Kenwood Kac-8104D Problem

[bigpun4fubu]

When powered up first it would come on for a sec then blow the 10A safety fuse on my power supply, ( i have this to keep from overloading it , while i use it for testing purposes) all of the mosfets seem to check good, its almost like it is shorting out though, but now i can power it up and the blue light comes on and the red light does not, i was thinking it was a power mosfet but the seem to check good. anyone familiar with this problem?

 

[brandes.cm]

Cycle the power really fast the first 3-4 times to let the caps charge, you might be experiencing a huge in-rush that is taking out the fuse.

 

[Perry Babin]

What type of fuse are you using?

 

[bigpun4fubu]

ato 10 amp

 

[Perry Babin]

The ATO/ATC generally hold but you may need a 15 amp fuse for this amp (brandes was likely correct). A 15 amp fuse is generally safe for all amplifiers as long as all power semiconductors are clamped to the heatsink.

 

[bigpun4fubu]

even with no signal input?

 

[bigpun4fubu]

yeah all semiconductors are clamped during power

 

[bigpun4fubu]

and also a time or 2 it would overload the powersupply and cause it to shut down if it didnt blow the fuse

 

[Perry Babin]

What sort of supply did it shut down?

 

[bigpun4fubu]

its a 20 amp 12v powersupply, i just use it for test purposes , ive had way stronger amps on it and havent had any problem testing them , im just trying to trouble shoot this amp it should power up and stay on with that power supply

 

[Perry Babin]

I think the most important thing is the idle current after the amp is up and running (producing power supply voltage and able to produce audio).

If the current draw isn't significant (2 amps is common), I don't think there is a problem.

Did the amp power up and produce audio?

 

[bigpun4fubu]

im assuming its in protect mode if the red light isnt on and the blue light is on .. the red light comes on and goes out , blue stays on , i changed the fuse to a 20 its staying on now but im assuming in protect mode

 

[bigpun4fubu]

no audio yet

 

[bigpun4fubu]

it is staying powered up but no red triangle light just the blue is lit

 

[bigpun4fubu]

im getting 12v on the gates but 0v on the drains on the power mosfets

 

[Perry Babin]

Are you sure that you have the pin configuration of the FETs right?

If the amp is in protect, the voltage is likely to be 0v, 12v, 0v on the FETs.

When the amp attempts to power up, do you see a pulse of DC on the speaker terminals?

 

[bigpun4fubu]

voltage is 0v 12v 0 a pulse up to 15v thru speaker terminals then back to the mv scale

 

[Perry Babin]

The DC offset is what's causing the amp to shut down. Double-check the output transistors to see if any are shorted. Check between all legs of each individual transistor (leg 1 to leg 2, 1-3 and 2-3). If none appear completely shorted but give low readings, set the meter to ohms and find the one or two that read the lowest from leg 1 to leg 2.

 

[bigpun4fubu]

from right to left the first two 32k and 33k ohms and the other two read 76k and 85k

 

[Perry Babin]

There's nothing conclusive there. I think you're going to have to remove them from the circuit to check them. Start with the ones that read ~30k. There should be no leakage (infinite resistance) from leg 1 to either of the other legs.