View Full Version : Help to identify an unknown component (at least for me)

March 8th, 2014, 02:03 PM
It can be found in the guts of a IBM JX:


Is it an EMI filter capacitor?, and the meaning of the code is 270pF/+-20%?

March 9th, 2014, 05:22 AM
IBM loved to use the 3-legged caps... I'm 99.9999% sure its one. Not sure how to read the code.

March 9th, 2014, 07:37 AM
Yes, those would be 270pF caps. You could replace them with similarly rated two legged caps as long as you made sure to get the polarization right. The outer solder pads would probably be connected together on the motherboard, otherwise use whichever one is hooked up. IBM used capacitors like these to avoid assembly problems where they would put polarized components in backwards.

Those probably are there for EMI/RFI type purposes, and replacements would need to have similar EMI/RFI properties.

March 9th, 2014, 09:07 AM
Given that these are ceramic caps and 270pF, polarity is no matter. Just insert one so one leg is connected to the center pin. Probably less of an EMI filter than an anti-ringing measure.

March 9th, 2014, 11:27 AM
They might need to be 3-lead if the board passes current through the capacitor. I have a pile of these I got in bag of assorted capacitors many years ago. Always wondered what they were for. I found this interesting page with more info.


March 9th, 2014, 11:48 AM
If that's the case, then a simple jumper joining the outer legs. Best check with a continuity tester. It's difficult to believe that the inductance of what amounts to a half-inch of wire would matter in this case.

I note that IBM was fond of 3-terminal caps on the early PCs, even electrolytics.

March 10th, 2014, 12:54 AM
Thanks for the responses!, indeed it seems to be a pair of inductors and a capacitor in "T" shape. Once cutted, I've wired outer legs and seems to work without problems (it's tied to the CGA's red color video component, don't see the reason to filter a TTL signal).

Basically, I was trying to fix a faulty video output, the JX seemed to lack the red component. At first I believed that one of these filters was the culprit; but after extracting it, there was no change at all :-P. The real problem laid in a partially blowed 74LS240. Now is working again.

As Murata is still selling these kind of filters, I'll try to get a replacement...