replaced irfz48n transistors.. still blowing fuses

[DcMaN]

Hey guys, i thought i would start by attempting to fix the jl 300/4 but that might be too advanced for me right now. I figured id start with something that is less complicated. I had a directed d800 that was popping the amp fuses, not the inline ones. So i took the amp apart and noticed there was a irfz48n that was shorted out (see pic) Anyway i replaced all 4 of them, and i powered the amp on it worked for a few minutes but i noticed the ground wire was getting really hot. So i took it off checked everything out, everything looked ok. But then when i hooked the amp back up it kept popping the inline fuses not the amps fuses. Any ideas ?

Thanks

-Rob

 

[Perry Babin]

Can you post a photo of the inside of the amp?

 

[DcMaN]

Ill post a picture as soon as i get home from work.

-Rob

 

[DcMaN]

 

[Perry Babin]

Remove the rectifiers. Does the amp blow the fuse (15 amp fuse suggested below).

Insert a 15 amp fuse in the B+ line and clamp all remaining components tightly to the heatsink before applying power.

 

[DcMaN]

so remove those 4 that i just put in (the ones with out the heatsink on)?

 

[Perry Babin]

The Z48s are the power supply FETs. The rectifiers are near the large capacitors (possibly fully encapsulated -- difficult to see in the photo).

The heatsink is the large aluminum outer shell. The missing parts in the photo are clamps.

 

[DcMaN]

they are indeed fully encapsulated im removing them now ill let you know in a few min

 

[DcMaN]

ok, so after removing the rectifiers, the inline fuse doesnt blow, but it puts my regulated powersupply into protect.

 

[Perry Babin]

What's the current rating of the power supply?

 

[DcMaN]

14v 10 amp constant 12a surge.

i have a 12-15v adjustable power supply regulator that surges at 52 amps that will be here tommorow.

 

[Perry Babin]

There's very likely a problem if it trips the protection in the 10 amp supply.

Remove the 4 power supply FETs and measure the DC voltage on the gate pads for all 4 of the power supply FETs.

 

[DcMaN]

Q11 5.05 / 13.8 / 0
Q12 5.05 / 13.8 / 0
Q13 5.05 / 13.8 / 0
Q14 5.05 / 13.8 / 0

Thats what i measured at the gate pads.

 

[Perry Babin]

Is it possible that you had a solder bridge on any of the power supply FETs?

Did it shut down the 12v power supply before remote voltage was applied?

 

[DcMaN]

There could have been a bridge, i cleaned up all the contacts on the bottom. If there was a short are all those fets dead most likely? i also noticed that on the bottom of the board near the amp fuses there was some corrosion, so i cleaned all that off too.

 

[Perry Babin]

Check the FETs while they're out of the circuit.

 

[Kip McGee]

You may try putting some sort of a current limiter on the B+ power cable. I use a 100 watt 2 ohm resistor...Be carefull unless you like buying more parts..

 

[DcMaN]

So heres a small update, i replaced all the powersupply fets, and put the rectifiers back in. Amp turns on, protection light comes on for a sec then shuts off (not sure if thats normal) Amp stays on and doesnt blow fuses anymore. But now there are other issues i have no audio output.

 

[Perry Babin]

Is the relay engaging?

 

[DcMaN]

I do not hear the relays turn on. The amp does get warm after a few minutes of it being on though.