17 Mar
2013
17 Mar
'13
12:56 p.m.
------------------------------
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
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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
Tantalum caps are notorious for failing catastrophically on equipment that
is left unused for a couple years. If the power supply has lots of current
capacity, then you get the symptom you experienced. I'd change to
aluminum electrolytics, then you don't have to worry about a repeat
performance in a decade.
If you let the board sit for a few years and then power it on, you will
likely have another failure.
Jon
17 Mar
17 Mar
1:04 p.m.
On 03/17/2013 12:56 PM, Jon Elson wrote:
Tantalum caps are notorious for failing
catastrophically on equipment that
is left unused for a couple years. If the power supply has lots of current
capacity, then you get the symptom you experienced. I'd change to
aluminum electrolytics, then you don't have to worry about a repeat
performance in a decade.
A year or so ago, when I was working on a new design for work, Dan
Roganti noticed that I had designed in some tantalum capacitors in a
switching regulator. He razzed me pretty good about it, and talked me
into investigating some of the newer low-ESR high-capacitance-per-volume
capacitor technologies. I've since been using some really fantastic
aluminum organic polymer capacitors. They're cheaper than the tantalums
(tantala?), they do not contain a liquid electrolyte (which means they
cannot boil and burst, hence no "K" scoring on the top) and their ESR is
almost unbelievably low.
Just FYI, for those replacing tantalums.
-Dave
--
Dave McGuire, AK4HZ
New Kensington, PA
2:07 p.m.
On 03/17/2013 10:04 AM, Dave McGuire wrote:
A year or so ago, when I was working on a new
design for work, Dan
Roganti noticed that I had designed in some tantalum capacitors in a
switching regulator. He razzed me pretty good about it, and talked me
into investigating some of the newer low-ESR high-capacitance-per-volume
capacitor technologies. I've since been using some really fantastic
aluminum organic polymer capacitors. They're cheaper than the tantalums
(tantala?), they do not contain a liquid electrolyte (which means they
cannot boil and burst, hence no "K" scoring on the top) and their ESR is
almost unbelievably low.
Yes, great stuff making its appearance in a lot of new gear. However,
the range of values seems to be quite limited for through-hole (radial
wire leads) devices in comparison to what's available in SMT.
I suppose that's only to be expected.
--Chuck
4:50 p.m.
On 03/17/2013 02:07 PM, Chuck Guzis wrote:
Yes, but many of them are what I call "pseudo-SMT" packages, which are
basically radial-lead metal cans with wires coming out of them in the
normal sense, but they've been flattened out and made horizontal for SMT
soldering. They can be bent back out. :-) It's a bit kludgy, but I can
see situations where it'd work for replacement of through-hole parts.
-Dave
--
Dave McGuire, AK4HZ
New Kensington, PA
A year or
so ago, when I was working on a new design for work, Dan
Roganti noticed that I had designed in some tantalum capacitors in a
switching regulator. He razzed me pretty good about it, and talked me
into investigating some of the newer low-ESR high-capacitance-per-volume
capacitor technologies. I've since been using some really fantastic
aluminum organic polymer capacitors. They're cheaper than the tantalums
(tantala?), they do not contain a liquid electrolyte (which means they
cannot boil and burst, hence no "K" scoring on the top) and their ESR is
almost unbelievably low.
Yes, great stuff making its appearance in a lot of new gear. However,
the range of values seems to be quite limited for through-hole (radial
wire leads) devices in comparison to what's available in SMT.
I suppose that's only to be expected.
4145
days inactive
4145
days old
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