LG Hifi System - LM-K3960

[phlox]

Hi All, Hope you all are well?

So a friend of mine asked if i could have a look at his hifi.
Its powering on but the sound is very soft even at full volume.

So at first i suspected a blown amplifier IC but upon further testing its the amplifiers power supply thats at fault.

Its supposed to give out 35v but theres nothing.

I stripped it and found and traced the 35v line to the transformer( it uses a smps for the whole system)

The secondary side has no output but the primary side has around 45v AC.
Then after further probing and checking voltages , i probed a zener diode on the primary side while negative of the multimeter was connected to earth and the thing starting working again, but gave out about 20v on the secondary but the amp is working.

if the unit is switched off, even in sleep mode then after about 3 or 4 minutes and switched on again then theres no output on the secondary unless i just probe it with the meter then it works for as long as the unit is on.

Any ideas where to start looking? im new to smps supplies, checked the caps, none leaking and they are samxon caps(good quality?)

any help would be appreciated.
Attached is the circuit diagram of the SMPS (i have the workshop service manual but its over 10MB)

 

[Alec_t]

i probed a zener diode on the primary side

Which one? I can see at leat 3 on the schematic.

 

[phlox]

Ive attached a image of the diode as well as the board view.

Basically the multimeter is hooked to ground and i just probe the zener cathode and the amp starts working.
I can repeat it each time.

First time it started working i just probed across the transformer and it started working.

 

[Alec_t]

the multimeter is hooked to ground

The circuit seems to have several circuit commons. It's not clear which one is 'ground' in this context. Please use the component designators on the schematic when referring to test points and components.
Can you measure (EXTREMELY CAREFULLY, hands free,in view of the LETHAL voltages present in the circuit), the operating voltage between pins 3 and 4 of IC901 and between pins 3 and 5 of IC902 and post the results?

 

[phlox]

attached find requested voltages for IC 901 and 902

I attached wires to the pins for easier measurement and as for taking photos easier
I marked in the Photo which is IC 901 and IC 902

 

[Alec_t]

Looks like the Vcc of IC901 is too high. The Vcc of both ICs should be about 14V, given that both are regulated by the same spec 14V zener diode, a 338. I'd be replacing ZD911 and C909.

 

[phlox]

hey guys... so parts finally arrived. i replaced both 338 diodes and C909. Now i dont have standby power anymore and not switching on. IC 902 though now has about 13.5v and IC 901 has about 0.5v . If any of the two diodes are replaced with original it switches back on but no power to amp

 

[Alec_t]

Looks as though something in the area of IC901 is wrong then. Don't know what else to suggest.
Poking which zener enabled things to 'work'? 911, 905, 906 or 902?

 

[phlox]

If i recall correctly it was zener 911, what i also saw was i can just poke it to "ground" and it starts working. Also its not the zener as i poked one leg of the opto-coupler (nearest to the 911 Zener) and that also made things work.

 

[Alec_t]

Sounds as though your meter resistance or body capacitance was affecting the circuit enough to get oscillations going. Strange. Or perhaps there's a dry joint somewhere near which conducts only if the pcb is pressed/contacted.
Several of the components (e.g. ZD911, C927, R911, ZD901 etc) seem from the schematic.pdf image to have 'spark gaps' between their terminals?? If those are real, check that they haven't been bridged by fluff or other gunge which might create a leakage path.

 

[phlox]

as far as i could see there arent any spark gaps on any of those components. What i also noticed though is that every point going from IC 901 vcc pin to the opto-isolator, if i just touch it with the meters probe it works even if the other probe is lying on the table. I can also touch VCC pin of IC 901 and it'll work, so basically every component on that line from VCC pin to the opto-isolator makes it work.

What I have also noticed it that the DC Voltage of C905 is only about 315v where in the toubleshooting guide it stated that it should be over 400v. Where would that problem lie? almost all the voltages like the amps 35v and 12v are lower than spec.

For instance the amps 35v is only about 16v to 20v but still works fine if i probe VCC pin to get it going.

Another thing if the hifi is working and i put it to sleep mode and switch back on immediately then i works fine, but if I leave it off for a minute or two then the SMPS starts making a very faint humming/buzzing sound and if i switch it back on then it will work for a few seconds then volume slowly fades down until it goes off and i have to probe it again to get it working. Sounds almost like a capacitor slowly draining.

 

[Alec_t]

the DC Voltage of C905 is only about 315v where in the toubleshooting guide it stated that it should be over 400v. Where would that problem lie?

Well, according to the guide you should then check BD901, LF901,902.

 

[phlox]

i desoldered and checked BD901 which is the rectifier, ill try and get a replacement and see if it makes a difference and LF901 is two coils in one which im not sure what its for and how to test it, both coils have the same resistance though. and there is no LF902 , on the PCB its just bridged with two jumper wires

 

[Alec_t]

there is no LF902 , on the PCB its just bridged with two jumper wires

Well that's one suspect eliminated .

 

[phlox]

so i checked everything again. for some weird reason i cant measure the voltage on the 35v caps. And something mustve shorted out coz one of the caps blew , taking 2x 10ohm smd resistors next to the amp chip and the amp chip with it. Will order new parts , replace and check again.