflowed the solder joint but it made no difference, so I will replace it.
What is puzzling me is on the circuit diagram the capacitor, C102, has 100p, same as C103 but the components are marked different, C102(the one to be replaced) has 1uF and C103 has 47uF.
Am I right thinking I need to get a 1uF 50vdc non polarised capacitor?
What does 100p mean?

Capacitor 1.JPG

Capacitor 2.jpg

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It's difficult to see from the photo but I doubt that the replacement cap needs to be a non polarised electrolytic. If you can see a polarity marking (+) or (-)adjacent to either leg this would confirm that it is just a standard electrolytic type. If there are no polarity markings then it could well be a non-polarised version, however I think that NP just possibly refers to the manufacturer.

The schematic arrangement looks about right, ie C103 / C103 being set at around 100pf. 100P means 100 pico farad where as the cap you have taken out is 1 uF or micro farad. C102 / C103 along with R104 and VR101 will affect the negative feedback loop on the ic. They will set the gain and frequency response of the respective amplifier sections. The values indicated in the schematic look typical to me.

I frequently come across discrepancies between schematic diagrams and silk screening on pc boards so I wouldn't let that worry you. You've clearly identified that the electrolytic cap has an intermittent problem so just replace it with like for like and hopefully that should resolve things.

 

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The difference between 1uF and 100pF is...x10000...ie 4 decades so something is 'out' there.

Not going to do the maths now but 100pF would be typical in that position - op amp stability and hf bandwidth limiting.

Are you certain about the Silk Screen reference to component matching - it can sometimes get congested due to physical spacing.

imo 'NP' does likely indicate non-polarised. Look at CPC if you want to replace as is.

But tbh a 50V rated cap will be okay with any reverse voltage it is likely to see in this application - a few volts ?

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That new schematic makes a lot more sense in relation to C102 which is a non-polarised capacitor, so I stand corrected. 

Readily available on Ebay so should pose too much of a problem to replace.

Good luck with the fix.

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