AOC LM2028 - no voltage

[neutrino68]

Re: AOC LM2028 - no voltage

Ok, here's my readings, all relative to pin 1:

Pin 2: 1.49V stable
Pin 3: 16.65V stable
Pin 4: 1.29V stable
Pin 5: 0V
Pin 6: 0.01V to -0.01V (constant and fast fluctuation around 0V)
Pin 7: 13.48V stable
Pin 8: looks like Pin 6

[jetadm123]

Re: AOC LM2028 - no voltage

You might try checking the current sense resistor off of pin 6. It's probably a 2 Watt resistor that's 0.33-0.39 ohms. Check the resistance. If this resistor is bad, it will prevent the SG6841 from starting up.

[neutrino68]

Re: AOC LM2028 - no voltage

Here is what I think is the relevant portion of the datasheet.

== snip ==

Under-voltage lockout (UVLO)

The turn-on and turn-off threshold of SG6841 are fixed internally at 16V/10V. During start-up, the hold-up capacitor must be charged to 16V through the start-up resistor so that SG6841 will be enabled. The hold-up capacitor will continue to supply VDD before the energy can be delivered from auxiliary winding of the main transformer. VDD must not drop below 10V during this start-up process. This UVLO hysteresis window insures that hold-up capacitor is adequate to supply VDD during start-up.

=end snip ==

This means 13.48V on VDD is too low for the controller to be enabled, then? Or is it 16V initially and drops to 13.4 after the startup process?

[neutrino68]

Re: AOC LM2028 - no voltage

Ok, I think that resistor is between the heatsink and the huge cap beside the rectifier bridge, it's covered with heat-shrink tubing so I can't see its resistance value. I'll peel it tomorrow and check it. Now to bed.

[retiredcaps]

Re: AOC LM2028 - no voltage

This means 13.48V on VDD is too low for the controller to be enabled, then? Or is it 16V initially and drops to 13.4 after the startup process?

The same thought you had occured to me as well.

If this were the enterprise networking world, I could easily, in a few seconds, tell you by hooking up a couple of protocol analyzers. The protocol analyzers would capture the initial packets and I could tell you.

We may be able to deduce if the SMPS has started or not by looking at the other voltages.

Back to some more reading.

[PlainBill]

Re: AOC LM2028 - no voltage

OK, I'm going to come to the rescue. You've done a good job of troubleshooting, mainly following the same path I went through the first time I ran across one.

The idea of a bad cap on pin 7 is good, but if it's dried out and the capacitance has dropped the usual symptoms are a pulsing voltage at the outputs and at pin 7. The voltage at pin 3 comes from the big cap, dropped through one or more resistors. The app note calls for a total of 1.5 megohms. I think this uses a pair of 430K SMD resistors and a 680K through hole. Notice that the voltage on pin 3 is sort of low. The datasheet recommends a target of 20 volts with a maximum of 30 volts. There is a zener diode in the chip to set that limit. Note that at 16.65V it really can't pull pin 7 up to the start threshold.

Now there are two possibilities. Either the resistors have increased in value, or the current draw has increased. It was about this point where someone else stepped in and pointed out that he had repaired a number of monitors by replacing the SG6841.

PlainBill