25 Sep
2014
25 Sep
'14
4:18 p.m.
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