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whitejohnny 9th June 2012 06:56 PM

Kicker KX1200.1
I just replaced the LM361 on a kx1200 I was working on,and it powers up and plays but its a little distorted and sucking way too much power - the L1 inductor and the first set of caps (c09,c10,c11,c12,c13,c16) are getting hot. It seems to oscillate some when its cranked up too. Its producing a lot of transformer whine, so I figuring its pushing as hard as it can to keep the rails up. Rails checked at +-72volts.

I've read before, a good grounding point for my meter is the negative speaker terminal or the centertap of the power supply transformer secondary, right?

I'm seeing -70 and 70 volts on the gates of the IRF640s and the IRF9640s respectively. No DC on the output of the amp.
All output transistors checked ok, will check again tho.
It almost seems like the two sides are running full blast to cancel each other out.... but there's no bias pot, so is there possibly a circuit a need to be checking?

I won't have a scope for another couple weeks, but I do have a soundcardscope setup in the meantime.

Any suggestions where to start or does this problem sound familiar to those of you who have worked on this amp before?

BTW, this is a red board with 100ohm gate resistors.

Perry Babin 10th June 2012 11:44 PM

At idle, what is the DC voltage on the 3 terminals of the driver transistors (Q05 and Q11)?

whitejohnny 13th June 2012 09:05 AM

Sorry for the delay, Q5 (KTA1504?) 1-B 4.74 2-E 4.77 3-C 0.03 Q11 1-B 4.75 2-E 4.78 3-C 0.03 The 'full-tilt whine' stopped when I pulled it from the heatsink and powered it up - that's when I took these readings. It runs cold at idle, and plays ok off the sink, although something seems off, it's protecting as I raise the volume, although I'm not seeing a voltage swing at my bench supply, so I don't think it's 'power-sag' happening. Should I see the red led light when I first turn it on? Or should it go straight to green? It goes red a moment, then the green starts dimly, and then pops bright. Sometimes the red LED is dimly lit, I believe I had it in the sink when that was happening... BTW, originally, amp would light up but not play, TL361 was getting very hot to touch almost immediately, replaced, powered up whined, acted funny, retouched some bad solder, now its sort of

Perry Babin 13th June 2012 09:52 AM

The drivers appear to be OK.

The RED lights for a second then the green LED switches on (if I'm not mistaken, I don't look at the LEDs generally).

When the amp is in the heatsink, you MUST have the grounding wire/screw tightly screwed down. If it's only a screw, you must make sure that the washers are in place so that the screw can contact the board.

whitejohnny 14th June 2012 12:34 PM

Trying to eke out some free time to get back to this in the afternoon... Huh, there are plastic spacers between the board and the sink, and it appears that when you insert the screw it spreads the spacer and holds the board in place, without the board contacting the screw, the no circuit from the board to the sink. And on the ground wire, it goes to a ceramic cap. So, I guess what I'm asking is, is the voltage too high for the little insulator/heatsink tape? And the excess AC travels back through the cap?

Perry Babin 14th June 2012 07:42 PM

The wire shunts high frequency noise from the heatsink to ground. It has nothing to do with high voltage.

In some amplifiers, there is a pad around the screw that must contact the screw (generally uses a lockwasher to bridge the gap). The screw connects the grounding pad to the heatsink. In amps without the pad (using a wire to make the connection), the screw makes the connection between the wire and the heatsink.

whitejohnny 16th June 2012 02:05 AM

So the noise interferes with the power generation process? Cause it was literally sucking the power out. I bolted it in and connected the ground to the sink, and it behaves right now. Bizarre. Thanks for your help Perry, much appreciated!

Perry Babin 16th June 2012 07:13 AM

The noise gets picked up by the audio circuit and causes it to malfunction. I'm not sure why it does it but it's common on many class D amps and very common on Kicker amps.

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