smps fault finding

Thread Starter

Stockleys

Joined Jun 18, 2019
16
afternoon/morning all.
a little help if you please

i am trying to fix an smps from a digital mixing desk.
it has 5 voltage rails from 2 transformers.
1 transformer is clicking (hickup) and the output voltages are fluctuating

what i ahve done so far.

all electrolytic capacitors replaced.
replaced opto coupler and tl431
replaced ob2269c pwm ic (last resort)
all resistors tested for correct value.
all diodes tested against short.
zenners tested as far as not short.

confimed aux winding voltage output.
confirmed signal into and out of opto isolator

i cannot find any shorts or open circuits.
i have no way to ,confirm values of smd ceramic capacitors only that there are no shorts.

what am i missing?

thankyou in advancem32r smps.jpg
 

Juhahoo

Joined Jun 3, 2019
300
What happens to DC output voltages when the "click" happens? how much they dip?
Is clicking random or constant, what rate?
Can you isolate the load from the DC power supply (probably requires some cuts to PCB)

I'm thinking it is not the power supply, but the load is overloading the supply and it "resets".
 

Thread Starter

Stockleys

Joined Jun 18, 2019
16
The 15v rail falls as low as 11v the click is consistent probably just shy of 1hz. No load. No way I’m connecting the mixer pcb to the psu while it’s acting up. Spares are rarer than unicorn poop.
 

Thread Starter

Stockleys

Joined Jun 18, 2019
16
300v rail is stable. The other transformer that does 5v and 24v is working perfect (running from the same hv rectifier and cap.
 

Dodgydave

Joined Jun 22, 2012
10,826
Ok sounds like it's going into overcurrent mode, lift the diodes D4/5/9/14/18 these are the rectifier diodes for the outputs , leave D15 in for the time and see what happens. If it stops pulsing put the diodes back one at a time and power up each time to see which output is pulling.
 

Dodgydave

Joined Jun 22, 2012
10,826
So the only output is the+15V , see what happens if you remove D15, it might be on the primary side if it keeps ticking ,like the transformer or R12..
 
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Juhahoo

Joined Jun 3, 2019
300
Did you check R12 value and its feedback voltage? If it is out of window PWM controller goes to shutdown and restarts at a constant rate. This could be due to the faulty transformer or some other parts arount the PWM controller.
You can also use the other fully functioning power supply on that 300V rail to compare how the wake up should go and end and at which point the click comes during the wake up.
 
Last edited:

Thread Starter

Stockleys

Joined Jun 18, 2019
16
R12 resistance is within spec. Not checked voltage from it as I’m not keen on probing around 300v dc area while live. I don’t have an isolation transformer unfortunately
 

Juhahoo

Joined Jun 3, 2019
300
R12 resistance is within spec. Not checked voltage from it as I’m not keen on probing around 300v dc area while live. I don’t have an isolation transformer unfortunately
Unfortunately, if the secondary side doesn't show any problems you need to dig into 300V primary side...
 

Thread Starter

Stockleys

Joined Jun 18, 2019
16
Unfortunately, if the secondary side doesn't show any problems you need to dig into 300V primary side...
On the 300v side, it powers the other section just fine. (5v and 24v)
On the pwm ic section, all resistors test in spec. The zener diodes are not short but can’t confirm that they are switching at the correct voltage. I can’t find any open circuits or broke via’s.

I am getting 30v ac from the aux winding.
1.8v from octo coupler (but fluctuating with the output voltage)
 

Dodgydave

Joined Jun 22, 2012
10,826
R12 resistance is within spec. Not checked voltage from it as I’m not keen on probing around 300v dc area while live. I don’t have an isolation transformer unfortunately
Okay you removed the output diodes as i suggested the only options are to unsolder all the outputs and see if it still ticks, check the small capacitors in parallel with the diodes on the +/- 15v and 7,48v rails.

Where are the 5V and 24V on the diagram?
 

Thread Starter

Stockleys

Joined Jun 18, 2019
16
Okay you removed the output diodes as i suggested the only options are to unsolder all the outputs and see if it still ticks, check the small capacitors in parallel with the diodes on the +/- 15v and 7,48v rails.
I did also remove the capacitors that are in parallel with the diodes at the transformer. Same symptoms. I believe the fault lies around the pwm ic. But I just can’t find anything.
 

Dodgydave

Joined Jun 22, 2012
10,826
I did also remove the capacitors that are in parallel with the diodes at the transformer. Same symptoms. I believe the fault lies around the pwm ic. But I just can’t find anything.
What is the model of the mixing desk, is this the full circuit diagram?
 

Juhahoo

Joined Jun 3, 2019
300
You need to perform some measurements to see what happens at different points. I'm interested to see the waveform from the 15V line.
Is this the power supply in the pic?


1678349647774.png
 
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