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Mac Classic fuse blown after TDA4605 IRFBC40 replacement

ironborn65

Well-known member
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

Well-known member
Replace the optocoupler, this usually goes first well before the TDA4605 and IRFBC40
 

ironborn65

Well-known member
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?
 

360alaska

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

ironborn65

Well-known member
@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.

 

ironborn65

Well-known member
@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.
 
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