Background
I'm repairing a monitor with a SMPS utilizing a UC3842 IC. I've tested the monitor in my shop and worked fine. When I get to the customer's site, it wouldn't startup. The difference is the input AC (My shop = 120 to 130 VAC. He's shop is 118VAC).

At his site, I tested the Vcc voltage to pin 7 of the UC3842. As soon as I touched pin 7, the monitor started right up, which made me believe that the conditioned triggered the UVLO to go at or above 16 VDC threshold to startup. I think this issue has something to do with a component in partial failure pulling down the voltage enough to stay around 15.25 VDC and not trigger the UC3842 to start

I've tested the startup resistors (2 x 82K Ohm), capacitors, and the MOSFET, but am stuck and not sure what else could possibly causing the issue. Any suggestions, or test techniques I'm not thinking about, would be greatly appreciated.

View attachment Neotec NT-500DX - SMPS Issue - UVLO.png

 

Probably the customer lower line voltage is the culprit. OR the thermistor/fusistor in series with the DC source could be a bit out of spec. You could try a small cap )0.001MFD) across one of the 82 K resistors to give it just a bit more startup voltage.

 

I replaced the NT501 (NTC) and one of the startup resistor (was a little out of spec but within the 5% tolerance). This appears to have resolved the issue. The monitor will now startup and run through its specified input voltages (90VAC thru 130VAC).

 

In my picture the green arrows show startup charging current. There is a small chance the two resistors have changed values. I design these to start up at 90V but .......
The red arrows show places where current might be going when it should not.
There is a diode on the right side that might be leaky. Probably not.
There is an Ele cap on Vcc that could be leaky. More likely.
There is a protection Zener diode that could have been damaged.
The IC could be damaged and pulling too much power.

I tested the Vcc voltage to pin 7 of the UC3842. As soon as I touched pin 7,

Maybe there is a bad solder joint at pin 7 or near it.

 

In my picture the green arrows show startup charging current. There is a small chance the two resistors have changed values. I design these to start up at 90V but .......
The red arrows show places where current might be going when it should not.
There is a diode on the right side that might be leaky. Probably not.
There is an Ele cap on Vcc that could be leaky. More likely.
There is a protection Zener diode that could have been damaged.
The IC could be damaged and pulling too much power.
View attachment 333789

Maybe there is a bad solder joint at pin 7 or near it.

Thank you for the diagram. I did check all those components and pathways as well (in/out of circuit) and they all checked out good. Seems it was the NTC501

 

NTC devices in malfunctioning equipment are always suspect, but not always the guilty part. God job on the diagnostics.