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Old 3rd April 2013, 02:27 AM   #1
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Default Lanzar MXA 242

This amp looked like a beach was in it. It pulls current with remote applied until it goes into protect. You can hear the transformer making noise till it goes into protect.

I cleaned it all up and checked all the ps and output transistors. They all check good. The rectifiers seemed to read both directions but after removing from the circuit read fine so I reinstalled them.

There is no DC on the output terminals. I have the following readings on the 2 IC's:

TL494CN

Pin 1: 0.122
Pin 2: 5.00
Pin 3: 4.76
Pin 4: 0.082
Pin 5: 1.474
Pin 6: 3.437
Pin 7: 0.000
Pin 8: 12.68
Pin 9: 0.000
Pin 10: 0.000
Pin 11: 12.68
Pin 12: 11.79
Pin 13, 14, 15: 5.00
Pin 16: 9.48

2904D:

Pin 1: .001
Pin 2: 11.16
Pin 3: 5.89
Pin 4: 0.00
Pin 5: 10.15
Pin 6: 5.00
Pin 7: 10.21
Pin 8: 11.79

Any service manuals available? It looks like pin 16 is way too high? Not sure whats going on with that 2904...

Last edited by Deadly Sones; 3rd April 2013 at 02:30 AM.
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Old 3rd April 2013, 07:17 PM   #2
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Post a good quality photo of the area around these two ICs.

Is pin 7 of the 2904 driving pin 16 of the 494?
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Old 3rd April 2013, 07:56 PM   #3
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yes it does. Pin 7 connects to Pin 16 via D13 and J7. Gotta love how these amps have the traces traced on top.
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Old 3rd April 2013, 08:29 PM   #4
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Lifting D13 should defeat the protection circuit which should allow you to determine what's causing the amp to go into protection.

Have all transistors clamped tightly to the heatsink and have either a current limiter or a relatively small fuse in the B+ line before applying power.
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Old 3rd April 2013, 10:53 PM   #5
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I lifted D13. Amp powers without protection. It popped a 20 amp fuse. It draws hard on the current limiter (heats up pretty fast) showing 3.0amps on my clamp meter between amp and limiter. Using the limiter I rechecked the output terminals for DC.

RT ch has -1.75vdc, LT ch has -7.95vdc on it. Bridged yields -9.65vdc.

I now have the following readings on the 2 IC's with D13 lifted.

TL494CN

Pin 1: 1.49 was 0.122
Pin 2: 5.01 was 5.00
Pin 3: 0.071 was 4.76
Pin 4: 0.096 was 0.082
Pin 5: 1.482 was 1.474
Pin 6: 3.436 was 3.437
Pin 7: 0.011 was 0.000
Pin 8: 6.88 was 12.68
Pin 9: 2.181 was 0.000
Pin 10: 2.172 was 0.000
Pin 11: 6.87 was 12.68
Pin 12: 6.21 was 11.79
Pin 13, 14, 15: 5.01 was 5.00
Pin 16: 0.013 was 9.48

2904D:

Pin 1: 0.001 was .001
Pin 2: 5.86 was 11.16
Pin 3: 3.10 was 5.89
Pin 4: 0.012 was 0.00
Pin 5: 0.014 was 10.15
Pin 6: 5.01 was 5.00
Pin 7: 0.01 was 10.21
Pin 8: 6.20 was 11.79

I now have both square wave on ps transistors and a clean sawtooth pattern. Pics attached.

I did check all the outputs in circuit. They def have high ohms and don't have any broken legs. Should I pull them all and check out of the circuit?

What else can cause this neg DC on the output terminals? This amp could have had some water damage. I cleaned a lot of sand out off the board/components.
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File Type: jpg IMG_0615.jpg (593.7 KB, 36 views)

Last edited by Deadly Sones; 3rd April 2013 at 11:13 PM.
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Old 3rd April 2013, 11:30 PM   #6
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Measure the DC voltage across the emitter resistors of both channels. You should read 0.000v DC. It's likely that the left channel will read higher.
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Old 4th April 2013, 12:22 AM   #7
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Yes. All 4 of the emitter resistors for the rt channel are 0.000v. The 4 for the left channel read:

#1 = -0.035vdc
#2 = +0.036
#3 = +0.037
#4 = +0.025-.035
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Old 4th April 2013, 12:24 AM   #8
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If they're all about the same, I'd suspect a shorted or leaky driver transistor.
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Old 4th April 2013, 12:34 AM   #9
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only real diff between the 4 is #1 being a neg value and the rest being a positive value. Ok I'll pull the transistors from that channel and recheck them incase ones leaky. If I find one I'll just replace all 4 transistors in that channel and see if that takes care of it. The values of all 8 emitter resistors vary.... from .1ohms to one as high as .4ohms. Gonna replace all them too.

thanks perry. I'll update when I get the parts in.

Last edited by Deadly Sones; 4th April 2013 at 12:36 AM.
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Old 4th April 2013, 12:54 AM   #10
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The plus/minus isn't generally a concern when taking these readings. If you're going to look at the polarity, you have to take note of which probe is on the emitter leg of the resistor. The board may be laid out so that the resistors are connected differently (with respect to their terminal connections).
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