Macron ATX9912 standby issue

[Majestyk]

I recently had some fireworks inside one of my Macron MPT-350P psus. The switching transistor for the standby voltage had failed, 3 resistors were burned, the driver transistor exploded, one zener-diode shorted and finally the 2R2 MOX resistor interrupted the power supply for te standby circuit and put an end to the destruction.
I cleaned everything, replaced all defective components, turned the unit on and the damage happened all over again a second and a third time.

The switching transistor Q2 is a 2SC5027 (Fairchild). I ordered some that are made by "isc".
These "isc" types are obviously fake transistors, they overheat and fail within a couple of seconds if they are exposed to the voltages they are originally designed for.
I tested 2 Fairchild 2SC5027 from my other Macron units (DTK labelled) and everything´s working flawlessly.

Is there any source where I can get true Fairchild NOS transistors or does anyone of you know a good equivalent for this transistor that is still available for a reasonable price today?

[goodpsusearch]

Re: Macron ATX9912 standby issue

2SC3150

It is used in Macron Power 300W version, so definitely compatible.

[R_J]

Re: Macron ATX9912 standby issue

It could be Ksc5027 by Fairchild (On semi)
I just looked at the specs on iscsemi 2sc5027 1.3 w .1 amp 300 Vceo. not even close to what you need, I think they are the Toshiba 2sc5027 (horz. output transistor)
If you go to iscsemi and compare the 2sc5027 and the ksc5027 you will see the difference. so not fake just wrong transistor

[everell]

Re: Macron ATX9912 standby issue

I had a similar experience on the very first power supply I worked on, a Bestec ATX-250 12E. I replaced bad parts, turned it on, and it blew up. Did the same thing again with same results. Being frustrated, I gutted the 5vsb two transistor circuit, then built an outboarded IC circuit on a small piece of perf board using a DM311 chip. I wired it into the existing transformer. Powered it up, it worked, and has been working ever since. I have it in an old "standby" computer, and it is still working since 2009.

I tried making the same outboard 5vsb circuit on Antec Smart Power, but found that the DM311 chip did not work very well due to circuit loading in the feedback. So used a Viper22A and all worked well.

Once a two transistor 5vsb circuit fails (and takes out several components), I don't attempt a repair any more, I just build an IC circuit.