Chinese 12v Power Supply Repair [Solved] - Page 1
Forum section: electronics / repair

author: blackbrayn
author_rank: Contributor
timestamp: January 02, 2019, 02:29:57 pm
content: Hi , i'm trying to repair a 12v power supply that i keept for a long time in a box , i already eliminated the usual easy suspects like the output caps , transistors are not shorted and the main rectifier and caps are ok , i get voltage across them.Digging just a bit deeper , it seems the controler (ka7500) has no voltage at the suplly pins (measured across pin 7 and pin 12).The whole 2 transformer deal it's a bit confusing , what i can tell for sure is that there si no AC voltage across the input of the small transformer responsable for generating the aux voltages.Transistors are "D209L" NPN switching power transistors , not mosfets.I've attached some pictures hopping somebody points me in the right direction or gives me a ideea to continue the investigation. Thanks for your time&atention.ps:i don't have the schematic for the power supply.

author: orbanp
author_rank: Regular Contributor
timestamp: January 02, 2019, 02:46:16 pm
content: It looks like that the KA7500 is equivalent to the TL494 controller chip.The TL494 was widely used and lots of application info is available on those.I am sure that based on this would be easy to trace out the circuit and debug it.Looking a bit further, this one looks similar:http://imajeenyus.com/electronics/20151028_smps_variable_voltage/index.shtmlGood luck, Peter

author: blackbrayn
author_rank: Contributor
timestamp: January 02, 2019, 02:50:25 pm
content: I was looking at exactly the same page!Thx for the hint.

author: Edison
author_rank: Frequent Contributor
timestamp: January 02, 2019, 03:01:27 pm
content: There is no stand-by voltage - check the yellow capacitors next to the supply fuse - The schema from the link is C10

author: blackbrayn
author_rank: Contributor
timestamp: January 02, 2019, 03:34:32 pm
content: The rectifier part is working , i have voltage across the main electrolytic capacitors , the yellow caps near the input are part of the emi filter , they are not the problem , i eliminated them just to be sure , the power supply can work without them.

author: Edison
author_rank: Frequent Contributor
timestamp: January 02, 2019, 03:51:02 pm
content: The capacitor C10 of the schematic is not part of the EMI, it is a the coupling capacitor and is designed to shrink tensions similar as a resistor in a DC circuit. Do you have such a capacitor?Checking Transformer Pins, Voltage Switch, Rectifiers -(both halves work) - All OK?

author: blackbrayn
author_rank: Contributor
timestamp: January 02, 2019, 04:33:40 pm
content: I saw the C10 reference after i checked the emi capacitors and posted the results , i identified the capacitor in question on my power supply , oddly enough it was C10 as well , it had a very high ESR value , so i procedeed to measure all the small caps around that part , in total i found 4 X 4.7uF capacitors and a 47uF one all with high ESR values ranging from 30ohm to 140ohm.Power supply is alive now , thanks a lot for the help , i attached a picture with the capacitors i replaced , maybe it will help others.

author: blackbrayn
author_rank: Contributor
timestamp: January 02, 2019, 07:32:23 pm
content: Attached the referenced schematic from the above link.

author: Edison
author_rank: Frequent Contributor
timestamp: January 03, 2019, 04:13:25 pm
content: Quote from: blackbrayn on January 02, 2019, 02:29:57 pm

author: blackbrayn
author_rank: Contributor
timestamp: January 05, 2019, 05:57:07 pm
content: Quote from: Edison on January 03, 2019, 04:13:25 pmAgain , thanks for the help and pointing me in the right direction.Sent from my ANE-LX1 using Tapatalk

author: Edison
author_rank: Frequent Contributor
timestamp: January 07, 2019, 03:21:21 pm
content: You're welcome