Lanzar MXA 242

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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...
 
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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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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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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.
 
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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).
 
Awww, ok... was gonna say I didn't switch probe positions as I went across them but that is likely the case (board layout/position). I'll order up the replacements in the morning. thx vm

One thing I did notice was that 3 of them emitter resistors kept a solid reading and #4 varied between .025vdc and .035vdc... could this be a sign of the "leaky" transistor?
 
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The outputs in this amp are A1941 and C5198 and I can't seem to find any without buying 50 or 1000. Can they be subbed with 35/36C's? or is there a better sub?

Can I use 3watt dale resistors for these? Or are these ones (please see pic) 5watt? These are the largest output resistors I've seen in an amp.
 

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I replaced all outputs with the FJA4213/4313 subs. I removed the gate resistors and they seemed to read the same as the new ones I bought... 0.1- .2 bouncing to .4 even though they should be 0.15ohm. The 5w wire wound ones I bought are much smaller than the ones in the amp so I reinstalled the old ones in case the wattage is higher.

Now, its no longer in protect, but is pulling a lot of current (current limiter gets hot fast) and the output is extremely distorted. It is audible but sounds terrible. I have over 1.5vdc on the outputs bridging terminals and when checking across each gate resistor now 3 of the 4 banks have DC where as before it was only one channel (2 banks).

I'm going to try the 5w resistors I bought tomorrow (have to get to work right now) just to see if they are the issue... could the problem be driver oriented?
 
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