Kicker KX750.1 Repair Help - diyAudio
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Old 20th August 2009, 06:53 PM   #1
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Default Kicker KX750.1 Repair Help

I have a kicker ZX750.1 that just cycles from one to protection (about 3 second cycles). I have read the post here: Need schematic for kicker zx750.1

Only thing I haven't tried compared to that thread is changing the power supply gate resistors from 100 ohm to 47 ohm. All of the power supply and output FET's measure fine. The gate of the power supply FET's get a square wave. The CT pin on the TL494 control chip has a clean ~44.5kHz sawtooth wave. The dead time pin (pin 4) on the TL494 does get a 5V signal during the short time the protection light is on before it resets and tries to power on again. The thermistor also was outputting 4.1V, but the output of the comparator was low so I do not thing that was tripping it. Thermistor failure seems rare anyway.

In a random test, I just grounded the deadtime pin to see if it would stay on. It did, but the amplifier was not happy. It made a clicking noise from somewhere in the power supply section and the power supply I was using would hit the current limit (tried at .5A, 1A, 1.5A, and 2A). An interesting observation was the power supply dropped to 6.5V each time that it constant currented, no matter the current limit.

So it looks like it is the protection circuit is tripping (seemingly due to overcurrent), but I am not sure how it is setup on this particular amp. Anyone have any ideas or schematics for this particular amp? Everything else seems original on this amp as the pictures below should show. My next plan would be to unsolder the 2 rectifier chips so I can disconnect the power supply from the rest of the amp. Any reasons not to try this?

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Old 20th August 2009, 11:42 PM   #2
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Did you check for shorted output transistors?
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Old 21st August 2009, 12:12 AM   #3
luka is offline luka  Slovenia
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He did...
Quote:
All of the power supply and output FET's measure fine.
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Old 21st August 2009, 12:42 AM   #4
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Yep, measured them all. They were all similar too between different legs.
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Old 21st August 2009, 04:41 AM   #5
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With the black meter probe on the negative speaker terminal and the red probe on either terminal of either of the large output filter inductors (near the relay), do you read any DC voltage?
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Old 21st August 2009, 04:50 AM   #6
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Quote:
Originally Posted by Perry Babin View Post
With the black meter probe on the negative speaker terminal and the red probe on either terminal of either of the large output filter inductors (near the relay), do you read any DC voltage?
I'll check that tomorrow. I had no DC on the speaker terminals. Verified with a meter, and a cheap speaker (did not suck in or push out). I have not checked from the negative terminal to the output filter inductor though. I'll report the results...
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Old 22nd August 2009, 05:27 AM   #7
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So I checked what you asked Perry... there was no DC voltage. So I went and pulled out the rectifier and it still pulses in and out of protection. I think that pretty much rules out the output section unless I am forgetting something. Is there an easy way to test the TL494? It looks like on this amp it is setup as an emitter-follower because I get the pulses output on E1 and E2 (pins 9 and 10) The collectors seem to start to ramp up towards VCC, but reset once the protection trips. I also verified the comparator works for under voltage protection. I think now think that my problem does not involve any of the regular built in protections (thermal, undervoltage, over current) because all the outputs of the comparator stay low (unless I do undervoltage on my bench PS).

Long story short... I think I have issues somewhere in the control circuitry of the SMPS. Any ideas where to check next? I have some TL494's on order to see if that will fix the issue, but I'm running out of ideas!
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Old 22nd August 2009, 11:57 AM   #8
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I think Q04 drives pin 4 of the 494 high to shut the IC down. Try lifting two legs of that transistor or removing that transistor. Clamp the power transistors tightly to the sink (<<< VERY important) and insert a 10-15 amp fuse in the B+ line (<<< VERY important). Power up the amp. Since this will likely completely defeat the protection circuit, you'll have to watch carefully for problems.

Does it draw excessive current?

Does it produce positive and negative rail voltage on the center legs of the rectifiers?

Does it have any DC on the output inductors or on the speaker terminals?

Does the relay engage/click?
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Old 22nd August 2009, 03:39 PM   #9
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I will check those things later today. I use a nice power supply that has adjustable current limiting. I'm assuming that would work fine in place of a 10A fuse on the B+ line? For the early stages of all my repairs I use this supply because 1) It has good overcurrent and 2) It has low output current (9A) capability. I'm glad you know which transistor (Q04) controls the protection... that is what I have been trying to find so far! I'll report the results later today.
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Old 22nd August 2009, 11:14 PM   #10
jol50 is offline jol50  United States
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Just in case....2A PS may not be enough to start that amp, if I read your original post correctly. Or, if the PS goes down to about 10v some amps protect.
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