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Saving Computer SMPS Part-11

By Jestine Yong on January 4, 2025

Case (1) Foxin F500S : This was brought to me in dead condition in August, 2024. Surprisingly this was bought in the mid of 2023! That means it did not last even for an year! On opening I found that capacitors had burst indicating a major power surge.

The condition of solder was so badly flimsy that I could just pull out the defective components without any need for desolder!

On checking the tank capacitors, I was not surprised to get following readings:

The 330uF/200V caps were showing 183 & 195uF! You can imagine the compromise that is going on for the sake of making the product competitive. I replaced all the electrolytic capacitors on the board and did a thorough retouch of the board to give it a fresh life. I did this only after doing the ring test on the primary and looking for any defective components either in the primary or secondary. Now just see how the board looks:

After checking for any man-made shorts on the board by closely inspecting it under a magnifying lens, and making sure that everything was alright, I applied power with my ATX Power Supply tester, the FAN and a 12V/25W bulb loaded. I was glad to see that it was working perfectly. I allowed it to run for sufficient time before fixing the covers.

Case (2) iBall LPE223-400 : This SMPS was attended to by me before in November, 2023 and I had submitted a report of it, which can be seen in the following link: https://jestineyong.com/saving-computer-smps-part-8/

This time (in August, 2024) this was brought to me with the complaint that it was getting on/off. After opening and doing a preliminary cleaning of inside, I looked for any dry solder or bulgy capacitor. Nothing was found. When the SMPS tester was connected all LEDs glowed and then it went off with only the 5V remaining lit. Now, as we all know, getting on/off indicated that the SMPS tries to start but shuts off due to some defect. If the SMPS starts with all outputs coming, in all probability the primary section was ok. Something in the secondary section was amiss!

Now, who was the culprit or who were all the culprits? This is where our interesting investigation begins! I checked the primary switching voltage and was surprised to see 13.7V in place of 5V!!!! I immediately desoldered the PWM control IC and placed an IC socket there. On a preliminary inspection of the feedback circuit in the primary and secondary side, I could not get any defective component. I decided to check the TL431 using a tester made on a breadboard as per circuit given in https://www.eleccircuit.com/experiment-tl431-circuit-shunt-regulator/

Then tested the TL431 applying 12V to the circuit:

I found the TL431 working very well on turning the pot by switching off/on. Then combed the area with both of my analogue and digital multimeters and could not find any diode short or resistors of high value.

As the trouble shooting progressed and by the time I got rather exhausted in doing it, I had already replaced the 4.3V Zener, a few fixed caps and a couple of resistors in the primary feedback section. In sheer desperation, I replaced even the 817 opto and TL431 though tested ok. The fixed caps indeed showed slightly different values and that is why I replaced them. Because, any change of wave form would also affect the output. I even removed the transistor there for checking. Then kept the pins 3 & 4 of opto coupler lifted, removed the switching transistors mounted on the heat sink and used a three wire SMPS module to check whether the output was coming alright. It was correctly showing the output. (I have already explained about use of this module in several of my previous articles and hence skipping!)

Then my techie friend suggested removing the resistors one by one and checking, which I did in the secondary feedback circuit and found the culprit, which was a 1.5K resistor showing value as 6.6K resistor:

Thanking my friend for his timely advice, I replaced this resistor. Then the question would be how did I miss to notice the change of resistance when I combed the area again and again. By a sheer coincidence, it almost matched the resistance when checked on the board, perhaps through some interconnected path. Now you see my friend’s experience and expert suggestion really helped! The stupidity was that I did not apply this knowledge, though I was fully aware of such a possibility from experiece! Then when I applied the power, I got exact 5V instead of the 13.7V.

Then, switched off the power, waiting for all caps to get discharged and with reinvigorated enthusiasm, I inserted the control IC and applied power to see that it the SMPS was working very well. I just lubricated the fan as it was found to be a bit sluggish.

Was I glad to take a sigh of relief?

Well, I cannot be blamed! I fixed the board back and the put the covers putting a seal on this stupidly made complicated case!

Double missions accomplished with satisfaction wet with perspiration getting collected!

This article was prepared for you by Parasuraman Subramanian from India. He is 74 years old and has more than 30 years’ experience in handling antique equipment like Valve Radio, Amps, Reel Tape Recorders and currently studying latest tech-classes conducted by Kerala State Electronics Technicians’ Association. He has done graduation in BBA degree, private diploma in Radio Engineering and retired as MD of a USA company. Presently working as Consultant to Hospital and other institutions.

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You may check on his previous article on Modification Of DB Meter In Technics Receiver Amplifier Model SA380S

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14 Comments

Yogesh Panchal

January 4, 2025 at 4:15 pm

Good Fix! Sir,
Most of the time we think more theoretically rather than testing components one by one in small circuits, because we assume that the surrounding components are fine as it shows no physical damage.

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- Parasuraman S
  
  January 4, 2025 at 7:33 pm
  
  Yes, true! We develop a tendency of complacent attitude because of weariness and over-confidence! Thanks, Yogesh Bai for your valuable comments!
  
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Waleed Rishmawi

January 4, 2025 at 5:31 pm

ah,the job of replacing capactiors and the joy of seeing the device comes to life is very satisfying..lol. good job my friend on both devices repaired although sometimes when we grow older (LOL) we loose our concentration and be corrected by someone els but that is ok because we are humans and we learn and adapt every day to the new situations..have a blessed day my friend and thanks for sharing.

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- Parasuraman S
  
  January 4, 2025 at 7:34 pm
  
  Yes, dear friend! Very practical! Thanks for your wonderful comments!
  
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Albert van Bemmelen

January 4, 2025 at 8:00 pm

Another very interesting study on fixing those difficult defective smps unit cases. This time one that also made you replace some good parts invain which I take it you kept for future repairs afterwards.

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- Parasuraman S
  
  January 5, 2025 at 10:01 am
  
  Yes, you are right Albert! Some efforts do go in vain a few times in our quest to find the root causes! It is a byproduct! Many thanks for your expert comments!
  
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Charles

January 4, 2025 at 8:08 pm

I had missed this blog because of time the last 2 years and indeed was at a great loss. I am glad i just received this technically beneficial easy to follow article well explained.

Happy New Year sir I happy to see you still active on this blog you are adding a lot value to technical people.

Stay blessed

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PHILIP

January 5, 2025 at 12:01 am

Thank you, Engineer Parasuraman. Its wrong to work while you feel really tired. Fatigue brings about lack of efficiency and forgetfulness, and momentarily your memory may go blank and that may leave one in a staring mood, which may also make someone to give a completely unrelated answer to a question asked. Its not because of st+++++ty. Exhaustion may well cause the kind of desparation that led you to replace the 817 opto and TL431 though they had passed the test. One Yogesh oftenly warns us to stop working until another day when the going is tough.

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- Parasuraman S
  
  January 5, 2025 at 10:03 am
  
  Very analyticial and practical comments! I do practise putting off and taking up later. Many thanks, dear Philip!
  
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Imoudu.O

January 5, 2025 at 2:51 am

Sometimes human brain tends to diminish in functionality has it advance in age whether we accept it or not and it can't be calibrated as we prefer it to be.This i have experience a few times.Well the most important thing is you got the two smps back to function.Good job sir.

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- Parasuraman S
  
  January 5, 2025 at 10:04 am
  
  Many thanks for your wonderful comments, dear Imoudu!
  
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Mark J

January 5, 2025 at 4:37 am

Parasuraman great job well done.

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- Parasuraman S
  
  January 5, 2025 at 10:05 am
  
  Many thanks, dear Mark!
  
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Hollis LuQuette

January 6, 2025 at 9:23 am

Good job ..... For some reason that bluish green resistor looks like a choke / inductor...

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