Mac Classic fuse blown after TDA4605 IRFBC40 replacement

[ironborn65]

Hi everyone,
I got a Classic with a blown fuse.
I replaces the TDA4605 and the IRFBC40 and I was pleased that is was properly working, but for an our or so when the fuse popped again, I replaced the fuse and it popped instantaneously, the TDA and the IRFBC are probably gone.
Now: I will re-test an replace the TDA4605 and the IRFBC40 but the culprit is probably somewhere else in the high voltage section.
Can I assume the optocoupler is innocent?
What do you recommend to test?

thanks

 

[Byrd]

Replace the optocoupler, this usually goes first well before the TDA4605 and IRFBC40

 

[ironborn65]

I'll bench test it, but still I'm not 100% sure that it can be responsible for blowing the fuse, usually it affects the stability of the voltage.
Do you have different experience?

 

[bibilit]

Bridge rectifier maybe (happened to me once)

Never seen the opto popping a fuse.

 

[360alaska]

You need to check the fuse output with an ohm meter, if it pops instantly you have a short. Find out why.

 

[ironborn65]

@360alaska thanks for the tip. There was a short when I got it, as I wrote.
Then, after the replacement of the TDA4605 and the IRFBC40 it was fine for about 1h or so when it popped.
Now it pops instantly, the short is very likely in the MOSFET/IRFBC40 but IMO it's a consequence of something else up in the circuit which stresses the MOSFET or the TDA4605. I was hoping for a direction, instead of checking every passive component in the high voltage area.

​

 

[360alaska]

Look HERE:

MacintoshClassicEarly-PSU-AB.pdf

drive.google.com

OR if your board is newer:

MacintoshClassicLater-PSU-AB.pdf

drive.google.com

 

[ironborn65]

@360alaska thanks, I got those schematics. I'll try to make my way, but I do not have a background in electronics, I'm not a practitioner, I just know the basics, I'm learning here.

 

[360alaska]

I see, It might be worth to check the rectifier(KBL08) and make sure it's okay:

https://www.parduebrothers.com/siliconbridgerectifiertesting.pdf

Also check the capacitors on the left side of the rectifier.

 

[ironborn65]

amazing, thanks pal