On 2011-04-24 13:26:49 +0200, "Stephen" said:

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.
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 design,
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 reference.

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

The Primary side is available on
https://picasaweb.google.com/1097499...7012 71247186
and

the numbers on the right refer to the pins on the transformer primary

The secondary side is available on
https://picasaweb.google.com/lh/phot...t=d irectlink
and

the numbers on the left hand side refer to the order of connections on
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, its
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.

you can make a quick esr analog meter to check all your caps
sure the small chemical capacitor next to the uc3842 is dry ...
my web site contains easy and more complex esr meter to build yourself
regards,

http://kripton2035.free.fr/esr-repository.html

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Kripton