Hi guys,
I 'm trying to repair a SMPS from one of those cheap micro systems with cd player etc. Even if it might not be worth the effort, it is kind of a challenge to me.
Hope this is the right board to ask for advice regarding smps.
The input fuse was blown, and the first thing I realised is that the input was shorted. I checked and three rectifier diodes were shorted (marked in red) so I replaced them. There was also a MLCC capacitor 100nf 1kv burned (marked in red) which I also replaced prior to connecting again the power supply. Once this was done, I installed a new fuse and tried the SMPS without load.
No smoke, no blown fuse, everything looked fine, until I checked for the -25v output. It only reads -5v.
I thought maybe the output diode or capacitor was bad. Checked the diode and it works fine, and replaced the capacitor (marked green) for ease of mind. Still -5v output.
The 12v output and 5v outputs measure fine.
Any advice?? What else should I be looking for? My experience with SMPS is very limited but I would love to repair this, since the system has a sentimental value to me.
Thank you;
Fran
I 'm trying to repair a SMPS from one of those cheap micro systems with cd player etc. Even if it might not be worth the effort, it is kind of a challenge to me.
Hope this is the right board to ask for advice regarding smps.
The input fuse was blown, and the first thing I realised is that the input was shorted. I checked and three rectifier diodes were shorted (marked in red) so I replaced them. There was also a MLCC capacitor 100nf 1kv burned (marked in red) which I also replaced prior to connecting again the power supply. Once this was done, I installed a new fuse and tried the SMPS without load.
No smoke, no blown fuse, everything looked fine, until I checked for the -25v output. It only reads -5v.
I thought maybe the output diode or capacitor was bad. Checked the diode and it works fine, and replaced the capacitor (marked green) for ease of mind. Still -5v output.
The 12v output and 5v outputs measure fine.
Any advice?? What else should I be looking for? My experience with SMPS is very limited but I would love to repair this, since the system has a sentimental value to me.
Thank you;
Fran
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Hi. At first , from picture for me not clear ,which output is -25V ,it is floating or ground referenced ? I see clearly -5V named output ,it has no load at all ,so its voltage would be high even with very short pulses.
You are saying - started smps without load .Because there is common regulation , all voltages decrease if no load . Try to add some load , like resistor ,led stripe or something you have ,maybe small incadescent lamp of proper voltage ,to one of regulated outputs , your "-25v" output should increase . When no load , pwm pulses are very narrow ,and all voltages are reduced .Adding some load does increase pulse length ,to keep regulated voltage at same value ,and this increase other unregulated outputs too. Group regulation works in such way .
You are saying - started smps without load .Because there is common regulation , all voltages decrease if no load . Try to add some load , like resistor ,led stripe or something you have ,maybe small incadescent lamp of proper voltage ,to one of regulated outputs , your "-25v" output should increase . When no load , pwm pulses are very narrow ,and all voltages are reduced .Adding some load does increase pulse length ,to keep regulated voltage at same value ,and this increase other unregulated outputs too. Group regulation works in such way .
Also it looks strange to me one thing , you have marked in red D5 ,but its not a part of mains rectifier diode bridge , but its going to smps ic .Failure of such diode would mean smps ic likely destroyed too .So i hope you just marked wrong diode on picture .Also i see error in schematic you have posted , at right bottom there is second regulator ,which is used in strange way ,giving additional load of smps .But it misses ground R30 R31 D18 bottom pins line .
Thank you ximikas!
-5V named in green is the expected -25v Output. In green I typed the measured voltage. Sorry, this output is marked as -25v but the picture has poor resolution and it is hard to see. I guess it is ground referenced since its output capacitor is connected to ground.
I'll add some load to a 5v output, and see if the measured voltage in the problematic output gets to the expected -25v
Unfortunately it is not a mistake and D5 was shorted too. If the smps ic was destroyed, would I be getting an output voltage at all?
I took this schematic from the Service Manual.
Please find it attached. Pages 20(schematic) and 28(board layout). There does not seem to be a ground connection in R30 R31 or D18
Thank you so much for lending a hand on this. It is much appreciated.
-5V named in green is the expected -25v Output. In green I typed the measured voltage. Sorry, this output is marked as -25v but the picture has poor resolution and it is hard to see. I guess it is ground referenced since its output capacitor is connected to ground.
I'll add some load to a 5v output, and see if the measured voltage in the problematic output gets to the expected -25v
Unfortunately it is not a mistake and D5 was shorted too. If the smps ic was destroyed, would I be getting an output voltage at all?
I took this schematic from the Service Manual.
Please find it attached. Pages 20(schematic) and 28(board layout). There does not seem to be a ground connection in R30 R31 or D18
Thank you so much for lending a hand on this. It is much appreciated.
Manual has very good quality schematic picture ,but its strange ,that mentioned bottom end of resistors is not going directly to ground. But there are two holes ,square , maybe there should be something like metal shield soldered ,who keep that ground connection .Without that ground , for me U6 and all associated parts looks useless .Its looking like comparator ,which enables additional load for +5V line if PVCC voltage is low. If PVCC is higher than some voltage ,U6 conducts and turns off that additional load .So smps is kinda of prepared for operation with low load case ,maybe this needed for standby operation.
If smps IC is not destroyed , it may be damaged and have improper output , question then why D5 shorted and how ic survived that ??? .With shorted diode D5 ,ic got mains voltage both polarities ,positive and negative ,which is not allowed .Or there is different numbering in schematic and pcb .Also i see screw holes near , maybe when assembled ,it gets screwed to metal case(if any) and it makes that connection to ground.
Anyway ,if smps is working ,all you have to do is to small load +5V and check -25 named output. +5V should not change , and -25V increase .
Before connecting smps to other circuits in this audio device , check lines for short circuits .Maybe amplifier has blown ,or something metal got droped inside and shorted something .Only that would explain D5 issue too and why ic still alive .
If smps IC is not destroyed , it may be damaged and have improper output , question then why D5 shorted and how ic survived that ??? .With shorted diode D5 ,ic got mains voltage both polarities ,positive and negative ,which is not allowed .Or there is different numbering in schematic and pcb .Also i see screw holes near , maybe when assembled ,it gets screwed to metal case(if any) and it makes that connection to ground.
Anyway ,if smps is working ,all you have to do is to small load +5V and check -25 named output. +5V should not change , and -25V increase .
Before connecting smps to other circuits in this audio device , check lines for short circuits .Maybe amplifier has blown ,or something metal got droped inside and shorted something .Only that would explain D5 issue too and why ic still alive .
You are totally right, yes there is a metal shield which I did not install for troubleshooting, that keeps that and other ground connections. I will install it , add a load in +5v, and check again.
I will let you know. Thank you! No idea about the IC, but I will check lines for short circuits and loose metal parts before connecting other circuits, as you suggested, if smps works fine now.
I will let you know. Thank you! No idea about the IC, but I will check lines for short circuits and loose metal parts before connecting other circuits, as you suggested, if smps works fine now.
You can temporarily solder short wire between points ,which had contact with metal shield instaed of shield ,then probably you will not even need a load on 5v line .But actually what manufacturer did - i think incorrect approach , it wastes electricity at standby ... If load is too low , make load higher ,to allow smps operate ,nobody cares for standby consumption ... From schematic it looks like +5v loaded with 11 ohms with transistor at idle , which makes up to 2.2W waste .If i'm correct , standby power should not exceed 0,3W by regulations.
Thank you guys, yes I tested the smps without load . I took the smps board to my workshop for troubleshooting, and do not have the rest of the circuitry available here.
I will solder a wire as suggested, since the shield is quite big and will make measuring with the multimeter probes harder.
I'll keep you posted.
I will solder a wire as suggested, since the shield is quite big and will make measuring with the multimeter probes harder.
I'll keep you posted.
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