Switchmode Power Supply Trouble - Help appreciated

[Sebi]

That's what i wrote first: the TL431 (=KA431) in the feedback. The multiple output SMPS have only one exact output voltage, others depend from load. ( so the stdby and on voltages can be different) Just follow which output have a line to KA431 and You can exactly adjust only this output voltage (probably this is the 5V).

 

[HandyMan]

Sebi, thanks for the info. I have found that the KA431 is connected to the 5V line, as you thought.

I've now found a couple of things wrong, but won't mention them yet for fear of confusing things even more than they already are. The only thing I will say for now is that I have found the reason for the receiver operating in reverse - power supply on when unit is in standby and power supply off when unit turned on - one of the transistors in the secondary, a big TO-220 package, is completely short-circuit, all three pins short.

I don't know why this is exactly, but have ideas. It may be why all the voltages suddenly became high when they were previously too low, so perhaps not an imperceptible dry-joint after all...

Anyway, this transistor which is short - KSA614-Y - simply marked A614-Y, is to do with the standby operation of the power supply amongst other things, I suppose, as it is connected to a smaller transistor - KSP2222A - which in turn has its base connected to the standby voltage from the mainboard, i.e. power supply turns on when the base has a voltage going to it and goes off when that voltage disappears when the mainboard stops sending it after on/off button has been pressed.

So, I need to replace the KSA614-Y. I don't suppose there is anything that might do on the old chassis I have in the shed from TVs and satellite receivers, etc? Datasheet says it's a low frequency power amplifier, power regulator. I'd like to just test it with anything I can find around that'd do for now until I source a proper replacement, if I can find one.

 

[john1]

KSA614 Y

 

[HandyMan]

Thanks for that. I found a replacement which is working as power supply is back up and running. It looks like low voltages may have just been bad joints. All voltages are good enough.

I didn't think it was working, but when the sat receiver was taken back and connected up, it produced perfect pictures!

So there it is, fully working again.