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My dad's poor old Philips PM3070 scope died in a sizzle and a puff of smoke a few days back, after being sat in the garage for a year or so. He donated it to me and it seemed to work at first, but unfortunately it didn't last more than a couple of hours.

Whilst I was taking a photo of my new toy, it started crackling and began exuding some pretty acrid smelling bright-white smoke. The screen remained on whilst this was happening, though I very quickly unplugged it and didn't get to test any functionality. I'm hoping that this indicates a non-fatal fault; it would be a shame to have to commit it to the great electronic beyond.

I've now got it at home and have taken it apart, and pretty quickly discovered the culprit:

Cap Pic 01 Cap Pic 02 Cap Pic 03

Well, no wonder it made such an awful stench! The cap's a bit dead it seems. Since the scale is not immediately obvious: I'd say it's about 22mm across, by 17mm high, by 12mm deep. The traces leading to it are about 2mm wide.

I've got a service manual that shows the circuit diagram as follows. I circled the failed capacitor in red.

PSU Circuit Diagram

(excerpt of circuit used under assumption of fair use allowance)

For clarity, X6014 is a standard Molex connector that brings the 240V mains in from the external plug. S6001 is the main power switch. I don't know a huge amount about power supply design, but I'm going to guess that this is some kind of filter or choke.

I'm looking for a replacement, but don't really know what to look for. I can't tell whether this is a standard cap, or whether it has some special properties specific to its role. I also don't know what current rating I need to look for. Any ideas on what I should be looking for, and how I should come to such a decision?

For example, is this an appropriate replacement? If not, which properties are incorrect?

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    \$\begingroup\$ What you should look for is another X2 rated capacitor of the same value. \$\endgroup\$ – Kaz Dec 30 '13 at 17:36
  • \$\begingroup\$ I'm not sure what X2 rated means. Is it a specification of current limit, resistance, tolerance, reactance, or something else? \$\endgroup\$ – Polynomial Dec 30 '13 at 17:44
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X and Y capacitors are designed especially for EMI filtering in AC power supplies.

Due to the safety issues they have to comply to some requirements from safety agencies

X capacitors are used between two lines ("across lines") and Y capacitors are used between a line and the ground/earth ("line to ground").

Here is a simple document that explain briefly : http://www.illinoiscapacitor.com/pdf/Papers/EMI_RFI_suppression_capacitors.pdf

This capacitor seems to be the same as the one which died : http://uk.farnell.com/evox-rifa/pme271m622kr30/capacitor-class-x2-0-22uf/dp/9717480 (Rifa seems to have been bought by Kemet, so don't freak when looking at the datasheet)

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  • \$\begingroup\$ Thanks for the answer. Scope is now working with the cap removed (just to hold me over for the week) for now. I have a couple of replacements coming from CPC - two identical to the failed cap, and two very similar caps that are more easily sourced. Always pays to have spares :) \$\endgroup\$ – Polynomial Dec 31 '13 at 11:24

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