A brand new cap might be a little better than that, but certainly
right ball park at 0.5 ohms, and would not be an issue at that figure.

IIRC,the values on the DSE meter are MAX values,so IMO,if a cap is reading
what's listed,it's marginal.


"Approx worst case ESR values for new capacitors at 20deg C"

is what it actually says. Bob once told me that when he put that chart on
the front, he took it directly from a manufacturer's data sheet - Pan IIRC

I guess that there are two ways you can take that. I have always taken it on
the positive side, rather than the negative view that you take. As I'm sure
you well know from your own experience, using an ESR meter is a black art
anyway, as much magic as it is science, but given that, in the many years
that I have owned and used daily, a Bob Parker (DSE) meter, I have always
found the chart to be an excellent guide. From many years of experience with
SMPS's, and having found and replaced many pwm chip supply decoupling caps,
I would not feel the immediate need to replace one of that value and voltage
rating, that went 0.5 ohms. I would not consider it 'marginal' and
potentially the source of the problem, until it had got a lot closer to 1
ohm. Typically, caps that have failed in that position, will be up around
the 10 ohm mark, or open circuit.




About the only other thing that you could try, assuming that it uses a
startup resistor from the 170v, is to disconnect the self-feed diode
from the switching transformer. That way, you will force it to run
from the startup supply only, just in case the self-run voltage is
fluctuating, and interfering with your reading on the pwm chip's
supply. Bear in mind though, that you can't run it for too long on the
startup resistor, as it will get quite hot, being normally intended to
supply current to the chip, only for as long as it takes the supply to
fully start up and settle. If the voltage supply to the chip still
jiggles around when it is only being fed by the resistor, then after
the 47uF cap, which may yet be faulty, but not in an ESR way, the next
prime suspect would have to be the chip itself.

Arfa



usually,the startup resistor doesn't supply enough current to keep the IC
running,just enough to start,that's why they have the housekeeping supply
from the XFMR. Otherwise,you get a "burst" or "chirp" mode of
operation;the
PS starts,then dies because the IC isn't getting enough power.
Often,it's the housekeeping supply electrolytic cap that's gone bad,as you
said.

I've also seen where the start resistor goes high in value,even
open,preventing starting.(but not this guy's problem)

--
Jim Yanik


Yes, agreed, but you've got to admit that the supply to the chip fluctuating
between 7 and 15 volts is a pretty bizarre state of affairs, and there's not
actually too much bar the startup R and it's decoupling cap, connected to
that pin. Of course, the self-powering supply, also connected to this point,
might be interfering with the reading, which is why I suggested
disconnecting it. As to whether the supply will run in this condition,
depends on the value of the startup resistor, but running or not, it might
at least give an indication of where next to look, depending on whether the
voltage on the chip side of the resistor is now steady, indicating that the
fluctuating voltage is coming back via the self-powering supply, and in
which case is a red herring, or still varying as a result of the chip
drawing current erratically. I might have felt inclined to think that the
fluctuation was the startup bleed via the startup resistor at 7 volts,
followed by the self-power coming on line at 15 volts as the supply gets
going, followed by the supply shutting back down and so on, due to some
secondary side problem. However, that then doesn't stack up with the
secondary side voltages being steady, but wrong, and worse than wrong,
unbalanced.

It is an odd state of affairs to be sure, but of course, as we all know,
some problems on switchers can be very obscure.

Arfa