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.
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
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.
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
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.
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...
More details to come, just a minute.
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...
More details to come, just a minute.
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In this picture:
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.
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?
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.
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?
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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.
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.
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.
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.
Read 'Beyond the Basics', item #8, the first 2-3 paragraphs. Also read the Operational Amplifiers page.
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.
I'm not positive on which opAmps are for audio, and which are being used as something else.
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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.
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.
Going partially on a hunch, I clipped off # 7, 8, and 9 above (U600, U601, and U501) because I thought the input pins were not matching correctly. When I did that, the christmas lights stopped working completely; however they were not working right to begin with. The amp now has clean audio on the FL/FR channels. RL/RR are not playing anything. The following voltages are on the pads of #7-U600, #8-U601, and #9-U501:
#7-U600:
Pad 1: 0.630
Pad 2: 0.630 (1 & 2 are connected to D603)
Pad 3: 2.491
Pad 4: -14.7
Pad 5: 3.832
Pad 6: -0.595
Pad 7: -0.154
Pad 8: -0.158
Pad 9: -0.158
Pad 10: 4.50
Pad 11: 15.08
Pad 12: 1.013
Pad 13: -0.158
Pad 14: -0.158
#8-U601:
Pad 1: 0.512
Pad 2: 0
Pad 3: 0
Pad 4: -14.7
Pad 5: 0
Pad 6: 0
Pad 7: -0.636
Pad 8: -0.636
Pad 9: 0
Pad 10: 0
Pad 11: 15.08
Pad 12: -0.795
Pad 13: -3.350
Pad 14: 0.358
#9-U501:
Pad 1: -0.045
Pad 2: -0.045
Pad 3: 0
Pad 4: -14.60
Pad 5: 0
Pad 6: 0
Pad 7: 0
Pad 8: 0
Pad 9: 0
Pad 10: 0
Pad 11: 0
Pad 12: 0
Pad 13: -0.050
Pad 14: -0.050
Do these look correct? Better readings then with the opamps in place...
#7-U600:
Pad 1: 0.630
Pad 2: 0.630 (1 & 2 are connected to D603)
Pad 3: 2.491
Pad 4: -14.7
Pad 5: 3.832
Pad 6: -0.595
Pad 7: -0.154
Pad 8: -0.158
Pad 9: -0.158
Pad 10: 4.50
Pad 11: 15.08
Pad 12: 1.013
Pad 13: -0.158
Pad 14: -0.158
#8-U601:
Pad 1: 0.512
Pad 2: 0
Pad 3: 0
Pad 4: -14.7
Pad 5: 0
Pad 6: 0
Pad 7: -0.636
Pad 8: -0.636
Pad 9: 0
Pad 10: 0
Pad 11: 15.08
Pad 12: -0.795
Pad 13: -3.350
Pad 14: 0.358
#9-U501:
Pad 1: -0.045
Pad 2: -0.045
Pad 3: 0
Pad 4: -14.60
Pad 5: 0
Pad 6: 0
Pad 7: 0
Pad 8: 0
Pad 9: 0
Pad 10: 0
Pad 11: 0
Pad 12: 0
Pad 13: -0.050
Pad 14: -0.050
Do these look correct? Better readings then with the opamps in place...
A couple of problems. It appears that the pin configuration is wrong. Pin 4 should have +15. Pin 11 should have -15.
One op-amp has no +15. I'd suspect an open resistor near the power supply pin of the op-amp.
One op-amp has no +15. I'd suspect an open resistor near the power supply pin of the op-amp.
Yea I realized I had everything upside down in the last posting. Basically just swap the numbers top to bottom.
OK on the opAmp not getting +15, I found a trace that was mis-connected through the board. I just threaded a very thin piece of copper wire through the itty bitty holes and now Pin 4 of U501 is getting +15vDC.
Of the other measurements, do you see any that pop out as being in error?
OK on the opAmp not getting +15, I found a trace that was mis-connected through the board. I just threaded a very thin piece of copper wire through the itty bitty holes and now Pin 4 of U501 is getting +15vDC.
Of the other measurements, do you see any that pop out as being in error?
I think the next odd-est one is U601, where there is 3.3vDC on Pad 2.
#8-U601:
Pad 1: 0.358
Pad 2: 3.350
Pad 3: -0.795
Pad 4: 15.08
Pad 5: 0
Pad 6: 0
Pad 7: -0.636
Pad 8: -0.636
Pad 9: 0
Pad 10: 0
Pad 11: -14.7
Pad 12: 0
Pad 13: 0
Pad 14: 0
#8-U601:
Pad 1: 0.358
Pad 2: 3.350
Pad 3: -0.795
Pad 4: 15.08
Pad 5: 0
Pad 6: 0
Pad 7: -0.636
Pad 8: -0.636
Pad 9: 0
Pad 10: 0
Pad 11: -14.7
Pad 12: 0
Pad 13: 0
Pad 14: 0
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Pad 2 traces to a 3323 resistor at R621 and then to the Anode of D602; a MMBD701 diode with 5H marked on it's back. The anode measures 0.378. Pad 2 also traces through two resistors totalinh 12.5k, and then to various spots on the board; one of which is a pin with 5vDC on the ribbon cable connector which has 5vDC on the main board with the sound board disconnected.
I think U601 pads are OK.
Maybe its time for me to order up a minimum of 3 x ST brand 074C opAmps and hope this amp gets fixed!
I think U601 pads are OK.
Maybe its time for me to order up a minimum of 3 x ST brand 074C opAmps and hope this amp gets fixed!
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Without a schematic diagram, no idea of how the op-amp is being used and no op-amp in place, there's not much you can tell about the voltages on the pads. Replace the op-amps and if it doesn't work properly, then post the voltages again.
I put three new opAmps into this amp and its now working fine. I cant be 100% certain if the opAmps themselves were the problem though, as I had also found a misconnected trace for U501 opAmp:
I found a trace that was mis-connected through the board. I just threaded a very thin piece of copper wire through the itty bitty holes and now Pin 4 of U501 is getting +15vDC.
This amp is fixed!
I found a trace that was mis-connected through the board. I just threaded a very thin piece of copper wire through the itty bitty holes and now Pin 4 of U501 is getting +15vDC.
This amp is fixed!
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