Macintosh Performa PSU wrong voltage

[greeneminence]

I recently bought a Performa 450 which was in very good condition, no visible leakages, blown-up batteries etc. When I power it on, I can only hear a clicking sound from the speaker. Fan is working correctly. I measured output of the PSU and so far it looks like I'm getting wrong voltage on +12V and +5V lines. It fluctuates around +8.5V and +2.8V respectively. -5V line shows correct voltage though. Do I have faulty voltage regulators or should I recap the PSU? It's one of the Dyna Comp (not TDK). Close-up photos attached.

 

[BadGoldEagle]

Hi

If you had faulty voltage regulators, you'd most likely have nothing at all on the 5V or the 12V rails. 
As ever with those old machines, caps fail. Even if they're not swollen it doesn't mean they're fine. Low voltage typically suggests weak caps, so recap first and if it still doesn't work, then we'll have to do some troubleshooting...

 

[greeneminence]

Thanks for the suggestion. I desoldered all caps in the DC voltage section and here's a complete list of values - maybe someone gonna find it useful:

• C15 - 330uF / 35V
  
• C16 - 82uF / 35V
  
• C18 - 2200uF / 10V
  
• C19 - 1000uF / 10V
  
• C20 - 1000uF / 10V
  
• C21 - 82uF / 35V
  
• C22 - 82uF / 35V
  

All caps are rated at 105 degrees centigrade. Manufacturer is NIPPON, series LXF, which is discontinued. They recommend LXY capacitors as replacement, as they're low impedance and long life-type caps. Unfortunately, I can't find a supplier of those in my country, so I guess ebay is my go to place for now.

 

[greeneminence]

Small update - I replaced all caps in the power supply (except the huge one) but it still doesn’t produce correct voltage. Should I replace the voltage regulators?

 

[greeneminence]

I also noticed something weird - there's a missing resistor (marked R12) in my PSU. Can someone confirm if this is by design?

 

[ravuya]

That's very odd. If it were simply unpopulated I would expect it to be just empty holes, not two resistor legs dangling in the breeze.

What does the other side of the board look like there?

 

[bibilit]

Dyna comp are not prone to capacitors issues. 

Low voltages are more a regulation problem. 

Have a look at the Opto 

 

[greeneminence]

My attempt to fix the power supply was partially successful. I replaced two diodes (B1045, STPR810D) inside the PSU and I'm getting kinda OK readings without a load (10.8V on 12V line, 4.6V on 5V line). When I attach the load (connect the motherboard) all voltages drop significantly (i.e. from 4.6V to 1.8V). I'm not sure if this is still PSU issue or something on the motherboard is shorted.

 

[seven]

Have you already equipped the R12 bracket?

I am currently assessing a dead Dyna PSU from a Performa 450, where R12 is populated with a 4.7k resistor (I assume, since the colours are somewhat hard to tell).

On my board the parallel resistors R12 and R13 added up to about 0.762k.

 

[Tarantulas]

Dyna comp are not prone to capacitors issues. 

Low voltages are more a regulation problem. 

Have a look at the Opto 

Opto?

 

[Nathan]

Opto?

Could be referring to an optical isolation device. They are a circuit component that allows transferring signals from one point to another without a conducting electrical path by the use of an optical sensor pair.  

https://en.wikipedia.org/wiki/Opto-isolator  

Assuming I am reading the markings correctly those two 4-pin ICs next to the transforming are probably these:  

https://www.farnell.com/datasheets/73758.pdf  

P.S.  

Just my barely informed two cents, but it's probably worth checking the supply voltages on all the ICs just to be sure they're getting the power eneded to be functioning correctly. The same goes for verifying the output of regulator ICs to make sure it's what it should be, at least voltage-wise.

 

[greeneminence]

Have you already equipped the R12 bracket?

I am currently assessing a dead Dyna PSU from a Performa 450, where R12 is populated with a 4.7k resistor (I assume, since the colours are somewhat hard to tell).

On my board the parallel resistors R12 and R13 added up to about 0.762k.

View attachment 32472

Thanks for posting R12 pictures and resistor value! I actually couldn't find any details on this missing part. I will try to solder in 4.7k resistor and come back with the results.

 

[Tarantulas]

Thanks for posting R12 pictures and resistor value! I actually couldn't find any details on this missing part. I will try to solder in 4.7k resistor and come back with the results.

So, what was the result?