Electronics > Repair

SMPS Troubleshooting. About to give up...

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Milmat1:
Background, this is a SMPS board from a DRO system on a milling machine. It's an YCL PCB153 board, No schematic available.
It is an KA3842 PWM chip. / triple output +5V, +36V, +6V   (all outputs dead)

At the bridge rectifier, I have 120VAC in and 165.3 VDC across the bulk filter cap. (100uf/400V)  So I assume everything ion the primary circuit is OK (??).
I have reverse engineered the part of the board dealing with the chip and found it different from any of the sample drawings on this chip. But still a basic SPMS design.
I recapped the board, found an "Open" startup resistor (180K) and replaced it. Found the startup cap(86uf) had a very high ESR . I have checked all the silicon, i.e. Diodes, Zeners etc.
An R/C circuit between pin 4 and pin 8 is supposed to set the output freq. They are fine.
The chip will not start. pin 6 (output) is just dead and nothing going to the gate of the MOSFET. (And voltage across D/S is exactly the same as the DC across the BFC).
 EDIT: Also pin 8 should be a 5VDC refference voltage, it has no voltage present. Should this be present all the time when Vcc is present ?
The Vcc to the chip (pin7) looks like the attached pic.  Is this normal for a Vcc startup until the aux winding can take over ?

How likely is it this chip itself has failed, is there a way to test the chip to confirm ?

Any help or direction appreciated...
Thanks, Matt

kripton2035:
there is usually a small cap (around 10uF) between pins 7 and 5. often this cap has high esr or too low capacity.
it's often worth to try to replace it.

Milmat1:
There is an 82uf between pin 7 and pin 5 (gnd).  It did in fact have very high ESR, but I replaced it.

Bud:
Check the chip for shutdown condition

--- Quote ---Shutdown of the KA3842B can be accomplished by two methods; either raise pin 3 above 1V or pull pin 1 below a voltage
two diode drops above ground
--- End quote ---

Milmat1:

--- Quote from: Bud on August 16, 2022, 04:24:57 pm ---Check the chip for shutdown condition

--- Quote ---Shutdown of the KA3842B can be accomplished by two methods; either raise pin 3 above 1V or pull pin 1 below a voltage
two diode drops above ground
--- End quote ---

--- End quote ---

Pin 3 = 105mv
Pin1 = 6mv

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