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TDK Power Supply -5V issue - recap done

chuck42

New member
Hi guys,

after fixing 2 Mac LC TDK (699-0153) power supply by re-capping, I am having a issue.
5V and 12V are perfectly stable, but -5V fluctuates "wildly" between -3 and -5V, averaging at -3.9V.
No changes if under load or no load.

Does anyone have schematics for the TDK power supply?
Any hints maybe ?
 
How clean did you get the board after the recap? Cap juice is both corrosive and conductive, so any left on the board can give variable results.
 

chuck42

New member
How clean did you get the board after the recap? Cap juice is both corrosive and conductive, so any left on the board can give variable results.
I did clean it really thoroughly with IPA on both sides... I had some suspicion on the Diodes CR50 and CR51, but the measured fine. I also changed the voltage reference IC TL431, but no changes so far :-(
 

chuck42

New member
I just recapped another TDK, Exactly the same. +12 and +5 are stable and fixed. But -5V swing between -3 and -5.

Any schematics on the -5V rail part ? Thanks
 

RetroViator

New member
Count me in on this problem. I have two TDK PSUs, one recapped and another not. Both have stable +5V and +12V, but -5V swings without a load, and when connected to the logic board, both PSUs hold steady around -2.2V or so. Since that's half what is needed, it's tempting to think some component is amiss. The schematics posted by @SuperSVGA at LC TDK Schematics are helpful, but I haven't found anything yet.
 

SuperSVGA

Well-known member
Did you take a look at IC51? Either it's not getting enough voltage on input pin 2, something is drawing too much current on the -5V (other faulty components such as capacitors), or the UPC78N05 is faulty.
 

RetroViator

New member
Well, the PSU was good. I was testing this on an LCIII, which I had recently recapped, replacing the electrolytics with tantalums. But, I feel prey to installing C22 as marked on the silkscreen, when apparently it should be installed backward as discussed here. After switching around C22, I'm getting a steady -5.1V. Thanks @SuperSVGA for weighing in.
 
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