Vizio M421VT LCD TV No Sound Issue

Thread Starter

Maverick923

Joined Feb 4, 2019
13
I really would appreciate some help with the Vizio M421VT. The tv has no sound, and I have done everything to check for audio including using an external speaker on audio out. As far as I can get is sound when flipping channels like a like when changing stations on a radio. I hear something for less then a second then nothing. The channel source is antenna.

I purchased a mainboard second hand, and when installed, the Vizio logo blinks 24 times, then goes solid. After going solid, the led goes dim, and starts the process all over again blinking 24 times in a loop.

The seller swears the board works, but I am unable to figure out why the original board works with no audio, and the replacement does not even produce a picture.

I was wondering if anyone can give me some helpful info?

Yes the parts matched by part number.
 

abrsvc

Joined Jun 16, 2018
52
Disconnect the speakers and try again. What we are looking for here is whether or not the TV proceeds to showing channels etc. If so, then perhaps a speaker is defective (shorted) and causing the amp section to draw too much current, thus shutting down the set. I believe that the audio amp section is on the main board, so at this point I would not suspect the board, just a speaker or two.
 

Thread Starter

Maverick923

Joined Feb 4, 2019
13
Disconnect the speakers and try again. What we are looking for here is whether or not the TV proceeds to showing channels etc. If so, then perhaps a speaker is defective (shorted) and causing the amp section to draw too much current, thus shutting down the set. I believe that the audio amp section is on the main board, so at this point I would not suspect the board, just a speaker or two.
I receive channels, but no audio. I will try your suggestion, and report.
 

wayneh

Joined Sep 9, 2010
16,773
Are there menu settings that could be muting the internal speakers and/or diverting the audio signal to an output port? If you crank the volume do you hear a slight hiss? That usually means the audio amp is working but has no signal.
 

abrsvc

Joined Jun 16, 2018
52
Muting the speakers would not address shorted speakers. The connection would still be there but with no signal. This would be similar to a regular receiver entering protect mode without any signal. The shorted outputs in that case would still cause a problem even without a signal.
 

wayneh

Joined Sep 9, 2010
16,773
Muting the speakers would not address shorted speakers. The connection would still be there but with no signal. This would be similar to a regular receiver entering protect mode without any signal. The shorted outputs in that case would still cause a problem even without a signal.
My point was just that it can be easy to make the sound go away with menu settings. I have a TV that does it fairly often and I have to try to remember the magic trick to fix it. Always exhaust software solutions before digging into hardware.
 

abrsvc

Joined Jun 16, 2018
52
OK Makes sense. I would agree if the TV performed normally but with no sound. That is not the case here. The TV is stuck in a loop that suggests that it is being told to reset. A shorted speaker(s) could do this, thus my recommendation to disconnect them.
 

Thread Starter

Maverick923

Joined Feb 4, 2019
13
Disconnect the speakers and try again. What we are looking for here is whether or not the TV proceeds to showing channels etc. If so, then perhaps a speaker is defective (shorted) and causing the amp section to draw too much current, thus shutting down the set. I believe that the audio amp section is on the main board, so at this point I would not suspect the board, just a speaker or two.
So I tried what you said, and I get nothing. I did hear something for a sec when changing the audio settings to variable, and switch between PCM, and Dolby produces audio using an external speaker. All this is what is going on is with the original mainboard.

I had purchased a used replacement which will not do anything at all besides making the Logo blink 24 times in a loop, so I was thinking perhaps there was something I needed to do in order to get the replacement to function correctly. I have not found any answers.

The original mainboard is the one with the audio issue. With that installed, I get the Vizio logo to blink 36 times before it goes solid, then dim, and then the tv can be turned on, but with no audio. I am using the antenna, and I get no audio no matter what I do. Yes I have made sure the speakers were enabled, and not muted. I have tried bluetooth, and that does nothing. The only way I hear anything for a spit second is with the external speaker.
 

wayneh

Joined Sep 9, 2010
16,773
The only way I hear anything for a spit second is with the external speaker.
It produces 'normal' audio for that split second? Your symptoms suggest the audio amp may be working OK but it's not getting a signal from the main board. I know that's not very helpful in getting to a solution.

In that Service Manual I linked, there are a couple of procedures to follow for checking the DC-DC converter and to diagnose the audio amp. Have you looked at any of that?
 
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