Magic Smoke: 15uf/20v bypass (tantalum?) capacitor

------------------------------ Message: 7 Date: Sat, 16 Mar 2013 16:08:37 -0400 From: Paul Birkel <pbirkel at gmail.com> To: "General Discussion: On-Topic Posts Only" <cctech at classiccmp.org> Subject: Magic Smoke: 15uf/20v bypass (tantalum?) capacitor Message-ID: <CAJj77ru8UqBprno+xXahX09J-ZZbU9DbCH7J3onv6h0HwWPz=w at mail.gmail.com> Content-Type: text/plain; charset=ISO-8859-1 I'm slowly bringing some olde DEC PDP-11 boards back online. In this case a pair of Unibus Plessey PM-1116 B core memories. Fortunately I have really nice documentation for them. Their inhibit drivers are driven from the -15v rail and individually bypassed at that point by 15uF 20v +-20% axial capacitors. The specification is for Sprague 150D156X002082 parts. One board uses these, or a very close facsimile, *electrolytics*. No smoke on power-up. The other board has KEMET axial capacitors that are bullet-shaped and solid plastic. I've never seen anything like them before. Has anyone else? Searching through parts sheets @Mouser it looks like this is a *tantalum *capacitor (but apparently a discontinued line). One of these smoked mightily on power-up, splitting across the middle. (Fortunately this seems like a non-fatal failure as regards the remainder of the circuitry.) I'm unsure whether there's any good reason to replace it with a small electrolytic as used on the first board (and, perhaps, all of the others like it on the board), or whether I should replace *just* it with another KEMET capacitor instead? Since no other KEMET capacitors failed (there are 17 others) I'm thinking that I should replace *just *it, and with a comparable modern KEMET tantalum capacitor; *e.g*., the T110B156M020AT