Hisense F42K20E Hum

[revenerd]

With the TV on and not connected to any inputs, I get a hum. Increasing, decreasing, or muting the volume does not change it. It hums through the headphones as well.

The hum slowly and slightly goes up in pitch, then comes back down. On the blue screen, a click is heard every time the words "no signal" moves.

If I connect a device to the HDMI port, the hum gets much louder. The video's audio still plays and if turned up enough, it drowns out the hum.

I read the thread about this model and a static noise. I listened to the audio and it is different than the hum I'm getting. I tried that solution of making sure the boards were clean and I did not help.

[revenerd]

Re: Hisense F42K20E Hum

I have a little more time to look at this issue. The sound IS more static-like than I remembered.

When powering on with no devices connected, I get the blue screen with the words "no input" jumping around. The words are motionless, then jump to a new location. On each jump, I hear the static.

Any change on the screen results in the static sound. I.e. When the numbers change while adjusting the volume, pressing mute, changing the channel, moving around in the menu.

I muted the TV and the static sound is still there in the same way.

When I power off the TV, I hear the static for a few seconds after the screen turns off.

Using an HDMI source with a video, the static becomes constant. Audio is heard, but the static is loud.

I have reset the TV (static is there throughout the initial setup), increased the brightness to max, removed the power board and cleaned with alcohol and toothbrush (worked for another post about this model), and listened through the headphone jack (static is the same).

[revenerd]

Re: Hisense F42K20E Hum

Attached is a recording of the noise. The pop is when the 'No Signal' text moves on the screen.

[triplefour]

Re: Hisense F42K20E Hum

interesting problem. never seen this on any tv i worked on but i havent fixed a lot of hisense tvs. somehow the electrical noise maybe from a coil or transformer? from somewhere (psu or mb) is making it into the speakers. Hopefully someone smarter than me can offer you more help!

[revenerd]

Re: Hisense F42K20E Hum

There is an inductor on the power board I can hear buzzing. It is located to the right of the long cap on the power board and is labeled LEI-302B.

Could that be the issue?

[Diah]

Re: Hisense F42K20E Hum

There is an inductor on the power board I can hear buzzing. It is located to the right of the long cap on the power board and is labeled LEI-302B.Could that be the issue?

buzzing at PSU caused when there are load.. the most i found its at ribbon cable between T-CON and panel or LVDS . as yours LVDS are wires.. i would say start to clean pins and check after soldering... also if you have T-CON just disconnect the panel from it and turn on see the sound / buzzing if there any change.

[revenerd]

Re: Hisense F42K20E Hum

Buzz is still there after disconnecting TCON. Also attaching TCON board picture.

[Diah]

Re: Hisense F42K20E Hum

then keep remove load and turn on.. for example wifi BT speaker
BTW yours photo of MB C70 100uf 16V its formed on the top shape or its light shadow

[revenerd]

Re: Hisense F42K20E Hum

I disconnected everything I could. I also unscrewed the boards from the panel and placed a rubber mat under them to see if was something coming through the chassis. Even with the speakers disconnected, the sound comes through the headphone jack.

I checked the cap C70, it looks ok. No different that the others.

[CapLeaker]

Re: Hisense F42K20E Hum

Poke around with an oscilloscope.

[revenerd]

Re: Hisense F42K20E Hum

Poke around with an oscilloscope.

Unfortunately, my knowledge, experience, and tools do not go that far.

[revenerd]

Re: Hisense F42K20E Hum

The digital out is clean audio. I also noticed the left side of the screen is slightly darker than the right, it's almost imperceptible. If any of this could be related.

[Davi.p]

Re: Hisense F42K20E Hum

I think there is something related to the microprocessor, maybe a partial desoldering or a leakage between pins (dirt?) of it or the amplifier chip.. take a proper bay, sorry i don't remember the name, put the main board inside and fill it with alcool and let it stay for many minutes, then wash it and dry it, or another way let it boil a few seconds with distilled water or water + vinegar

[alfatv]

Re: Hisense F42K20E Hum

Those electrolytic caps on main board are likely to fail. Could be some other causes. Even with a scope it could be tough to troubleshoot.

[Davi.p]

Re: Hisense F42K20E Hum

is there a silicon pad on the cpu?

[revenerd]

Re: Hisense F42K20E Hum

Yes, there is a pad on the underside of the CPU heatsink.

I removed the soldered on metal cage around the CPU chip and removed the heatsink. I cleaned around the CPU with alcohol and a toothbrush, saturating the area well. I allowed it to dry then reconnected. The hum and click was still present but drastically reduced. I connected a video source and when audio was playing, the noise was imperceptible. I reassembled everything.

After reassembly, I conned a source and started playing video. It was normal, however, after a few minutes of playback, the buzz returned. It is there, distraxtively, through all of the audio.

Also, on a white screen, the left half of the screen is slightly darker. It's not noticeable during normal playback.

I've attached a picture of the CPU.

[Davi.p]

Re: Hisense F42K20E Hum

Are you able to reflow the cpu with soldering iron stick?

[revenerd]

Re: Hisense F42K20E Hum

I've never tried that on pins that small and close together. Is it just running the tip across them to heat them up?

[Davi.p]

Re: Hisense F42K20E Hum

apply much flux, use a salami flat round tip, or a common conical 1mm tip, ad a bit of solder tin, run over the pins slowly, but there are plenty YouTube videos about..