Hello
I am trying to repair a power supply that came out of a piece of equipment used to sterilize hospital surgical instruments. The initial problem was that relays on I/O boards fed by this power supply were cycling open/close non-stop. After cycling the sterilizer's main power on/off (120V AC) a couple of times the power supply's fuse blew. We then isolated the power supply from the I/O boards that it feeds and then turned the power on - the fuse blew a second time. Finally, we removed the power supply from the sterilizer and set it on the bench, looked over the board carefully for signs of damaged components (none found), and applied the line voltage again - the fuse blew a third time and an AMETHERM SL12 10004 NTC THERMISTOR broke in half. Replaced the thermistor and looked for other issues. Found that the THD215HI HIGH VOLTAGE FAST-SWITCHING NPN POWER TRANSISTOR was shorted. Looked online for a replacement and chose an ST1510FX high voltage fast-switching NPN power transistor whose ratings seemed to be very similar. After reading about repairing power supplies online I decided to replace the 4 200V 100uF caps that are grouped together and 2 of the 25V 2200uF caps with 50V versions simply because the new replacement power supply had done the same. Maybe I should've replaced all the caps? At this point I decided to power up the power supply, still sitting on the bench, still no load connected. A brief power on (5 seconds) and the fuse held. Connected my meter to the output looking to see if there was a DC voltage and powered it on again - no output voltage showed on the meter and the fuse still held - turned off the power again. Checked my meter connections and tried it again - this time the fuse blew and sure enough the ST1510FX NPN was shorted across it's leads. With the ST1510FX removed I powered on the PS again but this time I used a variac, started the voltage at 80V, and gradually brought it up to 120V (I did this after reading that some power supplies don't like full supply power instantly - probably adding an extra variable at a time when I shouldn't). The fuse held but I'm not sure where to go from here. I'm a little hesitant to attach another ST1510FX until I have a better understanding of what is going on. Probably didn't do a great job of troubleshooting this power supply from the outset which is why I'm here looking for some guidance. Any help would be appreciated. Thank-you.

 

Photo of circuit board:

 

What you have there is a switchmode psu, it probably is an in house design and will use a chip to provide the pwm signal, which is on the pcb somewhere, better close up photos of both sides please..

 

What you have there is a switchmode psu

And pretty much impossible to repair without a schematic and experience repairing them. The problem is that when they go wrong there will be a cascade of faulty parts and if you don't find them all then it will go bang again.

 

What you have there is a switchmode psu, it probably is an in house design and will use a chip to provide the pwm signal, which is on the pcb somewhere, better close up photos of both sides please..

 

 

 

 

 

 

It is a virtual crap shoot trouble shooting a P.S. such as that, you may find some of the power semi's check defective and could be lucky in replacing them, In trouble shooting SMPS I have found al kinds of different causes of problems, the hardest being is a shorted turn in a small switching transformer, which is virtually impossible to detect and trying to get a replacement, also impossible.
Max.

 

And snubber capacitors that break down at voltage but are OK on a tester.

 

What about the track side, i cant see an pwm chips there, bet there is a smd underneath!!

 

What about the track side, i cant see an pwm chips there, bet there is a smd underneath!!

Nope

 

Other notable components:

SCRs - TYN225, P0102
Optocoupler - CNY17
Rectifier diode (connected to the other big heat sink) - BYV79E
Transistors - (3) 2222A, (2) 2907A,
Shunt Regulator - TL431

 

Must be a self oscillating transistor one, using the St1510fx with Tl431 zener regulator and preset on the opto-coupler, don't know what the thyristor is for, unless its being used for a crowbar regulator on another output, there are two presets,,, Wierd!?

 

OK, looks like I'll throw the towel in on this one. I have another PS that isn't working properly but is a less sophisticated design. I'll take what I've learnt and try my luck with it.

Thanks again for the feedback.