Blown fuse on Macintosh Classic II: Causes and potential other damage

[Vacaneu]

I have attached 2 images of the analog board, one marking cracked RIFA caps and the removed fuse.

My analog knowledge is poor, so I'd like to know what the potential causes would be, and what other damage may be inflicted.
I will recap the board once I have time so I do hope it's the RIFAs I'm suspecting. Please help point me in the right direction.

 

[Vacaneu]

*This is a 240 volt board

 

[SpocksBeer]

Well, a fuse that pops straight away is usually indicative of a fault on the primary side (240v) of the power supply. The safety caps (the white ones you've identified as RIFAs) do sit on that side, so it's possible that's the cause. I forget which one, the X or the Y, but one does fail short.

Could also be a shorted inductor, or winding in the main transformer which would be a bit more terminal. I'd replace the safety caps first and see how that goes.

 

[Vacaneu]

Well, a fuse that pops straight away is usually indicative of a fault on the primary side (240v) of the power supply.

The issue is I have no idea if it does pop straight away or if there was another anomaly because all I was told beforehand is a fuse blew due to a short. I would pop one in and test but I'm scared I'll break it more. I still do suspect those caps anyway

 

[cheesestraws]

Agreed, something has probably gone awry on the primary side.

Check whether those caps have failed short or not. Could be either the X or Y, if I remember correctly.

Also might be worth checking the diodes in the bridge rectifier to see if they are still diodes; I had a couple of power supplies that blew fuses when one of the diodes decided to become a resistor instead.

Obviously please be careful when poking around the primary side of any PSU.

 

[bibilit]

Rifa caps are always cracked on the AB of those classics.

Never bother replacing the caps myself, i leave them untouched.

The main reason for a blown fuse is the Mosfet RP40 being shorted QP2 I think

Using a multimeter you can check for shorts in the legs.
the bridge rectifier can be shorted too, but only had one in maybe 40 boards