TDK Power Supply -5V issue - recap done

[chuck42]

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 ?

 

[SpocksBeer]

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]

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]

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]

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]

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]

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.

 

[lostone]

Did anyone with the unstable -5V condition get things resolved? It looks like RetroViator found a unrelated fix.

I'm with chuck42 unloaded goes all over while +12 and +5 are rock solid. I've cleaned the board several times, but didn't depopulate the entire board which may be needed.

I saw the comment on IC51, but on the schematic in the other thread looks like they're using a positive voltage regulator C78N05 in the reverse configuration so regulator high output tied to ground supply, and regulator ground tied to -5V line. Or that's what I'd expect.

What's the expected input value on pin2? Looks like it needs to be at least 7V but expect more likely 12V. Where do I find the specs on the MT1311 and its pin 5 output? Should I isolate IC51 to check its output? I could hook it to a DC supply and test if needed.