Kenwood KA-5002 Protection board

[mpr2000]

Yes - that is the schematic for SN 43001 and above. Mine is below and slightly different. IE, the protection circuit is on a separate board (as shown above), and there is a separate board yet for Q15, the bridge, and a couple of caps.

Interestingly, my amp board has all the empty locations for the later board with the protection circuit built in.

 

[CAM300]

Yes - that is the schematic for SN 43001 and above. Mine is below and slightly different. IE, the protection circuit is on a separate board (as shown above), and there is a separate board yet for Q15, the bridge, and a couple of caps.

Interestingly, my amp board has all the empty locations for the later board with the protection circuit built in.

So does mine - the two circuit boards for protect and the empty locations.. All the drawings I have say that the relay is connected between pins 6 and 7 on the protect board. This makes sense because the loop contains both the 270 ohm resistor and the flyback diode. My unit was wired with pins 6 and 10 to the relay. I would hope that shorting the speaker terminals would open the relay but so far that hasn't happened. Is that a fair expectation, or does the protection only open the relay when there's an internal failure in the amp?

 

[mpr2000]

I was able to 'test' when I inadvertently installed a transistor incorrectly and the relay energized. A high-ish DC voltage would have been directly on one of the speaker lines. No damage was done, but proved that the circuit 'monitors' for DC above a level set by one of the pots and activates the relay. No short-circuit protection though that I can see.

And brings the point that the relay as used here is using the two NC contacts (one for each channel) and may never see an activation over the life of the unit. So - testing the circuit and relay is a must.

 

[CAM300]

Cool. Yes, I had a similar experience with a slipped probe, the relay tripped. The owner's manual talks about short circuit protection for the output transistors but that doesn't always mean its true. I have briefly shorted the speaker terminals without any damage.

Thanks for the input!

 

[Rebooted]

I can't even begin to tell you how much hair I pulled out trying to get this protect circuit to work "properly" before finding your post. I even have a parts unit to compare to that was exactly the same as yours. Thanks to your info, I was able to get it back up and running! Now to replace those noisy 2sc458's...

 

[mpr2000]

Excellent! Great to hear.