agree with that ...
search for a small capacitor 10-47µF next to the uc3842 and change it...
regards,

On 2011-04-24 16:44:15 +0200, "Wicaksono S." said:

This is a familiar error on UC384x based SMPS, 0.5 Hz ripple indicate
that the UC3842 is repeatedly goes on & off. There is an small
elecrolyte capacitor at UC3842 power supply pin that maybe went dry,
please replace it.

On Apr 24, 6:26*pm, "Stephen" wrote:
Hi all,

I have three identical Headend UHF channelised amplifiers that all use
identical switch mode power supplies. One is working, the other two are
faulty but appear to have an identical fault as the output voltages are t
he
same low values on both units.

So I've got my screwdrivers out and a multimeter. Upon taking the backs o
f
all three units there is a 8 wire ribbon lead between the SMPS board and
the
main board, comprising of 1 black wire and 7 grey ones.

There is a 8 wire ribbon cable that connects the SMPS to the main board,
which are the wire numbers in the middle of the image.

There are two columns of output voltages as tabulated below:

Probing for grounds and voltages gives me on both units:

Pin number * * * * * *Good unit * * * * * *2 Bad
Units
pin 1 * * * * * * * * * *ground * * * * *
* * * * *ground (this has zero
resistance to the metal case)
pin 2 * * * * * * * * * *ground * * * * *
* * * * *ground (this has zero
resistance to the metal case)
pin 3 * * * * * * * * * *-3.42v * * * * *
* * * * * * *0.4v
pin 4 * * * * * * * * * *0v * * * * * *
* * * * * * * *12.85v
pin 5 * * * * * * * * * *8.73v * * * * *
* * * * * * *0.76v
pin 6 * * * * * * * * * *18.32v * * * * *
* * * * * * *7.4v
pin 7 * * * * * * * * * *0v * * * * * *
* * * * * * * *8.8v
pin 8 * * * * * * * * * *0v * * * * * *
* * * * * * * * * 8.5v

There is approx 0.5Hz *ripple on the secondary voltage outputs of the t
wo
bad units and the green power indicating LED can be seen to ripple with t
he
naked eye..

None of the secondary side capacitors are bulging or have brown crud on t
hem
and I'm already aware of the bad caps issue, so I replaced all the second
ary
side electrolytics with Panasonic Low ESR FM series capacitors. This has
not
made any difference.

I have traced the circuit schematic from the transformer to the output pi
ns
of the SMPS board and the primary side from mains input to the transforme
r.
I don't *know the exact winding configuration of the transformer, and I
have
not yet traced the optoisolator board, but this is based on a 3 chip desi
gn,
TL400, CNY17 and UC3842AD, One is an opto isolator, one appears to be an
error amplifer/feedback chip and the third is a precision voltage referen
ce.

I'd really appreciate some comment amongst any SMPS experts amongst you a
ll
as to where the problem lies so I can get the two faulty units repaired.

The Primary side is available onhttps://picasaweb.google.com/109749938109
115860428/Apr242011?authkey=...
and the numbers on the right refer to the pins on the transformer primary

The secondary side is available onhttps://picasaweb.google.com/lh/photo/A
HQo6tOf_NwmsTV_8owu9IbByfimUi_...
and the numbers on the left hand side refer to the order of connections o
n
the
transformer secondary

The whole of this side of the circuit is isolated from the primary side,
except for the one feedback wire off pins 1&2 which go off to an
optoisolator on a small daugherboard on the SMPS PCB.

Any comment?

Also there is a diode between two of the secondary windings *that would
seem
to short ciruit the winding back out onto itself? I have checked my PCB
tracing and its definately the case.

I'm of the view that the output of pin 4 should be 5 volts due to the
LM7805, so why is the working unit producing 8.73V at this point?

Give the 2.2K series resistor for the LED, and assuming a 10mA current, i
ts
suggesting that the voltage coming out at this point should be 24.2 or is
there something subtle I'm missing? Also there are noi marked values for
any
of the inductors.

Regards,

Stephen.

--
---
Kripton