Dear EEV Friends,
I have many years' experience in SMPS and general electronics design and repair, and generally know my limits, but am a little stumped by this little bugger (see attachment).
Basically, there is a 5.6V standby circuit, which is used to produce a P_ON signal which switches everything else on.
Everything else does come on and is well within tolerance, except the +/-28V and -5V supplies generated by the MR4040. This will come on intermittently, most often not though.
All the standard culprits have been checked - electrolytic capacitance and ESR checked, diodes, optocouplers, resistances, dry joints. MR4040 has been replaced. Nothing. I've partially localised the issue (I think) - the measured Vcc on pin 4 of the MR4040 is at around 7V when it doesn't switch on, but is at +11V when it is functioning. I can't find any info on the Vcc requirements for the MR4040, though.
You can see that during the transition from standby to on, initially the Vcc is supplied from a rectified output on the primary of the ICE3B1565J via RB16, DB6 and RB10 (all checked), but I don't understand how this voltage can be too low if all the secondary voltages are correct.
I'm beginning to suspect the ICE3B1565J, a little out of desperation rather than logic...
Any suggestions or ideas?
This appears to be a common failure mode for this particular board based on internet searches (power LED flashes white/red, sometimes starts if allowed to "rest" for a few days), however I could not find a solution...
Cheers
Ilya