Switch Mode Power Supply (SMPS) Repair - Low output voltages whilst under load.

Thread Starter

MC22

Joined Sep 12, 2022
2
Hi,

I’d like help please troubleshooting some circuitry. I have some knowledge of electronics repair but are eager to learn more!

I have an audio mixer with a faulty switch mode power supply. When switching on the desk the buttons will flash and the screen lights up, but the unit does not boot. Switching the power on/off a few times will eventually get the mixer to boot up but not operate fully. Only half of the boards would light up and respond.

I have since went ahead with the obvious fix and replaced all 21 of the Chinese branded electrolytic capacitors with comparable Panasonic and Nichicon caps purchased from Digi-Key.

No change! Still the same non-working status.

The output voltages under *no load* are close to the specification, but when the 7 control boards are connected the voltages drop like a stone! Connecting up a single PCB on the output has the same affect. I have included the values measured below – in order.


Manufacturers Specification:
+48v
+16v
-16v
+6.6v
+6v

*No load*:
+46.2v
+15.1v
-15.1v
+6.6v
+5.0v

*Load connected (1 PCB)*:
+8.75v
+2.4v
-2.4v
+2.2v
+0.9V

My thoughts now are, could there be something failing in the IC feedback circuitry to increase the voltage when required? This would this cause all the voltages to be affected.

Am I right in thinking? How would I go about checking this?

I have added attachments to help diagnose.

Kind Regards,
Matt
 

Attachments

Dodgydave

Joined Jun 22, 2012
10,591
No that sounds like overcurrent sense , there must be more leaking current in the circuit which is sensed by pin3 on the switchmode chip (UC3844) .

It could be bad capacitors on the output side or output diodes.
 

Ian0

Joined Aug 7, 2020
6,711
Check that the value of R6 is really what it should be.
If it had previously suffered some fault which resulted in damaged components, then R6 could have been overloaded and its value may have changed. If it is now higher that it should be, the supply will current limit at too low a value.
 

Thread Starter

MC22

Joined Sep 12, 2022
2
Thank you to everyone who replied. Apologises for the late response. I am back to work again on the power supply!

I will check out the components suggested by renemonte and Ian0.

No that sounds like overcurrent sense , there must be more leaking current in the circuit which is sensed by pin3 on the switchmode chip (UC3844) .

It could be bad capacitors on the output side or output diodes.
I have replaced all of the electrolytic capacitors so they shouldn't be a problem.

If I test the output voltages individually with a load, would that identifiy which output diode (if any) is at fault?
 

Dodgydave

Joined Jun 22, 2012
10,591
Thank you to everyone who replied. Apologises for the late response. I am back to work again on the power supply!

I will check out the components suggested by renemonte and Ian0.



I have replaced all of the electrolytic capacitors so they shouldn't be a problem.

If I test the output voltages individually with a load, would that identifiy which output diode (if any) is at fault?
Remove the diode from the circuit and test it with a multimeter, and your method will work as well.
 
Based on what you've described, there may be a problem with the IC feedback circuitry that regulates the voltage. It would be good to check the voltage under load and the voltage when a single PCB is connected.
 
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