SMPS troubleshooting . .need help! - Page 1
Forum section: electronics / beginners

author: Giorg
author_rank: Contributor
timestamp: January 01, 2012, 08:13:44 pm
content: After spending a couple of hour trying to fix a power supply, I thought that maybe some expert guy here at the EEVblog forum, could help me figuring out how solve my problem.I know that replacing the whole PS will cost lest effort and money, but now it's all about learning and challenge.PS has on the left side from top to bottom : +5 (5A) | gnd | +15(1A) | gnd | -15(1A)on the right side from top to bottom: heart | AC1 | AC2 (220V)That's the story:this power supply comes from an old video sync generator. Everything worked fine until one day ..no power at all.I found blown fuses.. caused by a shorted transistor on the primary side. I've changed both with higher specs MJE13007G.Unit turned on until some seconds later a thyristor on the output side exploded. It was a NEC 2p4m which I've replaced with a X 0402NF after checking for shorts on the output capacitors.After more visual inspection I've found a bulged 220uF bulged cap. I've replaced two caps of this type with new one (light blue in the picture).Now if I try to switch the unit on .. the leds on the main board and on the main switch (connected to the 5v line) lit up for a moment and than turn off.If I disconnect the PS from the main board and I leave only the led connected to the 5v line, it stays lit and it slowly fade after switching off the unit.I've checked the main board using a pc power supply and everything was working as expected even if it had +12/-12 instead of +15/-15.I hope someone could help me .. any tips would be greatly appreciated.!A more scientific method for troubleshooting is neededthank you all!PICS:TOP:http://web.tiscali.it/giorg/eev/DSCN0673.JPGBOTTOM:http://web.tiscali.it/giorg/eev/DSCN0675.JPG

author: Fraser
author_rank: Super Contributor
timestamp: January 01, 2012, 11:12:33 pm
content: Hi,I am no expert on SMPSU's and, like you, have had the odd unit that resisted repairI bought a book on SMPSU's that fully explained the operation of them and demystified the designs.I just did a search and found a link to an electronic copy of the book so you may wish to do the same. Search for:demystifying switching power suppliesordemystifying switching power supplies ebookI found copies on rapidshare but I must advise you that these are likely not published with the publishers permission ! I have the paper copy of this book and it has been most helpful to me.When tackling a faulty SMPSU that is of some age, I always fit a complete set of new low ESR capacitors, not just one or two. The switching transistor is often U/S as you have found so that gets replaced also. That exploding thyristor is a worrythough.Using the above detailed book you should be able to work out how to test the various parts of the SMPSU without risking a repeat BANG! experience. I also fit a new switching controller and any associated driver components for the main switching transistor as they often get fried if the transistor fails short source to gate or collector to base.I have no magic answers for SMPSU repair and find them a pain to work on but others may have better advice and I would also wish to learn more as there is a lovely Tektronix 2215 CRO and a 2710 Spectrum analyser in my lab, both with dead SMPSU's that are resisting repair and eating expensive new componentsThey are on the "too hard for the moment" shelf !Good luckAddition:Have you looked here?http://www.capetronics.com/switching_power_supplies.htm#smpstocor here?http://forum.allaboutcircuits.com/showthread.php?t=7738

author: ipman
author_rank: Regular Contributor
timestamp: January 02, 2012, 10:01:27 am
content: Make sure that you replace caps with LowESR and same rating.Check the rectifier also, the SMPS detect an under or over voltage condition and protects itself. But make sure that you test it under load, at least a light load of some tens or hundreds miliamps.

author: Giorg
author_rank: Contributor
timestamp: January 02, 2012, 02:08:23 pm
content: thank you all for your great suggestions!Especially Aurora for the useful link.since the SCR exploded and his job is probably to protect from over-voltage on the output line, I thought it should be worthy to start from that part of the pcb.I've checked all the diodes and caps and no one was bad. Then I've removed the transistor connected to the heatsink and found it was a C2562Y ..a NPN transistor and it was faulty!To be sure I've also removed the same component on the -15 rail and it was ok.Now I've on my bench some TIP31C transistor.. do you think they will fit as replacement? Is it better to change both of them on the -15/+15 to have a "identical response effect" or the rails are completely independent?Thank you!in green : replaced componentsin red : faulty transistor

author: Giorg
author_rank: Contributor
timestamp: January 02, 2012, 07:47:40 pm
content: Update:installed the tip31C and it's back to life!!I'm really happy.. thank you all for helping!