Fluke187 LCD turns Off when measuring AC VOLTAGE

[Ayoublee]

Hello everyone,
I was repairing a TV and measuring voltages in AC+DC mode as usul, suddenly the lcd turns off then turned back on once I removed the probes. I figured out it happens only for AC voltage measurements (either in ac mode or ac+dc one).
When selecting mAC the lcd reading is full barre reading, but once measuring in AC or AC+DC an AC voltage the digits disappear as far as the probes are on the test points. DC voltage is measured without problem and doesn’t cause the lcd to turn off at AC mode.
I changed the batteries and made factory reset, nothing changed.
I opened the multimeter nothing suspicious, I saw an bridge rectifier Df068 and I doubt if it’s the cause of the problem. I don’t have any other multimeter to measure the components.
The readings are normal when the probes are shorted or open. Again the problem appears only when measuring ac voltage (case 220V ac)
Thank you.

[coromonadalix]

Ac voltage(s) in what ranges ?  not higher than 220vac as you wrote ??

Smps  supply sections ?
Flyback section ?
Spikes
Motor(s)  ??

[Ayoublee]

Hi coromonadali,
Thank you for the reply. I talked to a colleague of mine and he suggested a miraculous solution: clean the area under the knob. Believe me it worked !!!! 
No idea why but it could help the community so I posted hier.
Regards,
Thank you