[sdiy] "re-cap" when it's SMT?

Pete Hartman pete.hartman at gmail.com
Sun Sep 29 19:50:59 CEST 2013


Further examination of the area shows that the corrosion is more
widespread, but less directly associated with a specific cap.

All over the board, along various traces, there are little copper
circles.  Across most of the board, these are nice and clean.  All of
the ones in the immediate vicinity of the D/A chip (roughly 2" square)
are corroded.  Additionally, one of the SMT ceramic caps I overlooked
before has a lot of corrosion and junk on it--I only spotted it after
trying to take a macro picture of C175.

Here's the picture in question with the original concern circled, but
you can see lots of other corrosion at this scale.
http://elmegil.dynathome.net/~elmegil/XP-50-C175.jpg

Thanks...

Pete


On Sun, Sep 29, 2013 at 11:13 AM, Pete Hartman <pete.hartman at gmail.com> wrote:
> So what would cause digital crosstalk, in only one channel, for a
> synth that presumably didn't have this problem when it was new?
>
> The hint appears to be lost on me ;-)
>
> The only thing that occurs is tracing the path and seeing where it
> comes nearest to digital lines that the other channel does not...?
>
> I think I can step away from the idea of "re-capping" the synth; I've
> heard other people talk about doing such things, and I did fix an old
> Roland S-10 which had NO output by replacing a handful of likely
> seeming caps, so that's where I was coming from with that.  Clearly
> the weight of experience is that doing so willy nilly is a bad idea
> (and my own experience with other hardware at a larger scale would
> argue the same), and I'm not married to the idea.
>
> I appreciate everyone's suggestions, and I'll keep the list posted as
> to my progress; of course it will be a bit before I can proceed on, as
> I have no 10uF SMT caps handy, and I'm very doubtful I'll find them at
> Radio Shack.
>
>
>
>
> On Sun, Sep 29, 2013 at 8:21 AM, Magnus Danielson
> <magnus at rubidium.dyndns.org> wrote:
>> On 09/29/2013 04:36 AM, Pete Hartman wrote:
>>> To follow up on this....
>>>
>>> I did audio probes, and found:
>>>
>>> Only the left channel has the problem.
>>> The D/A chip (IC19, UPD63200GS-E2) left channel out (pin 11) has no
>>> sign of hiss.
>>> IC175, a 10uF SMT electrolytic--appears to be for blocking DC--has a
>>> bit of corrosion on the far side, and there is hiss there.
>>>
>>> First guess is then replace IC175 (we'll see how well that works for
>>> me; I'm not much good with wire trap connectors and there are LOTS of
>>> cables all around this section of the board).
>>>
>>> I'm curious if any of the more experienced members have an opinion on that.
>> Are you sure that it's a cap problem?
>> I would look at cross-talk from digital lines. Caps can help mitigate
>> that, but you need to look at it from a signal integrity way rather than
>> just audio by itself.
>>
>> That only one channel has the problem is a great hint.
>>
>> Cheers,
>> Magnus
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