Mean Well Model P25A-14E Power Supply Repaired
A local radio station asked me if I would take a look at one of their mixing consoles’ power supply. They dropped off the mixing console and its power supply.
They already confirmed the power supply to be at fault by substituting it with one from another mixer. The complaint was that the power supply would cease sending power to the mixer after a couple of minutes. Only by switching it off for a while, would it work again. My findings were the same as theirs.
I then proceeded to open the power supply. Luckily it only had two screws. There was no visual damage of the board or components. No cold solder joints either. All voltages check out good, and all capacitors tested fine on my ESR meter.
I powered up the supply, with the mixer connected to it, and waited. Soon enough, it cut out again. I then used some freeze spray on all the active devices to no avail. It was like something was latching on and putting the power supply in a shut down state.
Further investigation revealed a small SCR in the secondary section. It was hidden behind the TL431. The part number is MCR100-6, in TO-92 package.
SCR’s can latch on when used in DC circuits. I then waited for the power supply to cool down and powered everything back up again. This time I used some hot air from my rework station on all of the active devices.
First on the Top247, then the 7915 and the diodes followed by the TL431. Lastly, I heated the SCR and the power supply immediately shut down.
I then removed the SCR and tested it with my PEAK component tester. It showed up as a common cathode diode network.
I took a similar SCR from my parts bin, a MCR22-8 just to confirm. It tested properly as a sensitive gate SCR.
I measured the resistance from Anode to cathode on the faulty SCR. It measured 8 ohms. This resistance would change with temperature. Definitely a BAD component. Why it failed I can’t say for sure, probably it was just stressed over long periods of use. I was interested to know how and why the SCR was used in this circuit. I haven’t seen one in SMPS designs recently.
I could not find any schematics for this specific power supply online, so I decided to trace out the secondary side of the circuit by hand. This is what it looks like:
It seems like the SCR is connected across the TL431 and acts as a crowbar for overvoltage protection. I replaced the SCR with a similar one, a MCR22-8, and confirmed the power supply to be working properly. What I found interesting was how the +15V rail is generated. I left the mixer powered on with the power supply for a couple of hours with no issues.
This article was prepared for you by Riaan Diedericks. He runs his own electronics repair shop in Pretoria, South Africa. He specializes in Pro Audio repairs.
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You can also check his previous repair article on Kef PSW3000 Subwoofer Repair
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Albert van Bemmelen
April 16, 2022 at 7:18 pm
Repairs like this one with in detail included schematic make it a very interesting article to read! The Peak Atlas digital SCR100 tester and probably the one you used are the only few that are capable to also test the higher gate current requiring triodes (upto 90mA!) since most cheaper Chinese semiconductor testers are only able to handle the low gate types. And even Peak Atlas's own DCA75 Pro semiconductor is not capable to test Triacs with gate currents higher than 12mA!
Parasuraman Subramanian
April 16, 2022 at 8:38 pm
Excellent informative article! Many thanks for sharing
Rich Leonard
April 16, 2022 at 8:57 pm
Very nice repair - good catch! Have to add a Peak Atlas digital SCR100 tester to my bench.
Imoudu. O
April 16, 2022 at 10:55 pm
Wow!! It sounds new thing to me, for I have not handle with such component before.More importantly I have learned additional information from you.Thanks for sharing.
Robert Calk Jr.
April 17, 2022 at 11:21 am
Good job, Riaan.
Paris Azis
April 18, 2022 at 1:44 am
Very interesting repair indeed, Riaan.
The reverse engineering you did on the defective stage makes the mechanism of both the failure and the way the applied protection works even more easily explicable and understandable.
I find the circuit design very sophisticated as you already have explained.
Yogesh Panchal
April 20, 2022 at 12:16 am
Good Job! keep sharing,Riaan.