Problem with switching power supply

Thread Starter

lawrence101

Joined Oct 23, 2017
29
Hi folks haven't posted in a long while guess i am now cause i need something :) ,
My bro recently gave me an old Sony power amp home entertainment system , i had no use for it so i decided to dismantle it and get the power supply board before throwing it in the trash . After removing the boards ( there were 3 ) i plugged everything back together, plugged the power chord into the wall plug and it appeared to be working. The display reads " PUSH POWER PROTECTOR " when i press the power button it then reads " STANDBY " for about 3 seconds and then go's blank . Taking a voltage reading at the power amp output plug shows about 30 volts DC but only for about 3 seconds or so. Can anyone help diagnose the problem or should i just toss the whole thing in the garbage ? .

The SMPS is a Sony 06DAV-POWER-12 and i have the schematics for it after an online search . Used my DMM and took some voltage readings around the board .
 

MisterBill2

Joined Jan 23, 2018
18,535
Some switcher power supplies have an over-voltage shutdown and they tend toward over voltage if they are not loaded enough. So if you know what the voltage should be then that is one thing to check. Are all of the sections that should be drawing power connected??
 

Thread Starter

lawrence101

Joined Oct 23, 2017
29
Some switcher power supplies have an over-voltage shutdown and they tend toward over voltage if they are not loaded enough. So if you know what the voltage should be then that is one thing to check. Are all of the sections that should be drawing power connected??
Thank you Bill for replying.
I plugged in the main control board and the small display board which also has the push buttons on it for turning on the power etc. From DMM tests on the SMPS board's 12 pin socket ( CN906 ), which supplies power to the main control board, there are correct voltage readings, but pins 1 , 2 , and 3 ( P-cont1, 2 and 3 respectively) there is nothing ( 0v) . So it appears that the low power standby section on the board is working fine but there is no lasting response from the control board . The power amp output socket ( CN902 ) shows about +30Vdc briefly ( about 2 or 3 sec ) when the power is first turned on and then slowly drains away.
I really don't need the control or display boards , only interested in the power supply board , thinking to use it on its own as say a battery charger ? Was hoping there was some simple way to bypass the control board inputs via installing jumper wires, perhaps its more involved than that.
 

Thread Starter

lawrence101

Joined Oct 23, 2017
29
Do you know the normal voltages when the supply is connected to the original amplifier?
Thanks again Bill for replying, sorry for the long delay had the flu but better now.
Power for the amplifier board comes from the power supply board via socket CN902 and it has two wires on it a black and red ( #16 awg) , stamped on the board near the connector (CN902) is 31.5 volts. I assume that's what the normal voltage should be but when i power it up its only there for a coupla seconds or so and quickly fades away.
I haven't yet tried your suggestion of using a dummy load across the red and black wires on CN902 ( as i wasn't feeling well ) was thinking of using a small incandescent lightbulb as a dummy load ?
 
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MisterBill2

Joined Jan 23, 2018
18,535
OK, if you read 30 volts, and the marking on the board reads 31.5 v, then my guess is that there is a bit of a problem.
I also see what looks a bit like metal spray on the PCB next to the power supply connector.
Are you able to try the power supply board without the amplifier board connected??
Are there other connections to the power supply board? OR are these three photos of the power supply board?? Does the whole system work as intended when all of the boards are connected as they are intended? Did the system work when you got it??
 

Thread Starter

lawrence101

Joined Oct 23, 2017
29
No i don't think it worked when i got it ( prolly why my bro gave it me ) but the display comes on and i have front panel control somewhat.
I connected a 15watt incandescent bulb as a dummy load to CN902 but it made no difference ( sigh :( ) , DMM detects ~ +30volts ( briefly ) but thinking the meter doesn't respond fast enough to read the peek volt level which quickly fades away to zero.

From studying the schematics it appears that the control inputs ( P-cont 1,2 and 3 ) on CN906 come directly from the microcontroller IC on the main control board which supposedly outputs +3.3volts to these pins under normal operation.
My assumptions ( i hope ) is that there was some means of by-passing these control inputs via installed jumper wires to those pins ?

The metal spray stuff ? i guess you mean the solid white substance around the connectors and components ? I think its some type of glue to help hold certain parts from coming loose ? It's very hard to remove prolly need a chisel and hammer to get it off .
 
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MisterBill2

Joined Jan 23, 2018
18,535
Reference the photo of the power supply PCB in post #7, it is to the right of capacitor #394, between the big cap and the aluminum heat sink. It looks like a small ferrite inductor that overheated and blew off it's cover.
If you also have the circuit for the power suppy board then post the whole circuit and probably somebody can tell yo how to force the supply to switch on.
 

Thread Starter

lawrence101

Joined Oct 23, 2017
29
Thanks for pointing that out bill but it appears to be fine , photos can be deceptive at times. I also noticed a small burn mark near C942 when i first looked at the board and was thinking may be one of the surface mount voltage regulators had died but it turned out to be ok.
Has you suggested i've attached the schematic of the power supply board along with a pictorial of the solderside ( which also has some surface mounted components on it ) for reference.
 

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MisterBill2

Joined Jan 23, 2018
18,535
What I see is thatd downstream of the 31 volt supply are several secondary regulator circuits including a number of feedback loops, including at least two more opto-isolators. This is by no means a simple power supply.!! What I see, (or think that I see) is five additional regulators providing additional outputs, all linked by feedback loops.
Probably somebody else who is able to better display the circuit on a better screen will be able to expand on that analysis and see how the multiple loops can be satisfied.
It looks like some other portion of the system is forcing the shut down.
 

Thread Starter

lawrence101

Joined Oct 23, 2017
29
Yes i agree the board appears to be a bit complex but after doing some additional study it's really just several individual switching supplies crammed on to one board. As you said there are 5 main regulators ; 3 are surface mounted ( IC's 941 , 942 , 943 ) on the solder side , a low power standby IC921 and the main high power IC901. From taking some earlier voltage readings it was found that all are working except IC901.
In the block diagram it shows a switching transistor ( Q901 ) connected to pin 4 ( Vcc ) on IC901 which is controlled by photo coupler PC903 which itself is connected to two surface mounted transistors Q943 and Q947 ( labelled "voltage detect" in the block diagram ) which in turn is connected to pin #2 on CN906 . Pin 2 is the P-cont 2 control output from IC501 on the main system control board, but there is no voltage present on this pin when the board is powered on . I have a suspicion that one or more of those parts or its associated supporting components have gone out of spec.
Ive since tried forcing it to come on by putting jumper wires on P-cont 1, 2, and 3 to supply +3.3 volts to those pins but it didn't work :(
I guess the next step is to test each individual component in that series ?

P.S. Ive attached the block diagram for clarity.
 

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Hi folks haven't posted in a long while guess i am now cause i need something :) ,
My bro recently gave me an old Sony power amp home entertainment system , i had no use for it so i decided to dismantle it and get the power supply board before throwing it in the trash . After removing the boards ( there were 3 ) i plugged everything back together, plugged the power chord into the wall plug and it appeared to be working. The display reads " PUSH POWER PROTECTOR " when i press the power button it then reads " STANDBY " for about 3 seconds and then go's blank . Taking a voltage reading at the power amp output plug shows about 30 volts DC but only for about 3 seconds or so. Can anyone help diagnose the problem or should i just toss the whole thing in the garbage ? .

The SMPS is a Sony 06DAV-POWER-12 and i have the schematics for it after an online search . Used my DMM and took some voltage readings around the board .
First of all, check the amplifier board for faults. It looks like the amplifier goes into protection mode. Most likely there will be a =n issue in the main board causing it. The 30V line will only appear when the amplifier works properly. When it detects some kind of short or over-current the 30V line will be turned off for protection.
 

Thread Starter

lawrence101

Joined Oct 23, 2017
29
Update : Problem solved.
After nearly an hour of retracing the suspect circuit it was found that all components were fine. Turns out there was an open chasis ground between the standby and main power sections. Ran a jumper wire between those two points ( see attached photos ) , re-connected jumper wires from Pin 4 ( E+4volts ) and P_cont1 and 2 on CN906 and i now have a constant +31 volts DC on CN902. So relieved , persistence paid off.
Thank you to everyone that was interested.
Just have to figure out how to lower the voltage on CN902 from 31 volts to ~13.8 or 14 volts to be used as a 12 volt battery charger, my next project.

P.S. also attached is a rough hand drawing of the suspect circuit and jumper wires.
 

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MisterBill2

Joined Jan 23, 2018
18,535
The sections may have been connected when it was in the original enclosure. OR the connection may have become disconected in the disassembly process. Those things happen.
 

Thread Starter

lawrence101

Joined Oct 23, 2017
29
The sections may have been connected when it was in the original enclosure. OR the connection may have become disconected in the disassembly process. Those things happen.
Yes that's most likely what happened , apparently the board was using the chasis itself as a ground plane. What's odd though even with the ground wire jumper added , there is still no output from the control board to P-con 1,2 or 3. But i'm not interested much in the control board anyway, having the SMPS board working again i can move forward with my battery charger project.
Thanks Bill for responding.
 

Reloadron

Joined Jan 15, 2015
7,517
Looking at your schematic the supply provides 3.3 volts, 5.0 volts and 12 volts. IC941, IC942 and IC943 all show a sense line on pin 5. Where are those sense lines connected? Those sense lines should be sensing the outputs of their respective regulators.

Ron
 

Thread Starter

lawrence101

Joined Oct 23, 2017
29
Looking at your schematic the supply provides 3.3 volts, 5.0 volts and 12 volts. IC941, IC942 and IC943 all show a sense line on pin 5. Where are those sense lines connected? Those sense lines should be sensing the outputs of their respective regulators.

Ron
Thanks Ron for your interest, yes those sense lines are connected to their respective regulators via two resistors. One is connected to ground and one to Vout. They appear to be working fine tho , they are only low power surface mounted regulators and i dont really need them anyway, as the high power section is now finally working.

I was going to post another thread regarding how to modify the shunt feedback regulator ( IC931) located in the low voltage secondary section of T901 so as to reduce the output on CN902 to ~+14 volts. Its a TA76431AS adjustable shunt regulator IC , i've only briefly looked at the specs for it , but from what i've read so far it appears do-able.
 
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