Restored A Service Declined Board Of White-Westinghouse Humidifier Model WDE401
This belonged to the hospital where I work. I chanced to hear from our Biomedical Technician about power supply board failure in this Humidifier used in the CT Scanner Room. He reported that it was lying defective for weeks as the power supply board was defective and neither the company who supplied this to us along with the Scanner, nor a few freelancers and authorized service representatives did anything. They took the board, kept it with them and returned saying that only a board replacement was a solution and that too was not available immediately.
That’s how I got interested and asked him to bring the board to me. On inspection of the board on my table, I could not see any visual damages. Biotech confirmed that the fuse was intact, as he did check it. I brought it home for a detailed checkup. Let us have a look at the inside of this Humidifier and the power supply board along with an interesting electric circuit diagram label inside the machine: (The first two pictures and a few of them that follow are provided by the biotech!)
I observed that it was a simple power supply board with a transformer, a couple of relays, a 7805 regulator, one twin optocoupler IC, a few capacitors, with one SMD transistor and IC. That’s all! Nothing very complicated! Then I was wondering why the service providers as well as others declined to even do a trouble shooting on this board! Anyhow, chance came to me and I investigated the reasons for the failure by applying AC input.
There was no DC output. So I checked the primary of the transformer and found that it was open. Since these types of transformers would necessarily have a thermal fuse built in, I removed the transformer from the board and tore open the tape and traced it and, as expected, it had blown. However the primary winding was not open as it showed continuity. Let us have a look at the Transformer and the inside:
Then I shorted the thermal fuse and applied power, which you can see from the last picture above and got an AC output reading of 15.32 in place of 11V AC written on the transformer and 12V AC written on the board.
Then I kept the transformer connected to the mains without any load for a few minutes. Noticed that it was getting hot confirming my doubt that the primary was shorted. That’s the reason we were getting 15.32V instead of 11V. Then I probed the board for any defective components that could have caused it.
There were no defective components on the board to the extent checked with Analogue and Digital Multimeters. Then the next choice was either to get the transformer rewound or make a new one that will not take more space than theo riginal. The first option was not immediately possible as getting such thin wire was very difficult, as per the person to whom I took the Transformer for rewinding. Then he offered to make a new one, size of which will be slightly larger, but assured to do a proper winding with good quality materials with additional turns in the primary to take care of power fluctuations, with a secondary voltage of 12V AC at 500mA. The original transformer was only 350mA and so proposal was accepted by me and he made it ready the next day. I went and collected it and brought it home to find out how different it was and how to accommodate the slightly larger transformer. In order not to go near the AC inlets which was on the side of the transformer, I opted to keep it on its side, reducing the base size. Then covered the cores with cloth tapes and also pasted one on the board as a precaution. Then soldered the primary and secondary wires in its place (this transformer did not have soldering pins) and used a cable tie to keep the transformer tight on the board.
The output showed 13.95V, which was good enough and on load it might come to 12V to suit the relays rated for 12V. The 7805 showed an output of 4.990, which too was good enough. I did the checking on my table and not after connecting it to the machine:
I took the board with me to the hospital during my next visit and asked the Biotech to fix it in the machine and check the output voltage. He confirmed that the voltage reading was around 12.5 and that was very safe enough. He also confirmed that he connected the display and control board and allowed the machine to run in open condition for a couple of hours, before fixing the cabinets and moving the machine to the CT room for running it.
He sent me a video of the machine running, from which I extracted the following picture to show proof to you that the machine was working. Excited satisfaction jumped into the bag, providing additional thrill to me. Luckily the hospital saved a lot of money because of the delay in getting a replacement board and my chanced intervention!
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 Restored A 1996 ONKYO 6-CD Player Model DX-C330 To Working Condition
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Albert van Bemmelen
November 23, 2024 at 4:48 pm
That was another incredible done job Parasuraman! Which was no surprise after all those other incredible repair jobs you already have done!
The only surprise here was the name Westinghouse on a device in India that surely must have been a very famous name in the USA at that time in the past with the also great Nikola Tesla.
Parasuraman S
November 23, 2024 at 8:20 pm
Many thanks, dear Albert for your valuable appreciation and comments!
Albert van Bemmelen
November 23, 2024 at 11:01 pm
Thanks Parasuraman. Nice thing of these repairs on Jestine's Blog is that the zooming-in on photos and circuits still works after we have opened the as PDF document saved articles. Which a very welcome addition. Best regards!
Parasuraman S
November 24, 2024 at 10:54 am
Yes, Jestine Yong is very quality conscious and he takes care of everything with concern and dedication!
Waleed Rishmawi
November 23, 2024 at 5:31 pm
wow, totally agree with Albert here about the professionalism being done here with a successful repair outcome. it is strange why companies not willing to take a look at a power supply as simple as that...good job my friend, thanks for sharing and have a blessed day
Parasuraman S
November 23, 2024 at 8:21 pm
Many thanks for your continued support, dear friend!
Tayo
November 23, 2024 at 8:50 pm
You know... it's only because you wear glasses .... that they never realized that you are really SUPER MAN!
Otherwise they would have never bothered to look towards anyone else to save the day!
Well done Maestro!
I really learned a thing or two from the article.
Thank you very much!
Parasuraman S
November 23, 2024 at 11:33 pm
Many thanks for your SUPER MAN comments! It means a lot!
Francisco Maciel
November 24, 2024 at 1:16 am
Congratulations, another well done job !
Just to share my experience in MEXICO...
NORMALY all product design is intended for USA market. This mean: 110VAC +-5% which is... MIN: 104.5VAC, AVG: 110VAC, MAX: 115.5VAC {Also very stable voltage}
BUT in MEXICO we have 127VAC +-10%. This is, MIN: 114.3VAC, AVG: 127VAC, MAX:139.7VAC {AND fluctuating voltage!}
So, electronic design has to be able to disipate excess voltage to be consider as a GOOD product for MEXICO.
That is also the reason why I always advise the use of voltage regulators, specially if you can adjust working voltages !
Best regards !
Saludos amigos reparadores
Parasuraman S
November 24, 2024 at 10:52 am
Very valuable inputs and comments! Many thanks, dear!
Mark J
November 24, 2024 at 3:53 am
Parasuraman good job on the repair. The hospital where you work is luck to have you.
Parasuraman S
November 24, 2024 at 10:52 am
Tha's very nice of you, dear Mark! Many thanks!