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 the
same low values on both units.

So I've got my screwdrivers out and a multimeter. Upon taking the backs of
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 two
bad units and the green power indicating LED can be seen to ripple with the
naked eye..

None of the secondary side capacitors are bulging or have brown crud on them
and I'm already aware of the bad caps issue, so I replaced all the secondary
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 pins
of the SMPS board and the primary side from mains input to the transformer.