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Old 8th January 2011, 04:15 PM   #1
Dr Zeus is offline Dr Zeus  United States
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Default MTX Thunder 7804

This amp has got me a little stumped. It powers and plays, but the output on the Front Left channel causes the amp to shut down at medium volume.

Another strange symptom of this Amp may be related to this off-rebranded LM317; as after a few minutes it gets very hot. I pull this part off the PCB and tested it against a brand new piece and its measuring identical... so I put it back in it's original spot.

It also seems a previous repair replaced the 512 rectifier next to the LM317, and on the other side of the heat sync as well.

Click the image to open in full size.

I Measured just about all of the transistors and ICs except a lot of the SMD ones. Nothing pops out defective. The opAmps are all getting

No DC on the outputs. RCA shield resistance to ground is 100k
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Old 8th January 2011, 05:27 PM   #2
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Confirm that the source resistors for the outputs in the defective channel are within tolerance. Also make sure that there are no broken legs or bad solder connections.
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Old 5th February 2011, 03:52 AM   #3
Dr Zeus is offline Dr Zeus  United States
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Im working with this amp again and have not been able to find the problems 100% at this point, but i think I'm close.

The sound board may have some issues, as several of the opAmps have DC on the output pins. By comparison, I've also been working on a smaller MTX amp of the same series. Both sound board are pictured. On a hunch, I actually tried the smaller MTX model 404's sound board in place of the original Whats interesting is the MTX 404 is also a 4 channel amp as this one is, but the processor board is just lacking features. With the 404 board, the amp powers and plays. Also with this board, the LM317 pictured above stays cool to the touch under operation.

It is nice that I had two of these style amps in for repair...

Click the image to open in full size.

More details to come, just a minute.

Last edited by Dr Zeus; 5th February 2011 at 03:54 AM.
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Old 5th February 2011, 03:56 AM   #4
Dr Zeus is offline Dr Zeus  United States
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In this picture:

Click the image to open in full size.

From top to bottom, left to right in this picture, the opamp output pins are measuring these voltages referencing the amp's general ground.

1. OpAmp near C406 has ~0.005vDC on all output pins. +-14.75vDC on power pins.
2. OpAmp near R440 has ~0.005vDC on all output pins. +-14.75vDC on power pins.
3. OpAmp near C500 has ~0.010vDC on all output pins. +-14.75vDC on power pins.
4. OpAmp near C400 has ~0.010vDC on all output pins. +-14.75vDC on power pins.

5. OpAmp near R502 only has +14vDC on power pin in the top row (near grey ribbon cable). Two of the outputs have 0vDC, the other two have 12.5vDC. Input pins all have ~0.005vDC on them.

6. OpAmp near R602 has -13.4vDC on 3 outputs, and +4.5vDC on the top right output. Power pins have +-14.75vDC on them. The input pins have between 1vDC and 5vDC on them, mostly 3-5vDC on them all. Only one has 1vDC on it.

7. OpAmp near C650 has 11.2, 1.7, -9.6, and 0.5 vDC on output pins (Left to right, top to bottom. Power pins have +-14.7vDC on them. The input pins in the top row all have 2-4vDC on them.

*The diode at D605 gets warmer than most components after running for a bit. Its the diode in the bottom right corner of this board. in the picture above. it measures fine (0.675) on the meter using diode check.

Click the image to open in full size.

8. OpAmp near C509 (left) has -13.3vDC and -14.5vDC on it's likely not getting power correctly.

9. OpAmp between L605 and L606 has -12.6vDC on three of the outputs and +12vDC on the bottom right output pin. Power pins have +-14.75vDC on them. 4 (left) of the input pins have -12vDC on them, and the other 4 (right) have 2.7vDC on the top two, and 0vDC on the bottom two.


So... it looks like # 5, 6, 7, 8, & 9 have some issues.

Do you have a schematic for this board?

Last edited by Dr Zeus; 5th February 2011 at 04:21 AM.
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Old 5th February 2011, 04:28 AM   #5
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Correction to #5 above:

5. OpAmp near R502 has +-14vDC on power pins (near grey ribbon cable). Two of the outputs have 0vDC, the other two have 12.5vDC. Input pins all have ~0.005vDC on them.
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Old 5th February 2011, 04:31 AM   #6
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Theres on more opAmp on this board. The last image on the top just cut off is an 8 pin opAmp. Voltages are all correct on it.
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Old 5th February 2011, 04:39 AM   #7
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I wish I new a way to test opAmps. #4 above is measuring 5 ohms between pins 6 & 7. The pins dont seem to be connected but maybe through a series of resistors.
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Old 5th February 2011, 04:50 AM   #8
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Read 'Beyond the Basics', item #8, the first 2-3 paragraphs. Also read the Operational Amplifiers page.
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Old 5th February 2011, 02:42 PM   #9
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I read those pages last night and I likely have to re-read again when I get back to this amp. I'm now thinking of just ordering up replacement opAmps for this amp. Of the above readings, which opAmp<s> do you think may have failed?

I'm not positive on which opAmps are for audio, and which are being used as something else.

Last edited by Dr Zeus; 5th February 2011 at 02:44 PM.
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Old 5th February 2011, 03:21 PM   #10
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If you need me to try to determine which op-amps are bad, post the voltages on all pins in list form. Copy and paste the following as needed for each of the op-amps.

IC#
Pin 1:
Pin 2:
Pin 3:
Pin 4:
Pin 5:
Pin 6:
Pin 7:
Pin 8:
Pin 9:
Pin 10:
Pin 11:
Pin 12:
Pin 13:
Pin 14:

For audio op-amps, the inputs for each op-amp should match. For op-amps being used as comparators, you can predict the output by looking at the inputs. The comparator demo on the power supply troubleshooting page (item #12) should help you understand how op-amps work when used as comparators.

I don't have a schematic diagram for this amp.
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