Macintosh LC TDK PSU troubleshooting

[bibilit]

I am trying to repair a TDK Psu, i want the LC to keep as original as possible.

new caps around, the PSU is temperamental.

most of the time the PSU won't work, touching the solder joints kick the PSU, but 12 and 5 volt rails are not very steady, the only stable rail is the -5 volt.

Opto has been replaced, but problem remains.

any idea ?

 

[techknight]

Sounds like a hallmark sign to a drifted or open start resistor. 

There is a resistor that kickstarts the power supply by providing temporary VCC to the controller until the SMPS can get into regulation, which then takes over and powers the controller. 

Or the controller IC itself is bad. 

 

[bibilit]

Ok, Thanks, any location for this resistor ?

have not been able to locate schematics for this PSU.

 

[techknight]

Nope. I am speaking in general from my knowledge of repairing SMPS units. 

What I typically do in cases where I dont have schematics, is to look up the chip numbers on all the chips and pull datasheets. Most engineers design circuits using the chips supplied datasheets, sometimes they take the reference design as-is, or sometimes they may make changes and add things to it. 

 

[bibilit]

Issue solved. 

The shunt regulator (HA17431P) was shorted. 

Working fine after replacement. 

 

[techknight]

Thats a first, for me anyways. 

At least you figured it out. 

Is it a TL431? 

 

[bibilit]

Yes it is 

 

[marcelv]

had a similar problem but a different solution

the power supply would not start after the recap, only sometimes, when working it remained working. But next day (a cold start) t would not power on.

Problem appeared to be cap goo under the diodes and resistor next to the shunt regulator.

The cap goo was not visible but when I desoldered the parts it came out sissling (if that is the correct term)

After cleaning and placing the parts back, the power supply is now working ok.

on the pictures the cap goo is visible after desoldering and present on the legs of the diode

 

[bibilit]

Any idea what reference this diode can be ? think i have a leaky one on another PSU