[mythtv-users] FE replacement recommendations

[Brian Wood]

On Friday 29 May 2009 09:58:27 Calvin Harrigan wrote:
> Alan Marchiori wrote:
> > Well, I swapped out the RAM, no change.  Then I swapped the PSU and
> > low and behold it has been stable ever since.  I have been working
> > with PCs for a long time and have never seen a "bad" PSU where you
> > could still boot the machine (usually it's just dead).  It is a
> > Seasonic PSU with a 5 year warranty but they say if the fan turns on
> > the PSU is not bad (and the fan does turn on).  I guess it could have
> > been a loose connection somewhere.  I could swap it back and try to
> > fiddle with it, but I'm just happy to have my myth box working again.
> >
> > Thanks for your help
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> > mythtv-users at mythtv.org
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>
> I had a similar problem with an antec powersupply intermittent boot up
> random crashes.  Turned out to be bad caps.  Replaced caps, problem
> solved.  Usually I'd just get another PSU but it was a special unit
> designed for the case.  One of the things the tipped me off to bad caps
> was the hissing/whistling sound the unit was making, that usually means
> filtering is sub par aka bad caps.  Had a bunch of 2000uf caps measuring
> in the hundreds.

Be careful how you test capacitors, especially electrolytics. Unless you test 
at the actual working voltage you may not get meaningful results. Leakage 
tests especially, must be done at the actual working voltage, and preferably 
higher (up to the rated max.). New electrolytics need to "form", creating the 
extremely thin chemical layer that serves as the dielectric, and testing a 
new cap at lower than the working voltage will almost always show low 
capacitance, unless the cap has been "pre-formed", something found on 
MIL-spec components, but not usually consumer grade parts.

But a 2000uf cap. that has been in use for a while and measures in the 
hundreds should definitely be replaced. It will probably also show excessive 
leakage, and a bad ESR (equivalent series resistance).

For those not familiar with MIL specs, here's an example:

MIL-TFD-41

(Make It Like - The F***ing Drawing - For Once)

-- 
beww
beww at beww.org