Kenwood KAC-7285 amp, blows fuses as soon as turn on lead is applied

[bobotech]

I got this amp for next to nothing knowing it would be a repair job. Basically it blows fuses only when you apply the remote turn on lead.

I'm pretty sure that one of the transistors developed a short but when I use my DMM, the all show up as okay however I have not removed them from the board.

The board looks spotless with no burn marks anywhere and does not have that acrid burn smell that most burned electronics have.

I hooked it up to my car and did have speakers hooked up to it. I have not tested it without speakers (didn't think to). It was hooked up the same way my Rockford amp is hooked up.

The 2 20 amp fuses on the amp do not blow until you apply the remote turn on lead then both of the fuses snap.

Any suggestions? I'm a newbie when it comes to full blown diagnostics but I am not afraid of using a soldering gun when someone points me in the right direction. Done a lot of computer cap replacing and board level repairs on laptops.

Thanks in advance!

[Perry Babin]

I'm not familiar with this amp but kenwood amps have problems with broken solder connections on the bias transistors. They are typically small transistors clamped to the heatsink in-between the large transistors.

Do not power up the amp unless all of the transistors are clamped to the sink. I'd recommend that you insert a 10 amp fuse in the B+ line before you apply power again. This will protect the power supply transistors better than the two 20 amp on-board fuses.

[bobotech]

Okay, I will take the board out and check the solder joints where the small transistors meet the board and touch them up with my gun. Then I shall power up with no speaker and a 10 amp (or a pair of 10 amp?) fuses.

Is that a good plan so far?

[Perry Babin]

Yes as long as you clamp the transistors back to the sink before applying power.

[I Am An Idiot]

A single 10 amp fuse would be much safer than (2) 10 amp fuses.

[bobotech]

Originally Posted by I Am An Idiot

A single 10 amp fuse would be much safer than (2) 10 amp fuses.

I just didn't know if the amp required 2 fuses to operate.

[Perry Babin]

Sorry, I missed the other question. In most amps, you can use a single fuse but there are a few that require multiple fuses to operate. Since I don't know every amplifier that needs more than one fuse, I recommend inserting a 10 amp fuse (or 15 amp for larger amplifiers) in the B+ line feeding the amp. That prevents having possible problems with an amp that won't operate properly on one fuse.

[bobotech]

Okay, I went and took all the screws off the mainboard and removed it. Well what do you think I found?

I found a bunch of water damage by the power circuits and fuses. What is the next plan of action? Obviously, I'm going to clean it as best as I can using electrical cleaner. I didn't see any cold solder joints but I'm going to reflow them anyway just in case. I'm also going to check the small transistors by the power circuit that have corrosion by them.

But in a nutshell, can corrosion cause dead shorts and be repaired easily just by cleaning it all off or does the corrosion usually cause other parts on the board to fail?

Thanks for everyones help here. This is my first amp repair and I hope to have more in the future.

[bobotech]

Still no go. I cleaned it all up, looks great and all the traces look good to the naked eye.

Went and put it into my test battery and as soon as I hit the remote turn on lead, the fuse popped.

Where should I go from here?

[Perry Babin]

Do you have a large (25+ watts) 2 ohm resistor or an automotive headlamp (h6054 or similar) that you can use as a current limiter?

The problem may not be a short. An open circuit can cause an amp to draw excessive current.