Kicker KX750.1 Repair Help

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I have a kicker ZX750.1 that just cycles from one to protection (about 3 second cycles). I have read the post here: http://www.diyaudio.com/forums/showthread.php?t=143940&highlight=kicker

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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An externally hosted image should be here but it was not working when we last tested it.


An externally hosted image should be here but it was not working when we last tested it.
 
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...
 
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!
 
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?
 
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.
 
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.

I used a nice PS (Agilent 6654A) with overcurrent protection in place of the fuse (12V with a 2A limit). It only can output 9A, so its perfect for initial tests :) I removed Q04 like you suggested. Below I have answered your questions in red. The first answer is with the rectifiers out and the second is with them in.

Does it draw excessive current? No (0.7A idle) / No (1.4A idle)

Does it produce positive and negative rail voltage on the center legs of the rectifiers? N/A / +63.25V and -54.75V (should they be that much different?)

Does it have any DC on the output inductors or on the speaker terminals?
No / 4.1V from negative terminal to ground (not applicable??) .015V between + and - speaker outputs

Does the relay engage/click? Yes / Yes

As a sidenote, I tried hooking up a speaker to see if I could get some sound out of it, but no luck. Using a scope on the "output" pin of the preamp card showed nothing was coming out. It looked like the signal dies at the JRC 13600, but I don't know exactly how it is setup, so I can't verify that chip is the problem. Can removing that Q04 possibly have anything to do with it?
 
Update: (Still having moderators approve my posts makes it seem like I'm posting every couple minutes... but there is a decent amount of time in between :)) I just resoldered the JRC 13600 opamp, and I now get my test tone signal out from the preamp daughter board down to the amp... still no sound out though!
 
The rails should be almost identical if you used the negative speaker terminal for the reference (black probe).

Do you have ±15v on the preamp board?

Are you sure the preamp connector is plugged in correctly (not misaligned)?

Are the gain and crossover frequency set fully clockwise?

Well I guess I didn't use the negative speaker terminal as the reference, because I just redid it making sure I used that, and I get +/- 69V now.

I do have +12V/-12V on the daughterboard (that's how its labelled also). I got the preamp board to work fine now, I just had to resolder one of the opamps. I sitll get no sound out at the speaker though. So it looks like the signal is dying somewhere down on the main board. All the gain/crossover/bass boost settings are fine. Also, the board pretty much can't be misaligned because the connector is on one end of the board and only fits in one way if the screw holes line up.

Further suggestions on where to look so I can finally get some audio out of this thing? And what are my options for fixing the protection circuit? Thanks for all the help so far guys!
 
Do you have a rail to rail square wave on the tabs of the output transistors?

I certainly do not have this... below are pictures from my scope (using the negative terminal for the ground reference for all the output transistor measurements and the power ground for the SMPS transistors). It looks like I have 0V on the drain.... but -70V on both the gate and the source. Something is definitely not right... I did these measurements in haste because I had to go somewhere. I will try them again tomorrow unless you have some idea from these pictures what might be wrong. All the FET's still measure mine (resistance wise) between the legs with the amp off.

IRF3205 Gate Pulse

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IRF3205 Drain Pulse - Should these edges be more square? I did replace the standard 100 ohm gate resistors with 47 Ohm as suggested in a previous thread

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Rectifier Input

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IRF640 Drain

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IRF640 Drain Small Scale

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IRF640 Gate

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IRF640 Source

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It seems really weird to me that the rectifier is putting out the correct voltages, yet the drains are not seeing them. Is there something in this (or Class D in general) that I am missing?
 
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