[time-nuts] FE-5680A - Newbie capacitor question

[Didier Juges]

Tantalum capacitors are known to occasionally short for no good reason.
Give them a reason and they'll be happy to oblige.
Things that can damage a tantalum cap in a hurry include excessive voltage,
reverse polarity, overheating (likely with hand soldering) and current
spikes.
If it were me, and assuming it is at the input of  a linear regulator, I
would put a ceramic multilayer capacitor in its place.
It will likely be much smaller (requiring a short jumper to match the
solder pads of the larger tantalum) but a lot more reliable, and you are
less likely to damage it by hand soldering.
(some linear regulators are unstable with a large ceramic capacitor on the
output without proper damping, but there is typically no issue at the input)

Didier KO4BB

On Sat, May 2, 2020 at 2:59 AM Matthias Welwarsky <time-nuts at welwarsky.de>
wrote:

> On Freitag, 1. Mai 2020 23:42:46 CEST John Bievenour wrote:
> > I'm an amateur electronics hobbyist who's still a newbie (especially to
> > SMD), so please forgive the basic question.
> >
> > I recently acquired a FE-5680A that was failing to lock on frequency
> (pin 3
> > never went low). It will power up, sweep up and down, and finally settle
> to
> > fluctuate around 9.9999999 to 10.0000000.
> >
> > When I opened the case, I found that one of the capacitors had blown,
> which
> > I believe might be the reason. Looking at the top of the board, with the
> > DB9 on your left, the capacitor in question directly across from the
> right
> > hand 2941 voltage regulator. It has 105 on the first row, 35 followed by
> a
> > stylized K on the second row, and 333 on the last row.
> >
> > I'd like to attempt to replace the blown cap. After doing lots of
> searching
> > and reading, I'm pretty sure this is a Kemet 35 VDC 1uF SMD Tantalum
> > Capacitor (never knew these existed until yesterday). I've narrowed down
> > the options on Mouser, but I can't figure out:
> >
> > 1) is my identification correct?
>
> Sounds about right, going by the images I found.
>
> > 2) is it a T489, T491, T494, T495, T496, T498, or T499 (the band at the
> top
> > makes me think it's a T498, T491, T494, T495, T498, or a T499) ? 3) is
> the
> > tolerance 10% or 20% (I think it's 10%)?
>
> > 4) what is the ESR (I have options from 1 Ohm to 83 mOhms)?
>
> I don't think you need to worry about that too much. The capacitor is
> probably
> for filtering on the input of the regulator. For output filtering it's too
> small. The datasheet for the LM2941 I found says at least 22µF is needed
> for
> stable operation and ESR between 100mOhm 1 Ohm. So, the output filter cap
> must
> be somewhere else.
>
> Just make sure the size matches. The band on the top only marks the
> positive
> side of the capacitor.
>
> > 5) for future reference, how am I able to tell the tolerance, ESR, and
> > series just by decoding the writing on it?
>
> Without knowing the exact part number and looking up the values in the
> data
> sheet it's impossible to know these details. I usually compare  the top
> markings against manufacturer data sheets until I find a close match and
> then
> go from there.
>
> >
> > Thank you very much in advance.
> > - John
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