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Ralph Mowery

In article ,
says...

On Friday, May 6, 2016 at 3:02:30 PM UTC-4, wrote:
I have a computer that won't turn on unless it is warm enough. The
problem is in the power supply. If I use a blow drier to blow some
warm air through the exhaust holes in the power supply for a minute or
two the power supply starts working. I was told by someone here or on
the basics group that there is probably a bad electrolytic capacitor
causing the problem. I also have a CNC machine that has a servo amp
that acts the same. The machine must be on for a while and when the
cabinet that holds the servo amps gets warm enough inside the amp
start working and will work fine as long as the machine is kept
powered up. If the machine is turned off for an hour or so and then
powered back up the amp still works. But if the machine is off
overnight the amp acts the same as above. I replaced the amp so there
is no problem running the machine but I would like to repair this amp
so I can have a spare on the shelf. Anyway, it has only 3 physically
large electrolytic caps. I unsoldered one lead from each cap and
measured the capacitance with my Extech multimeter. The two 10 uf caps
measured 11. something uf and the 33 uf cap measured 37. something.
Could these caps be bad anyway? There are also three blue colored
dipped caps that have printed on them: .068K 250. I don't what the dot
in the .068K means but I don't think it's a decimal point. Maybe it
means lead free. There are also a couple 47 uf wound polymer caps.
Could either of these types of caps need to be warm before they work
properly? I have no schematic for this amp and have not been able to
get one. Furthermore, the company that makes this servo amp has ground
the markings off of any device with more than three leads.
Thanks,
Eric

Yes, caps that read good value can still be bad. Most electrolytics that are bad will show high ESR readings (bad) before they show low capacitance. ESR meters are cheap enough that anyone who screws with electronics should have one. Despite the advice of others, I always remove the capacitor to test ESR with an "in-circuit" ESR tester.

Also, a good percentage of wonky electrolytics will show signs of venting at the top of the cap, if not an actual electrolyte leakage, then a discernible bulge. If you see any bulged caps, change them.

Most small disc and film caps will not become temperature sensitive. For a device to be warm up sensitive like your device it's usually a weak electrolytic , so concentrate there.

If there's only a few electrolytics, change them all with good quality (I like Panasonic) caps and they'll probably run.

I agree with changing all the electrolytics as there are only a couple
of them. Even though the value may check out ok , there could be other
problems with them such as the ESR value.

I would guess the .068K would be a .068 UF at 250 volts. They seldom go
bag. If only a few of the 47 uf you might as well change them while you
are at it.

While you could have other problems, the heating up seems to indicate
the classic bad capacitors in the power supply.