On 06/02/2015 19:13, Michael Black wrote:
On Fri, 6 Feb 2015, wrote:

On Friday, February 6, 2015 at 3:59:18 AM UTC-8, N_Cook wrote:
On 06/02/2015 09:37, wrote:

This Zenith DVD2381 has worked for years, except for a spell a few
years ago when the tray would shudder but not quite open and close.
I managed to free it up.

...
try googling model number and "badcaps"

Thanks for your response!

Nothing leapt out at me. The electrolytics in and around the power supply
also look pristine. They are marked "KME" and "SMS", so likely Chemi-Con.

But looks can be deceiving.

That Acer LCD monitor I brought home sort of worked, except it kept
resetting to the point where it was showing the logo on the screen. So
the power supply was nominally good. I changed the larger value
electrolytics in the power supply, none of them looked bad. And it's
worked fine since.

Do Chemi-Con electrolytics have a bad reputation? They are not called
out as bad on the badcaps site.


A little bit of information is worse than a lot.

There were instances of "bad caps" but electrolytics by nature can dry
out. It's about how they are used, and the heat they see.

In the tube days, most equipment had only a handful of electrolytics, in
the power supply and to bypass the cathode resistor(s) in the audio
amplifier chain. You could usually easily tell there was a bad
electrolytic, you'd hear hum in the speaker (because the power supply
filter capacitor went bad), or maybe lesser audio output.

Transistorized gear brought a lot more electrolytics, since tubes were
high voltage low current, and transistors were love voltage high
current. They needed electrolytics because the lower impedances meant a
need for larger valuce capacitors, and the only way to get that in a
small package was electrolytic capacitors.

And take transisfor radios from 40 or 50 years ago, and the
electrolytics generally need replacing because they are now old.

But the real change was when switching supplies came along. Power supply
capacitors before that only had to keep out 60 or 120Hz, pretty low
frequencies. Switching supplies got by with lower value electrolytics,
since they operated at a higher frequency, tens of KHz, but that meant
the electrolytics had to work harder. And that's why they are now
suspect when something with a switching supply goes bad. They are more
likely to go bad than a diode or other component.

Michael


In the old days you would never see 120 deg C rated caps, even with all
those glowing valves the caps did not get that hot. Combined with
smaller physical size for the same rating. Has QC of elecrolyte and
materials really got that good to allow for half size caps , compared to
sizws before?