OK, I'll get some 22uF 50V 105 degree C ones ordered in then Do they have to
be low ESR? if so I'' go for the Panasonic FM series that I used to replace
the secondary output electrolytics.

If a capacitor has gone dry, wouldn't there be a clue like bulging tops,
leaking brown crud, and a horrible smell when they get hot?

Regards,

Stephen

wrote in message
...
Yes. The small (100µF) electrolytics on the primary side of the
power supply are prime suspects for a problem like this. As a matter
of course, I replace them all. At $.30 each, it's cheap insurance.

PlainBill

On Sun, 24 Apr 2011 16:44:02 +0100, "Stephen"
wrote:

UPDATE: I have re-measured thge output voltages on the good working unit.

So here are the correct values:

Probing for grounds and voltages gives me on both units:

Pin number Good unit 2 Bad Units
pin 1 ground ground pin 2
ground ground
pin 3 -3.42v -0.4v
pin 4 30.2 12.85v
pin 5 8.73v 0.76v
pin 6 18.32v 7.4v
pin 7 23.7v 8.8v
pin 8 23.7v 8.5v

Also pins 7 & 8 are soldered together at the PSU end, pins 1 & 2 are
soldered toegether at both the PSU and the main board.

The UC3842AD, TL400 and the CNY17 are on a small daughterboard, which also
has a load of surface mount tantalum capacitors and resistors.

Asfar as electroyltics go, there is one on the main board, which is a
capxon
22uF 50V job. is this likely to be the capacitor thats gone dry?

"Stephen" wrote in message
news:rtGdnTSRqdpvlSnQnZ2dnUVZ8tqdnZ2d@brightview .co.uk...
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.