Macintosh SE/30 Sony PSU not working

[Jinnai]

I replaced all the radial caps in my SE/30 PSU except the three largest, but it still doesn't work. I measured the voltages with my multimeter, with the negative probe on the black wires and the positive on the red/yellow/orange ones, and the voltages I got ranged from about a half volt to about 1.5v. What could it be thinking? Is it very likely that I need to replace the huge 400v ones? I thought they were not very prone to failure.

 

[techknight]

Troubleshooting will likely be needed here. If changing all the caps didnt help, then you will need to find out what failed. Not everything fails due to "caps", as power supplies fit that exception. its likely a failed component. Diode, resistor, maybe even an IC. 

Luckily the schematics are available. 

 

[Jinnai]

Just an update - I've been looking at the schematic and tested up to the main voltage from the biggest filtering cap and subsequent inductor, its about 164v, which seems good. Voltage on the yellow wire - +12v - starts at about 0.9 and slowly and steadily drops when measuring, it got to 0.22v before I got tired of holding the probes. I measured the output at transformer T154 - if I understand correctly and mind you I know I might be 100% wrong but I'm trying to understand whats going on - which is connected to the PWM chip, and got nothing at all. If I understand correctly, is it DC voltage at that stage?

The switching PSU converts AC to DC, smooths it, and converts to high-frequency by switching via transistors Q151 and Q152, at which point it's switching, but still measurable with the DC setting on a multimeter, is that correct? There is a transformer later on in the circuit for the main 12v voltage - T151 I think - but if it cannot be used with the plain 164v output from the filter cap because a transformer won't do anything with flat DC current, it must be getting it's input from Q151 and Q152. How incorrect am I?

Schematic for reference:

 

 

[techknight]

Only thing a DMM will measure here is DC output from the DC stages. Switching stages would be seen by AC, and most DMMs arnt fast enough to see the switching going on. It would require an isolated scope. 

The fact that you have no voltage output tells me its not switching. Why isnt it switching? thats the part you have to figure out. Could be shorts, could be bad/open components, caps, anything. 

 

[Jinnai]

So if I try to measure the switching transistor output with the AC setting, I'll probably get nothing (or perhaps 0.9v?) whether it's working or not? That would be too bad.

I went back as far as D252 on the 12v rail and D251 on the 5v - thats immediately after T151, in the hopes that I would find some measurable DC output, but I got the same as the analog board connector voltages. At least it's consistent, but yes I don't think switching power is coming to T151. It seems I can't verify that without a scope.

Does the PWM circuit control the switching? If so maybe two things that could be wrong include the PWM components and the transistors themselves.

 

[techknight]

the PWM control does run the system, but there is a standby regulator that powers up that circuit and has its own transformer, if you look at the schematic. Once that circuit kicks in, it will create a feedback loop for the standby regulator as well. 

 

[Jinnai]

So I have been slow and all that but I have gotten around to checking the input voltages for the PWM chip and they were pretty good. So I swapped the two main fast switching transistors and saw a voltage much closer to normal.

I swapped in ones from a unit that already was suspect for having bad transistors, so before I proceed I want to replace them with a modern equivalent if possible. They are marked C3042, and I'm guessing that is equal to a Sanyo 2SC3042. I found one I think might work as a replacement - a Fairchild KSC5338D, what do you think?

The current problem I don't know if there's still something wrong with the transistors, but the voltage varies a bit when it's turned on, depending on how long its been off. Perhaps the issue will go away when it's under load. Also, -12v is not working, so I guess I'll be hitting the schematics again for that. I don't think I really understand what negative voltage is yet.

 

[JDW]

@Jinnai  If you still haven't swapped out those 3 large capacitors (as you said in your opening post), you should do that.  When I swapped all those caps on my SONY PSU, I found that the largest one had leaked.  You can see that at 18:43 in my Sony PSU Recapping video.  After watching that, jump to 24:17 in that same video where you can see me measure capacitance and ESR of that cap, where I didn't find anything unusual as per the meter, and yet that cap leaked and clearly wasn't holding up voltage well.  Ditto for the other caps too. I checked most all of them with my meter and the capacitance and ESR looked normal on almost all of them, yet after the full recap, the PSU started working normally again.  (Prior to the recap, the voltage was fluctuating wildly and the SE/30 wouldn't boot.)