On 09/23/2015 12:08 PM, Cydrome Leader wrote:
In sci.electronics.repair Phil Hobbs wrote:
On 09/22/2015 06:18 PM, Syd Rumpo wrote:
On 22/09/2015 21:40, John Larkin wrote:
On Tue, 22 Sep 2015 12:56:56 -0700, Jeff Liebermann
wrote:

On Tue, 22 Sep 2015 15:34:51 -0400, bitrex
wrote:

So I'm working on repairing a Korg MS2000B synthesizer for a friend
with
a dead power supply. Here's the service manual:

http://www.loscha.com/scans/Korg_MS2...ice_Manual.pdf

The first thing I notice when looking inside is that the small SMT
100uF
10V tantalum capacitor C109 has completely vacated - it appears to be
gone, blown right off the board. There are some little fragments
rattling around in the case.

I have little experience with tantalum capacitors. Any suggestions for
a more reliable replacement?

I don't think there's a reliability issue here. C109 (located on the
Power / MIDI board schematic, is the initial output filter capacitor
in a 3.3V switching power supply. There are other 100 uf caps
surrounding it, but it seems to be the only tantalum cap, which
suggests that the designer needed some specific characteristic of the
cap to make things work. Methinks a change of capacitor type might
not be such a great idea.

What would make the cap explode? My guess(tm) is that someone plugged
in a wrong voltage power supply, which blew up FET (F1) which then
applied overvoltage to the tantalum. It much have been quite a large
jolt as tantalums using produce a low resistance "short" which then
gets hot and starts smoking toxic fumes. I would look around for
other parts in the power supply section that may be been destroyed,
such as the other caps along the 3.3V rail.

Dry-slug tantalums across power rails are bad news. High dV/dT
literally ignites them; MnO2 is the oxidizer and tantalum is the fuel.

Derate them 3:1 on voltage, or use something else. Polymer aluminums
are good, and some come in a tantalum-like surface mount package.

+1

Cheers

Al polys can make voltage regulators oscillate. Tantalums have a nice
middle-of-the-road ESR that makes 7800s happy.

The ignition problem is quite real--see
http://electrooptical.net/www/sed/TantalumCapReforming_25272-what_a_cap_astrophe.pdf

Can somebody decode "postprocessing fixture" back into English? I have
idea what the author is talking about or what the "fix" really was.


He ramped up the supply slowly with a current limit, to give the tants a
chance to clear the damage.

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC
Optics, Electro-optics, Photonics, Analog Electronics

160 North State Road #203
Briarcliff Manor NY 10510

hobbs at electrooptical dot net
http://electrooptical.net