Lg 32LC4R no audio

[Grunder]

Hi, please if somebody could help. my LG32LC4R has good picture, but no audio.

I've measured all the marked voltages, and all but two are missing:
The 3v4 rail from the psu pcb, and
The "3v4-on" from the cpu pcb.

Is the missing 3v4 responsible for my audio loss or am I barking up the wrong tree?

The "psu on" from cpu measures 2v1, and the "inv on" measure's 4v4.
Could I use one of these two "on commands" to switch the 3v4 on?
(hoping to link "psu-on" to "3v4-on"?)

I have checked all the usual, like speakers on and selected PAL I, and made sure its not a user setting.

[dick_barton]

Re: Lg 32LC4R no audio

Service manual and flow chart for No Sound
https://www.manualslib.com/download/...Lg-32lc4r.html

[dick_barton]

Re: Lg 32LC4R no audio

What is the number of the power supply. Can you provide a sharper picture of it showing voltage labels.

[budm]

Re: Lg 32LC4R no audio

If you want to force on the 3V4 power supply, you MUST disconnect the 3V4-ON wire going back to the main board first and then you can jump the 3V4-ON pin of the power supply to the psu-on using 100 Ohms resistor for safety.

[Grunder]

Re: Lg 32LC4R no audio

Hi Guys,

Thanks Dick for the link, will pour over it once finished here. Thanks Budm, will disconnect lead 3v4 from cpu pcb before forcing 3v4 on.

I took closer photos of the Psu which I studied, and see they use a U3037 as a pwm controller driving two nchannel fets supplied from the 5v rail to achieve the 3v4.

The layout looks identical to the typical configuration supplied with the pdf for the U3037.

This set use two transistors Q202 (pnp) & Q203 (npn) to switch the 5v vcc to the U3037 (see photo). The 5v vcc (pin 2) supply for the U3037 isn't there.

The 3v4 "on" lead from the cpu connects to the base of Q203 (npn), so I guess it's waiting for a "high" from the cpu?

NOW THE STRANGE PART: when you trace the 3v4 "on" lead back to the cpu board, it gets to the plug, and is marked NC (no connection?). see photo

Using my meter on ohms, I tried to find where the lead goes to, but like they say..NC ?

I see that the emmiter of Q202 picks up the 5v rail, and it's collector goes to pin 2 of the U3037. Being a PNP transistor I reckon it needs to be grounded by Q203.

I am going to try taking Q202's base to ground. It would be grounded by Q203 anyway, when Q203 was switched.

Will report back later, thanks.

EDIT : I uploaded 4 photos, I don't know why they don't appear. Have got busy will try again later.

Edit2 have uploaded other photos, unfortunately dont have photo of psu complete. will try again later, maybe start new thread

[dick_barton]

Re: Lg 32LC4R no audio

On the Schematic CN801 Pin 1&2 provide P_19V to audio IC601 which I would think is 19V supply for the IC.

What voltages do you have on CN801 Pins 1 -> 10 ?

[Grunder]

Re: Lg 32LC4R no audio

Hi Dick, I must admit that i checked the 19v on the psu plug, but not on the cpu or the audio chip (didnt have the diagram, & heatsink over audio ic, so i couldn't identify it)

I have picked up another set in the meantime (snowed under), but will pick up again, and armed with new info surely crack it. thanks for all so long.

[Grunder]

Re: Lg 32LC4R no audio

On the Schematic CN801 Pin 1&2 provide P_19V to audio IC601 which I would think is 19V supply for the IC.

What voltages do you have on CN801 Pins 1 -> 10 ?

Hey Dick, sorry for delay in response (juggling a couple of jobs).

Down loaded diagram, page 10 shows 19v supplied to pins 35 & 36 PvccL
& pins 45 & 46 PvccR. there is 19v there.

I haven't checked voltages on other pins as you requested, I first tried getting the 3v4 going by disconnecting the 3v4 switch from the processor pcb and sourcing it from the Blon switching voltage, using a 120 ohm resistor to limit current draw in case of short on circuit being tested/switched

The 3v4 volts came up YAY! (so processor not sending 3v4 switch)
But... It pulled the Blon voltage down from 4v4 to 1v3.AAAW.
Causing the backlights not to switch on. AND STILL NO AUDIO.

I tried leaving the 3v4 switching lead disconnected to see if the circuit being switched wasnt pulling the switching voltage down, but 0 volts from the cpu.

At this point the fact that the Cpu wont switch the 3v4 on, and the fact that the audio IC is on the the Cpu pcb, I feel replacement of the Cpu board would be the best option.

Sounds like a bit of a cop out, but I think the battle of "cost of component" is beating the "cost of time."

Thanks all & Budm for the resistor idea. (prevented some domestic science/cooking)

Edit: there is a slight difference in the Cpu boards, but minor like the manual shows 2x scart plugs, whereas mine has Rca's, and mine more hdmi inputs.