25 Sep
2014
25 Sep
'14
8 p.m.
Now this varies based upon construction of the resistors, but it's almost
impossible for any typical 2W resistor to fail dead-short - they can drift
up, they can drift down - but a hard-short is just not what we see. And
wirewound parts tend to act like fuses - they simply go red-hot and open up
if they are forced to carry high fault currents.
If the PSU isn't built for rapid over-current shutdown, and the only other
alternative is the 6800uF cap, then I'd put the money on the cap.
Have you checked for build errors, like missing insulators between boards &
standoffs / mounts? The kind where all is well (often for many years) until
the board coating wears-though and creates a short to the standoff / mount
point?
On Thu, Sep 25, 2014 at 6:18 PM, Robert Jarratt <robert.jarratt at ntlworld.com
wrote:
> I am back home and have had another look at the problem board. The
> construction is such that it may prove really hard to get at all the
> components, because some transistors are screwed to a large heatsink plate
> that is on the non-component side of the board. I would have to
> successfully desolder 4 of these to get the plate off to be able to access
> the tracks so I can desolder other components. I suspect I would actually
> have to cut the pins on the transistors to remove this plate.
>
>
>
> From what I can see the possible remaining shortable components appear to
> be 3 parallel 150R 2W resistors and a large square Sprague 88D capacitor
> (value hard to read, perhaps 6800uF, looks a bit like this one:
> http://www.tedss.com/88D682M040BB). It should be possible for me to lift
> one end of the 3 parallel resistors to test them for shorts, but the big
> capacitor is going to be really hard to remove, for the reasons mentioned
> above (ie I can?t reach the tracks on the back of the board to desolder it).
>
>
>
> How likely is it that this capacitor, which I believe is an aluminium
> electrolytic, could have failed short?
>
>
>
> Regards
>
>
>
> Rob
>
>
>
>
>
> From: Jarratt RMA [mailto:robert.jarratt at ntlworld.com]
> Sent: 22 September 2014 23:35
> To: General Discussion: On-Topic and Off-Topic Posts
> Subject: Re: Value of a Tantalum Capacitor
>
>
>
>
>
>
>
> On 22 September 2014 21:43, Tony Duell <ard at p850ug1.demon.co.uk wrote:
>
> >
> > In investigating the fault on my H7874 PSU I desoldered a tantalum
> capacitor
> > which I have broken in the desoldering process. I can't tell its value,
> the
> > markings are 224 E5Z on one side and 038 ASF on the other side.
>
> The value is given by '224' You read that like the resisotr code, that is
> 22*10^4, or 2200000. The 'base unit' for cpaacitors is the picofarad
> (pF), so that's 220000 pF = 220 nF = 0.22 uF.
>
> That sounds low for a tantalum capacitor. I'#ve actually seen tantalum
> capacitors down to 0.1uF, but most are >1uF Are you sure it's tantalum?
>
>
>
> I think I have been convinced it is ceramic. I was fooled by the
> packaging, which looks a bit like what I think tantalum capacitors look
> like. One day I will learn to tell the difference more easily.
>
>
>
> Annoyingly, after desoldering three capacitors, and breaking the one
> mentioned above, I still see an apparent short across the outputs of the
> PSU.
>
>
>
> Thanks
>
>
>
> Rob
>
>