I have an AMW monitor F199B that occasionally shows a high frequency interference signal on the display screen. Have replaced the BLM1700M10112 board with no change in the behavior. The remaining board is an FSP035-1PI01ZT power supply. All the caps have been replaced on the remaining board with no change in behavior. The attached file has two schematics, the first referred to as (open frame) appears to be accurate to my case but the second referred to as the inverter uses different IC's relative to my case.

Since there is only 3 feeds from the power board to the open frame board, being a 12-volt supply, ground and inverter, I might guess that the high frequency is coming in on the 12-volt supply. Could the problem be the opto coupler 817B (IC330 on schematic)?

 

This is where I like to use my small hand held oscilloscope with a small piece of wire in the probe as a detector. Wand the probe around looking for the noise source. If it is on the board then narrow it down with an RF probe on the signal tracer. Basically is it on the board or not? Once found then go about attenuating the noise. I have seen external noise coming from keyboards and CB radios just to name a few.

 

This is where I like to use my small hand held oscilloscope with a small piece of wire in the probe as a detector. Wand the probe around looking for the noise source. If it is on the board then narrow it down with an RF probe on the signal tracer. Basically is it on the board or not? Once found then go about attenuating the noise. I have seen external noise coming from keyboards and CB radios just to name a few.

I may be wrong, but I do not think I would describe this interference as noise. There is more of a sign wave look to it, maybe even possibly a 60-cycle content. Maybe if I can capture a screen picture with the disturbance that would help.

 

I may be wrong, but I do not think I would describe this interference as noise. There is more of a sign wave look to it, maybe even possibly a 60-cycle content. Maybe if I can capture a screen picture with the disturbance that would help.

Here are some pics of with and without the interference,

 

All interference is noise which is why it's called the signal to noise ratio. The goal is to squelch/attenuate the noise to get a clean signal.

 

Here is the latest; I resoldered all electrolytic capacitors that I had previously replaced hoping that would solve my noise problem. Now I cannot even get the unit to turn on. Question, the power supply has an SG6841 (U320 in drawling) which controls a MOSFET to control a second phase of the power supply. Should the SG6841 supply the 12 volts when the unit is first plugged in? I get zero. And if it does not supply any power when first plugged in how does the attached additional card which has the on button perform that function. I have confirmed most of the components with exception of the SG6841 unit and the Q320 MOSFET. Any thoughts would be appreciated as I do not know how to verify the SG6841 short of replacing it.

 

Replaced the SG6841 chip and all is corrected.