Saving Computer SMPS Part – 14
Case (1) Zebronics ZEB450W :
This was brought dead. Internal cleaning was already done. I saw a fixed capacitor had burst in the primary section:
Since the SMD tester was flip flopping 0 and some other value when checking, I had to continuously click to capture the image that you see above.
Then I removed the tank capacitors and checked and look at what I found, which was not a surprise:
The 330uF was reading 220.6! Then I went ahead and replaced all the electrolytic capacitors along with the 105 Fixed Cap, as this SMPS was being opened for service for the first time. Then did a thorough dry solder patch up, upon which the board looked shining new:
Then cleaned the board with IPA and looked for any man made shorts and could not find any. I patted myself for having done a good job! (LOL) Then applied power giving a 12V 25W bulb as load and checker as usual through a series bulb. I smelt something was burning.
When I checked the 5V, it was showing 21V! So, disconnected the power. Saw two resistors burnt. One in the primary section and another in the secondary. The values were beyond recognition! One new 1000uF/10V had fumed in the secondary section.
While I could easily replace the cap, how to know the values of the burnt resistors. I tried using the method suggested by Jestine Yong in his book, but could not succeed. So, I looked through the collection of discarded Boards and found that these were 100 ohms resistors. So, I replaced both, upon which the voltage was steady at 5V. Then allowed the
SMPS in on condition with the fan connected for sufficient time, before closing the covers.
Case (2) Delta DPS250AB-22E :
This had undergone a service before in October, 2021. Kindly have a look at this link:
https://jestineyong.com/two-smps-saved-from-dump/
I had replaced all the electrolytic caps and did a thorough retouching at that time. This developed sickness and got admitted to my electronics hospital in December, 2024. This is one of the best quality SMPS with a unique side fitting. If we remove the cover, we can do the trouble shooting without taking out the board, like any Amplifier Board. The circuit also was very complicated with mostly SMD components.
On close observation I saw one 100uF Cap bulgy, which I replaced. As it was in the path for the SMPS fan drive, I studied the circuit and was amazed to see that it had a thermistor fit on the heat sink and sensed by two SMD transistors and one through hole. Then there was something that made me aghast! The collector and emitter of the fan driver transistor were shorted by overlapping solder! This was overlooked by me but the SMPS worked for more than three years in this way. Look at the transistor, which had suffered the short but withstood this bravely:
I removed the short and checked the transistors, all of which were in perfect condition. The fan had gone bad as it was stuck and though I removed it and lubricated it very well using silicone grease, it was wobbling so replaced it:
After checking for any more defective components, I did the same thing like the first case and applied power.
I was glad to see that the SMPS was working fine again and I allowed it to run for sufficient time before closing the covers.
Double missions accomplished with satisfactions pouring into the collection bag, which readjusted to accommodate the
new entrants! (LOL)
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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Waleed Rishmawi
August 9, 2025 at 5:35 pm
A surgery well done by surgeon Parasuraman..LOL. Love the expression “admitted to my electronic hospital… we are doctors but in a different way.. have a blessed day my friend.
Parasuraman S
August 12, 2025 at 9:15 pm
Many thanks, dear friend!
Maurizio C.
August 9, 2025 at 6:15 pm
Dear Parasuraman,
nice work, thanks. But shouldn't it be titled Part 15, instead 14?
Best regards
Parasuraman S
August 12, 2025 at 9:16 pm
Many thanks!
Albert van Bemmelen
August 9, 2025 at 8:51 pm
Although only the base emitter diode inside the bridged over collector-emitter Fan driver transistor could have been damaged, I somehow hardly think that the collector emitter could have been damaged in any way? Since there was no current going through the bridged over collector-emitter at all and made the Fan powered on at maximum speed all the time.
The number of smps units you already saved and gave a second life must be enormous by now?
Parasuraman S
August 12, 2025 at 9:17 pm
Many thanks for your expert comments and inputs, dear Albert! Yes, the SMPS servicing has reached records! Thanks to Jestine Yong for his book!
Andy Shiekh
August 9, 2025 at 10:28 pm
21V on the 5V line is scary; not sure I would trust that supply.
Parasuraman S
August 12, 2025 at 9:18 pm
Yes, surely! Many thanks!
Mark J
August 10, 2025 at 1:48 am
Parasuraman well done. I always enjoy your SMPS repair articles.
Parasuraman S
August 12, 2025 at 9:18 pm
Many thanks, dear Mark!
Yogesh Panchal
August 10, 2025 at 11:47 pm
God job! Sir,
As much fun as it was to do TV repair fault finding, it is equally fun to do SMPS repair.
Parasuraman S
August 12, 2025 at 9:19 pm
Yes, it is indeed! Many thanks, Yogesh Bai!