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
The secondary side is available on
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.