R23 resistor charred ,400w power supply  - Page 1
Forum section: electronics / repair

author: playit
author_rank: Newbie
timestamp: March 16, 2020, 11:14:20 am
content: hi all,i did receive help from some good friends, and reached till here,just getting all help i can,please guide meplease see the pics ,i have attached the old and the present connections i have soldered,i am new to this,just started learning this as a try repairi connected the traces like in the pic(123_R23_connection),i get 16v at one end and 0.76v at other end of resistor connected to junction..with the psu tester i get only 5vsb ,and no other voltages.. the model is model mercury ckp-3400 gc14244 hope i get some guidance here ,thank you

author: tunk
author_rank: Super Contributor
timestamp: March 16, 2020, 12:05:51 pm
content: It looks similar to the 150-200W low quality PSU in the link below.So unless you do it as an exercise or are low on funds, I wouldn't bother.http://www.jonnyguru.com/blog/2012/12/03/death-of-a-gutless-wonder-iii-the-labors-of-hercules/Edit: If it's an ATX PSU it won't start until you short two pins:https://www.overclockersclub.com/guides/atx_psu_startup/https://www.instructables.com/id/How-to-power-up-an-ATX-Power-Supply-without-a-PC/

author: wraper
author_rank: Unknown
timestamp: March 16, 2020, 12:10:54 pm
content: It's a supper crappy PSU. It's best to get rid of it if hardware it's powering has any value. Because one day it may be gone thanks to this crap.

author: playit
author_rank: Newbie
timestamp: March 16, 2020, 12:41:57 pm
content: i did short it,it dint start,by the way i put it in psu tester which shows all the voltages and runs the fan in good power supply,for this one its showing only 5vsb.yes i am using this for practice,it was lying around for few months.. thought to use it for my case fans.

author: playit
author_rank: Newbie
timestamp: March 16, 2020, 02:18:57 pm
content: what would be the reason why i am not getting 16v or little less from the other end of resistor?

author: TheMG
author_rank: Frequent Contributor
timestamp: March 17, 2020, 12:51:56 am
content: A resistor with current flowing through it will exhibit a voltage drop. I can't really tell what part of the circuit this resistor is in from the pictures but with 0.76V on one end... is it connected to the base of a transistor by any chance? If so, then that reading is likely perfectly normal if this resistor is in fact biasing a transistor.Anyways, properly diagnosing such a power supply requires a good understanding of the topology and functional description of such a power supply. It's actually two power supplies in one. The +5VSB power supply, with its corresponding transformer T3, which is always active when plugged in to AC power, and the main power supply through T1. T2 provides isolation to drive the gates/bases of the main switching transistors, these switching signals would come from the main IC in the bottom right corner of the PCB (looks like possibly a TL494 or equivalent?).When you ground out the green wire, that sends a signal to the main switch IC to be enabled and produce switching waveforms to drive the switching transistors. It also handles voltage regulation and feedback, as well as over-current protection and other protections if applicable (again, depends on what the IC is that's being used).Since you have +5VSB, the next thing to check would be the output of the main switching IC for a switching waveform, using an oscilloscope (CAUTION: unless you have a suitable isolated probe or isolation transformer, DO NOT attempt to connect oscilloscope to any part of the primary side of the power supply under any circumstances!!!). If there is no waveform, the next task is determining why, and that will require a bit of reverse-engineering of the circuit (usually made easier by referencing datasheet for the IC and typical application diagrams).All this being said, when you're done using this power supply as a learning experience, PLEASE put this thing where it belongs: in the bin. The thing that really stands out as far as this being a complete piece of crap is the jumpers in place of LF1 and LF2. This power supply very likely does not meet EMC regulations, at the very least.

author: playit
author_rank: Newbie
timestamp: March 17, 2020, 03:15:49 am
content: thanks for the details,i dont have oscilloscope, the ic chip is wt7520 815 j22ab , could you tell me what points to check from the images i have sent.. ,any voltages to be checked at ic chip?

author: playit
author_rank: Newbie
timestamp: March 17, 2020, 03:20:29 am
content: that jumper goes to driving transformer through transistors from R23 (resistor).. should i change them? may be it would help?

author: playit
author_rank: Newbie
timestamp: March 17, 2020, 03:23:15 am
content: this is the closest schematic to the power supply i have.. was given by a online friend.. hope this might be of some help,this schematic has the same ic chip i have.