Wireless headset base transmitter bad voltage regulator

Thread Starter

Bwachtel

Joined Jul 8, 2020
26
I am currently trying to fix a bad voltage regulator on a board I have. I have verified that it receives 5v on the input and nothing is coming out of the output. On the backside of the board is a 3.3v test point and I verified it had continuity to the secondary side of the voltage regulator. So I assume it's a x-5vdc range in and 3.3v out. The only information I have on it is that there are 5 pins and the top has the text ex08. Does anyone know how I would find this component to purchase so I can replace?20200707_153818.jpg
 

Thread Starter

Bwachtel

Joined Jul 8, 2020
26
Which pin do you think is input?

Its looking, with that inductor alongside, like its a switching regulator.
The top left pin of the EX0B chip is continuous with the 5v input. Both pins on the output side are continuous with the 3.3v tesp point on the back of the board.

Can you check 'U10', It appears to do a similar function, please post result.
Good luck..
The U10 chip input is continuous with the 3.3v test point on the back of the board. Both outputs of it are continuous with the 1.8v test point on the back of the board.

I took a power supply and set it to 3.3v and it had a 500ma output. And I placed it on ground and the 3.3v test point and the leds came to life. It was difficult to hold like that so I couldnt test if it was functioning properly. But I beleive that chip functions as (voltage range, don't know max) to 3.3vdc voltage regulator chip that has failed open.

Before I remembered that the device used a USB port for power. I had found a 2A 5V power supply to turn it on. I can't beleive 2A caused the chip to fail open but that's the only thing I could think of that caused the regulator to fail.

Do aluminum capacitors fail short? Could I have a bad cap that failed short and spiked the current through the regulator?

This is the chip I purchased from Mouser after speaking with a representative. Do you guys think it would work from the information I have presented? I can't find the SMD code for EX0B. 3.3V Voltage Regulator
 

Irving

Joined Jan 30, 2016
1,006
No, I dont think so.

Its definitely a 3.3v switching regulator. I can't ID the specific manufacturer, but these, and other, Mouser parts fit the layout:
947-SC4626ZSKTRT
865-XC9258A33CMR-G
947-SC189ZSKTRT

Are there any markings on the inductor L12 next to it?
 

Thread Starter

Bwachtel

Joined Jul 8, 2020
26
Are there any markings on the inductor L12 next to it?
No there were no distinguishing marks on the inductor.

No, I dont think so.
I'm sorry, but what was this in reference to. I had went crazy with my posts before you messaged.

While I am an electrical engineer, I haven't touched micro-electronics since college back in 2015. Even then, I wasn't super great at the circuitry. So I appologive if I have a lot of novice questions.
 

Irving

Joined Jan 30, 2016
1,006
The part you got from Mouser. I can see where they were coming from, but if it were a linear regulator there would be no need for the inductor. That device is a 250mA part, the switchers are 1A.

Do you have a means of measuring inductance? We need to measure L12 to determine the likely oscillation frequency of the part, which will narrow it down to a more specific part. You might be able to do it in situ.
 

Irving

Joined Jan 30, 2016
1,006
947-SC4626ZSKTRT datasheet.
https://semtech.my.salesforce.com/sfc/p/#E0000000JelG/a/2R0000001NqE/Nc90mUvekLadiqtxH4dvLCDesI1AmnPRXEDzwQ0eumU
If you can get one of these, you could well be in business!
Good luck..
Could well be, but there are half a dozen or so parts that might fit - but they all have different switching frequencies. Without knowing the inductor value it may not work, or it could work but be overheating, or the regulation might not be sufficient...
 

Thread Starter

Bwachtel

Joined Jul 8, 2020
26
Do you have a means of measuring inductance? We need to measure L12 to determine the likely oscillation frequency of the part, which will narrow it down to a more specific part. You might be able to do it in situ.
I have a fluke 87 I don't beleive it measures inductance but will check the manual. Can inductance be measured in circuit or do you need a speacial meter to be able to measure in circuit.
 

Thread Starter

Bwachtel

Joined Jul 8, 2020
26
I also have an old bulky oscilloscope that I am not positive on its accuracy. Not sure if inductance is able to be measured from that.
 

Irving

Joined Jan 30, 2016
1,006
No I don't think it does, I have a Fluke 87 Mk3 :)

It can be done in situ, but it's likely to be more accurate out of circuit. If you were to remove U9 you could probably measure it onboard.

If you have a scope we could tack a capacitor across it and give it a pulse from a psu and see what frequency it rings at...

Its likely to be between 1 and 10uH

An alternative approach is to tack a wire onto the 3.3v output pad, and power up the 3.3 - 1.8v regulator and see what frequency that's running at... as they are the same family of parts.
 

Thread Starter

Bwachtel

Joined Jul 8, 2020
26
An alternative approach is to tack a wire onto the 3.3v output pad, and power up the 3.3 - 1.8v regulator and see what frequency that's running at... as they are the same family of parts.
Are you saying I should solder a lead onto the 3.3v test point on back and to the ground pad. Connect 3.3v to it with my variable output power supply (not a function generator). Then measure the frequency across the 3.3-1.8v output inductor?
 

Irving

Joined Jan 30, 2016
1,006
carefully remove U9,

measure resistance between pin 4 (lower RH pad) and ground (middle pin on LH side) - should be in kohm range.
measure resistance between pin 4 and +side of C51 (top pad of electrolytic on other side of L12) - should be 0ohm
solder the + wire to U9 pin 4
solder the - wire to -side of C51.(bottom pad)

connect u PSU @ 3.3v, set current limit to 750mA

power on, check current, should be <<1A
if it goes into current limit, turn up to 1A & try again.
If it still current limiting, have a listen, touch, smell or use IR temperature measure to try and ID whats drawing current.

check 1.8v rail (pin 4 U10)
if OK measure frequency with scope probe at pin 5 U10.

:)
 

Thread Starter

Bwachtel

Joined Jul 8, 2020
26
carefully remove U9,
Obviously I have to remove u9 because the regulator is bad. But should I go ahead and remove L12 and measure its Inductance with an lcr meter? Or. Is it easy to ruin the inductor with heat and should leave it alone?

Or... Since the regulator failed open, should I be able to measure straight across the inductor with an lcr meter?
 

Thread Starter

Bwachtel

Joined Jul 8, 2020
26
Sorry, didn't mean for my last comment to sound snarky. Just re-read it and I would change the wording if I could lol. I was just meaning, for testing purposes or for fixing the board I will have to remove u9.

My question was just "if I break the circuit by removing u9, shouldn't I then be able to more accurately be able to measure the inductor with just and lcr meter?

I got a smd soldering kit today to practice before I talking removing and replacing the part.

Just found the edit button... Oh well.
 

Irving

Joined Jan 30, 2016
1,006
If you have an LCR meter and can remove L12 then that would be better, but once U9 is gone one side of L12 is o/c anyway. Its just how you connect to it in-situ.

L12 can easily be removed with a soldering iron just carefully heating the pads on both sides, a chisel tip is best. if you use a hot air reflow tool practice on some gash boards first if you can. The trick is to heat the area gently then focus more on the device but not too close that you blow other parts off the PCB!

Use tweezers to grab the inductor and lift it clear as soon as the solder softens enough, then allow to cool. Clean the pads with lots of flux and a wick.
 
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