LG HT33S - Loud High Pitch Tone

[blackillusion]

Hi guys,
I have an issue with this. It appears to be coming from the PWM circuit
http://youtu.be/ryxeJLIIbaY

I had an inductor L606 the had come loose from the solder joints and replaced it back in circuit. I tested to see if it had shorted but it had not.

I tested the caps with ESR and they check ok.

Any ideas what could have failed. I'm leaning towards and IC

Service Manual/Schematic


PWM circuit is page 44

Please note I have a DMM and ESR tester, nothing more.

Thanks you

[blackillusion]

Re: LG HT33S - Loud High Pitch Tone

looks like some idiot put a 5v regulator in instead of a 3.5v/3.3v

it has been opened before by some shop. We got it through ebay.

schematic says it should be a 278R35, but its got a 78R05 in.

I cannot source a 278R35 at the moment. Looked on farnell.

the spec sheet says its a 2a ldo 3.5v
http://www.digchip.com/datasheets/pa...8R35PI-pdf.php

can I swap for a 1A? or would that be bad?

[blackillusion]

Re: LG HT33S - Loud High Pitch Tone

Ok so I finally got my 3.5v regulator exact match and still get the high pitch tone. It appears that I dont have the 35v on the other connector though Maybe I'm not testing correctly I think I need to check tomorrow as I only went over it quickly. Believe it stops a the switching transistor on the hot side.???

I will verify that. If a pwm is low or no voltage at a point would that cause the high pitch noise

[budm]

Re: LG HT33S - Loud High Pitch Tone

The 35v is for supplying the power for the POWER AMP ClassD the amplifirer, the IC's also require 12VDC, do you have the 12vdc?
If you are getting the high pitch from ALL speaker outputs, then it has to be something that is common to all the amplifier which will be the power supply. So you need to verify if you do have 35VDC or not.
The 35vdc is generated by transformer T902 on page 37, look at the connector C902 pin 2~4 with black probe of the meter on the chassis.
The power supply for this unit is ALWAYS running the minute you have it plugged into the AC outlet so it runs 24/7 which I do not like since it be exposing to any spike/surge 24/7.
You should have all the voltages as shown on the both connectors in page 37.

[blackillusion]

Re: LG HT33S - Loud High Pitch Tone

The 35v is for supplying the power for the POWER AMP ClassD the amplifirer, the IC's also require 12VDC, do you have the 12vdc?
If you are getting the high pitch from ALL speaker outputs, then it has to be something that is common to all the amplifier which will be the power supply. So you need to verify if you do have 35VDC or not.
The 35vdc is generated by transformer T902 on page 37, look at the connector C902 pin 2~4 with black probe of the meter on the chassis.
The power supply for this unit is ALWAYS running the minute you have it plugged into the AC outlet so it runs 24/7 which I do not like since it be exposing to any spike/surge 24/7.
You should have all the voltages as shown on the both connectors in page 37.

Hi budm thanks for your reply. I have just tested and only have 30.5v. All other voltages are present. Although 12v is showing 11.9v or something similar. But all are present very close to their specified

The high pitch comes directly from the PWM board, I don't have to have the speakers plugged in and its making the tone.

[budm]

Re: LG HT33S - Loud High Pitch Tone

The only thing I can think of that can make high pitch is from the inductors for the switching power supply. I need to look at the diagram some more, but you should also hook up the speaker to see if it also comes through the speakers.

[blackillusion]

Re: LG HT33S - Loud High Pitch Tone

Hi budm thanks again for your help. Well if the sub speaker is plugged in to the PWM circuit the tone comes through that speaker so I would assume it goes through the rest. I can confirm tomorrow?

[blackillusion]

Re: LG HT33S - Loud High Pitch Tone

I have taken another look into this and would assume that the 35v being 30.5v is the problem. Low voltage makes pwm squeal. ?

[midibob]

Re: LG HT33S - Loud High Pitch Tone

I can remember some time ago one particular make of PSU giving quite a loud high pitched whine when it was due to pack up.
This was invariably due to a low value cap in the oscillator cct drying up.
This maybe a red herring but worth checking.
Typical values are anything between 1uF and 4.7uF @ 50v and they are normally quite close to a heatsink for the switching transistor which is why they fail.

Good luck
Midibob

[ben7]

Re: LG HT33S - Loud High Pitch Tone

I can remember some time ago one particular make of PSU giving quite a loud high pitched whine when it was due to pack up.
This was invariably due to a low value cap in the oscillator cct drying up.
This maybe a red herring but worth checking.
Typical values are anything between 1uF and 4.7uF @ 50v and they are normally quite close to a heatsink for the switching transistor which is why they fail.

Good luck
Midibob

I agree, it seems like maybe a bad cap causing the audio amplifier to oscillate. Depending on how the amp is setup, all the outputs could oscillate too then.

[blackillusion]

Re: LG HT33S - Loud High Pitch Tone

Ok guys, I am in the process of setting my esr meter around the board I have two ceramic caps that appear to do nothing but I'm not sure if my meter doesn't support the value?

2a272J doesn't give a reading when using esr meter just - -

Again thanks guys

[blackillusion]

Re: LG HT33S - Loud High Pitch Tone

Well t902 has no output voltage. Ive tested components one by one and capacitors seem to be ok. Could it just be that IC902 is faulty?

[midibob]

Re: LG HT33S - Loud High Pitch Tone

Silly me,
I should have played the youtube sound in your first post.
It's definitely not a failing PSU noise as this was due to the oscillator whining. In your case you have some audio tone being generated from somewhere.

Checking with a software spectrum analyser it shows peak frequencies at 1.5KHz, 3.7KHz and 6.25Khz. I did originally think it maybe some kind of internal test tone but I can't find any mention of it in the manual.

Now, as you say you don't have any speakers plugged in, some kind of transducer must be generating the audio. I must admit I've never heard any components manage to produce audio like that barring transducers of course.
There doesn't appear to be one that I can see on the diagram either. All very odd.

Anyway, back at the ranch, if there's no output from T902 then ic902 has either failed or it's associated voltage supplies/ oscillator components are faulty.

I would check the 3 zeners Z902 and Z911 an 12 and possibly the dropper resistor R911. Are the voltages on the chip correct?
Also check there are no shorts on the output of T902 if you haven't already.

Good luck
Midibob