Grundig Vision 9 37 9880 T USB (Chassis SR) TV Repair Help - Distorted Subwoofer Sound and overall weird sound waveforms

[LEECH666]

I couldn't find any mention of it being LED based anywhere so I assumed it's still CFL based which may be incorrect. Anyway I connected the 1K between +5VS and STBY+BL pins. Here goes nothing.

[Diah]

Thank you again for your help so far. Your short hand style of writing is a bit hard for me to decypher. I assume when you say "if the BL didnt power on" you mean "if the back light (24V to the inverter board on the left and the CFLs) do not power on", is that correct? The rest I think I understand. Anyway time to bust out the soldering iron.

LOL.... yes BL we used here for back light... and MB to main boards.. yours TV LED one not CFL... so on the web specification

[LEECH666]

Thank you again for your help so far. Your short hand style of writing is a bit hard for me to decypher. I assume when you say "if the BL didnt power on" you mean "if the back light (24V to the inverter board on the left and the CFLs) do not power on", is that correct? The rest I think I understand. Anyway time to bust out the soldering iron.

[Diah]

so yours PSU Socket i will rename the pins to be more understandable
5VS = STBY .. STB= Power _ON BL = BL_ON DIM=DIM for BL
so STBY via 1KR connected to Power_ON & BL_ON ... if the BL didnt power on... you need to add DIM to the same rail of the 1KR.
PSU will start On. and you will have 12 & 24V... if the buzzing remain... disconnect the BL socket from PSU and check.the noise..
if PSU & BL power up and No buzzing... keep this mode test... disconnect the LVDS between the MB and T-CON if you have external T-CON.... take from the socket of PSU 12V line and fed the T-CON Fuse F1... check the buzzing... if no more buzzing mean the issue at MB.. i so we do narrow the issue to be sure which part make the overload on PSU

[LEECH666]

+5VS seems to be too be the standby supply voltage as it is the only voltage being present after the TV is switched to standby via the remote.

So if I understood you correctly I should connect the +5VS pin via a 1K resistor to the STBY and BL pin like shown in the attachment?

[Diah]

Hi Diah, thanks for your reply. I have to admit that I don't know too much about TV PSUs. I read in the basic TV repair guide that the TV PSU waits for an "ok turn on"-Signal from the logic board. I assume this is the STBY labled pin on the connector that goes to the logic board but how do I get it to power up when the logic board is not connected? The STBY pin measures around 2.9V during operation when everything is connected ... so do I just apply 2.9V to that pin via an externally connected lab supply to turn it on?

PSU with out Main boards can run as stand alone... disconnect the MB from the PSU socket.. plug in wall messure yours socket you will have only one V STB usually 3-.3V -5V. in yours case i think its 5VS will be... connect via 1KR to in yours case STBY ... and to too BL ( which i think it BL_ON and Dimm... yours PSU will start and power up the BL and you will have on the socket 12V pin & +24V ( this for sound )

you need to chevk first just plug in wall and the V which exsist will be PSU stand buy V. and the other Pins will be Power_ON ( this will switch 12V & 24V )

[LEECH666]

Hi Diah, thanks for your reply. I have to admit that I don't know too much about TV PSUs. I read in the basic TV repair guide that the TV PSU waits for an "ok turn on"-Signal from the logic board. I assume this is the STBY labled pin on the connector that goes to the logic board but how do I get it to power up when the logic board is not connected? The STBY pin measures around 2.9V during operation when everything is connected ... so do I just apply 2.9V to that pin via an externally connected lab supply to turn it on?

[Diah]

Buzzing Noise at PSU could be Filter leak.... or Overload to narrow the issue. run the PSU in stand alone test.. if the Buzzing Noise disappear... then the issue not at PSU

[LEECH666]

Some scope screenshots attached ...

[LEECH666]

Good day ladies and gentleman!

I am currently trying to repair a Grundig Vision 9 37 9880 T USB LCD TV for a friend that has a weird audio problem. Basically everything works fine except for the heavily distorted sound when the Subwoofer module is connected and there is a buzzing sound coming from the PSU PCB.

This seems to be a rather rare TV so there is not much prior on topic help to be found however a service manual is available at the usual sources (Electro Tanya i.e.).

I think the PSU buzzing comes from one of the coils near the bridge rectifier but I am not sure if this really is a problem or just a very noisy power supply. I do have professional experience with electronics but I am certainly not a TV repair guy. The output DC voltages from the PSU PCB seem to be fine. So while annoying for the moment I think this might just be severe coil whine and the issue lies with the sound chip or main logic board.

The audio system seems to be divided into a sound bar (sound projector) that's mounted beneath the screen and a subwoofer module that is mounted to the back shell / housing of the TV.

When the TV is turned on after being turned off at the power strip level a farting like noise is emitted from the sub module but that goes away shortly after.

https://www.youtube.com/watch?v=eqRuUCotLAI

My approach to test the audio was to listen to some music which sound distorted as if the sub speakers are damaged however I testes the subwoofer module on the bench with an external amplifier and the speakers seem to be fine. No damage to the rubber surrounds and tone is fine. No scratching. Coils seem to be able to move freely.

The next thing I tried was to feed a 20Hz to 20Khz sine sweep (found on youtube) to the system via HDMI3 and it sounds horrible. More like clicking or rattling in the lower frequencies. When the frequency gets higher the sound is more normal for a sine signal.

I also measure total garbage signals even with a steady 1kHz sine wave on all terminals (headphone out, subwoofer speaker terminals, tweeter speaker terminals) with my new Rigol DHO804 scope.

Measurements with DMM on the PSU PCB's connector to the logic board:


+5VS, 5.214V
GND, 0.01V
BL, 3.265V
DIM, 3.290V
+24V, 23.96V
STBY, 2.946V
+12V, 11.899V
GND, 0.0214V

Measurements with DMM on the PSU PCB's connector to the backlight inverter board:


GND, -0.0042V
+24V, 23.95V
BL, 3.265V
DIM, no wire connected

Some Videos that I've shot with some scope measurements down below.

Grundig 37 9880 T USB PSU Buzzing Noise

Grundig 37 9880 T USB Farting Noise
https://www.youtube.com/watch?v=eqRuUCotLAI

Grundig 37 9880 T USB 100Hz Sine Garbage at the Headphone Out
https://www.youtube.com/watch?v=5PlE8wRZYbk

Grundig 37 9880 T USB 100Hz Sine Measured (at Sub Module)
https://www.youtube.com/watch?v=_xRJSTvvYiE

Grundig 37 9880 T USB Thermal 1
https://www.youtube.com/watch?v=QBh1jmPa3t8

Grundig 37 9880 T USB Thermal 2
https://www.youtube.com/watch?v=e62o-2_bwEA


I suspect that the STV8357FSX sound processor is damaged but I don't know if this could also be some other issue related to the PSU buzzing. Help is much appreciated.