Trying to repair a bigscreen TV

tom66

Joined May 9, 2009
2,595
Ok, will do. later on this evening when i get home.
thanks
Test it with the line output jacks on the TV, if any. Make sure the audio preamp, if any, is not broken.


what about RCA? So far they account for 3 out the 5 TVs I have come across in my very short time as tv repair hobbyist
RCA probably OEM'd this model, like most manufacturers, you can't really say because one model had problems all RCA's are bad. But a good manufacturer will choose a good OEM, or even better, design the set itself.
 

Thread Starter

strantor

Joined Oct 3, 2010
6,782
Test it with the line output jacks on the TV, if any. Make sure the audio preamp, if any, is not broken.
good stuff!
One other thing I was going to test is if the problem is again with that cable input box.
I was going to hook up a DVD player to the aux input and see if I get sound that way.
 

Thread Starter

strantor

Joined Oct 3, 2010
6,782
If you have a scope get it to play some audio out of the speakers, then post the trace of the audio signal.


nothing above 20mV, just noise. dead.

No signal: burnt out power amplifier IC somewhere.



TDA?69 (TDA??69)

The red & green plugs on the left are where the speakers plug in. the PCB traces go straight to this IC.

The middle number (numbers?) were scratched off by the clip that holds the IC to the heat sink. I will look inside my other RCA TV and see if it has the same chip so maybe i can get the part# off of it.

Googled TDA*69 and all I could find is that TDA is audio ICs. Could not find anything that looks like this chip.

Test it with the line output jacks on the TV, if any. Make sure the audio preamp, if any, is not broken.
Didn't get around to testing that. battling with unrelated problems.

One other thing I was going to test is if the problem is again with that cable input box. I was going to hook up a DVD player to the aux input and see if I get sound that way.
I hooked a wii up to the aux input and same story so I guess either that power amplifier is bad or the preamp. I will need to test the preamp later.
 

Thread Starter

strantor

Joined Oct 3, 2010
6,782
Tom, just so I'm on the same page as you, you are saying that the audio signal present at the AUX jacks is same signal that goes into the power amplifier? and that this signal comes from the preamp?

so... sound signal from wherever ---> preamp ---> aux jacks and power amplifier

and

power amplifier ---> speakers

?
 

tom66

Joined May 9, 2009
2,595
Aux jack audio probably goes through basic protection and gets switched by a CD4066BE or similar.

If the set is more complicated, the audio will go through a DSP to have effects applied (e.g. digital volume control, tone/treble etc.)

In which case you need to trace the input signal going to the TDA7269. You could try removing it with a soldering iron then tracing the appropriate audio input pin.

Worst case - how much can you get a 5.1 system for? I got mine for free from someone who upgraded theirs - or you could look for a dead amplifier to fix.
 

Thread Starter

strantor

Joined Oct 3, 2010
6,782
Aux jack audio probably goes through basic protection and gets switched by a CD4066BE or similar.
through DSP, then to to the switch, then to the jack? what's meant by basic protection? a fuse?

If the set is more complicated, the audio will go through a DSP to have effects applied (e.g. digital volume control, tone/treble etc.)
I only just briefly looked up DSP so I get what it does, but is this going to be an IC, or an entire circuit?

In which case you need to trace the input signal going to the TDA7269. You could try removing it with a soldering iron then tracing the appropriate audio input pin.
I have the pinout from the datasheet Bertus provided, so couldn't I just scope the input pin and see if there's a signal there vise the output pin, without desoldering?

Worst case - how much can you get a 5.1 system for? I got mine for free from someone who upgraded theirs - or you could look for a dead amplifier to fix.
Oh I really would like to get this working, for the accomplishment. I have a 5.1 system. I could always swap out this TV with the one I already got working and sell that one, but then I would feel like a failure.

Thanks again for all your help Tom.
 

tom66

Joined May 9, 2009
2,595
through DSP, then to to the switch, then to the jack? what's meant by basic protection? a fuse?
Probably a resistor and zener. Just in case someone accidentally puts 5V into it. Nothing much more. The jack usually has basic protection circuitry around it, or on a cheap set, nothing. The CD4066BE lets the microcontroller for the TV select audio channels to match the video channel; it is an analog switch. Then the switched signal will go to the digital signal processor - probably one of the big ICs on the board. Anyway, whenever whatever it does is done, the signals end up on pin 7 and pin 11.

Also, use your meter to probe between pin 3 and pin 6. You should measure from 10V to 22V (max.) Any more or less, the amplifier function is not guaranteed.

And, the finger test is helpful: is the amp chip hot? Too hot to touch?

You could scope the input pin but if the amp is dead maybe the input is shorted or some other component around it is faulty, but it might work.

I understand about repairing it... But don't fix it if it ain't broke! ;)
 

R!f@@

Joined Apr 2, 2009
9,918
If you want to check the IC, there is a better option.
First check for proper Supply Voltage.

Next check pin 5, this often is the culprit. I bet the IC is in mute. The pin 5 voltage should switch when you go from stby to on
 

Thread Starter

strantor

Joined Oct 3, 2010
6,782
Also, use your meter to probe between pin 3 and pin 6. You should measure from 10V to 22V (max.) Any more or less, the amplifier function is not guaranteed.
Measured 31.6V, but I think that is normal considering this amplifier seems to be set up in what the data sheet refers to as the "split supply" configuration, with a +supply and a -supply.

And, the finger test is helpful: is the amp chip hot? Too hot to touch?
Not hot at all

You could scope the input pin but if the amp is dead maybe the input is shorted or some other component around it is faulty, but it might work.
Measured signal 8V Pk-Pk on the inputs

If you want to check the IC, there is a better option.
First check for proper Supply Voltage.
Supply voltage seems good, 15.2V pin 3 to 9, -15.2V pin 1 to 9.
Next check pin 5, this often is the culprit. I bet the IC is in mute. The pin 5 voltage should switch when you go from stby to on
R!f@@!!! are you a practitioner of black magic?! I measured -4.2V between pin3 (+Vs) and pin 5 (mute).

form the data sheet:
MUTE STAND-BY FUNCTION
The pin 5 (MUTE/STAND-BY) controls the amplifier
status by two different thresholds, referred to
+VS.
- When Vpin5 higher than = +VS - 2.5V the
amplifier is in Stand-by mode and the final
stage generators are off
- when Vpin5 is between +VS - 2.5V and +VS
- 6V the final stage current generators are
switched on and the amplifier is in mute
mode
- when Vpin5 is lower than +VS - 6V the amplifier
is play mode.
I checked the software settings on the TV and it is definitely not muted. Even went to the users manual and checked all the settings it recommends for no sound and its all good. Had trouble tracing out the circuit; I can't tell what components are involved with unmuting it. I was considering cutting pin 5 off and soldering a little voltage divider onto it to give it a constant -6V. I would never be able to mute it again, but that would be better than having no sound. Any reason I shouldn't do this?
 

tom66

Joined May 9, 2009
2,595
My error. Supply is rated to +/-22V so up to 44V between Vs+ and Vs-.

Anyway, I would try connecting that pin to Vs- (no need to use a voltage divider) and seeing if you get working sound. Whatever circuit that drives that is probably fried; no idea how though. It's quite possible muting is done in the TV by means of setting volume to zero. Maybe the mute function is only used in standby to prevent an audible click or pop when turning off? Just speculating.
 

R!f@@

Joined Apr 2, 2009
9,918
Hmmm!! Voodoo mojo is here, I am a bit more experienced....besides who has repaired TV's the way I did?

be mindful My young padawan learner :D

U need to put the IC in play mode and check for sound.
First test the IC to see if everything s A-OK besides the mute.

After that I will let you know how I do things.

Patience...
 

Thread Starter

strantor

Joined Oct 3, 2010
6,782
... I would try connecting that pin to Vs- (no need to use a voltage divider) and seeing if you get working sound. Whatever circuit that drives that is probably fried; no idea how though.
I jumpered pin 5 to vs-, nothing. also tried jumpering to gnd, nothing. installed a 50KΩ pot between Vs+ and GND, wiper to pin5 and adjusted for 6v, nothing.

U need to put the IC in play mode and check for sound.
First test the IC to see if everything s A-OK besides the mute.

After that I will let you know how I do things.

Patience...
One of the 3 things I did should have put the IC into play mode (I think) but none did. I think the IC may be bad. Also the datasheet says no more than 650μA should be drained from pin 5, which makes me think it is an output pin and not an input pin, which confuses me.

While trying to put my o-scope probe on pin2 (output) I shorted something to the IC case and let the smoke out. now I have no 15V + & - supply to the IC like I used to. DOH! Now I have 2 problems to troubleshoot.

Is it time to order a new amplifier IC?
 

R!f@@

Joined Apr 2, 2009
9,918
Why in god's name are u probing the scope for.
Audio trouble shooting is the simplest thing.

U should have done what I said.
Now you have blown a good IC and it's supply fuses too. tsk....tsk....

Try to find any fusible resistors around the chip or trace the power pins to the source, You will find blown resistors or fuses.

Keep the IC in place for now. Replace any faulty fuses or low value resistors and see if the IC is getting power.
 

Thread Starter

strantor

Joined Oct 3, 2010
6,782
Ok, power supply is back online. Installed a new amplifier IC TD7269, same issue. -4.2V on pin 5, when there should be at least -6V to bring it out of mute mode. I painstakingly traced out the circuit on pin 5 which goes through several vias, everything SMD, under heatsinks, PITA. this is what I got:

I suspected that the transistor might not be functioning, so I put the jumper shown in red, still didn't work, actually got -1.15V on pin 5, so going the wrong way. I think there might have been something in parallel with RA133 because now I'm measuring 80Kohm across it.

To me, this should be as simple as putting a resistive voltage divider there to give pin5 -6V but I already tried that with the last TDA IC and it didn't work. What the heck is the deal?
 

Thread Starter

strantor

Joined Oct 3, 2010
6,782
Just desolder the mute pin. Leave it disconnected and check
that was the first thing I tried (on the old IC). I:
Desoldered it, nothing
tied it to ground, nothing
tied it to -Vs, nothing.
attached a 50Kohm pot from +Vs to GND, with wiper to pin 5 and adjusted for -6V on pin5, nothing.

I have not done any of this with the new IC; should I try it again?

In addition to the voltage spec for the mute pin, there is an attenuation spec. (60-70dB). I don't know what that means; could that be the problem?
 

R!f@@

Joined Apr 2, 2009
9,918
Do you have an Audio tester.
It is a little handy tool. Much better than a scope.

You can make one with a 1W power amp OPAMP and a miniature speaker. I built the whole thing into a PC speaker with a volume. Added a shielded probe at the input of the amp.
So I can poke any audio line with it as I would measure voltage with a DMM.

Try is and check if any input or signal is available at the input pin of the Audio IC.

Another way is to desolder the input pin, power up and simply touch the IC input pin while it is disconnected. If you can hear noise from the speaker when you touch the input than that stage is fine. Problem would be that no Audio is coming into the amp
 

Thread Starter

strantor

Joined Oct 3, 2010
6,782
Do you have an Audio tester.
It is a little handy tool. Much better than a scope.

You can make one with a 1W power amp OPAMP and a miniature speaker. I built the whole thing into a PC speaker with a volume. Added a shielded probe at the input of the amp.
So I can poke any audio line with it as I would measure voltage with a DMM.

Try is and check if any input or signal is available at the input pin of the Audio IC.

Another way is to desolder the input pin, power up and simply touch the IC input pin while it is disconnected. If you can hear noise from the speaker when you touch the input than that stage is fine. Problem would be that no Audio is coming into the amp
I don't have an audio tester or a 1W opamp, but I do have a set of battery powered laptop speakers with a 1/8 headphone jack. Do you Think I could cut that headphone jack connector off and replace it with a probe to get the same result?
 
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