The signal goes from the board connector, through a 10uF cap, through the gain control, through another 10uF cap, through 100K ohm resistor to the + input of the 2068D. I suspect the three 2068D's.
Could it be the supply voltage going to them? It would be odd (not impossible if all from the same batch) for all 3 to fail.
Is one heating up?
If you set your scope to a low vertical amp setting and sweep it around on the circuit board (not on any conductors/pads), does the trace ever show any deflection?
Is one heating up?
If you set your scope to a low vertical amp setting and sweep it around on the circuit board (not on any conductors/pads), does the trace ever show any deflection?
Well only two are from the same batch, but I agree it would be odd for all three to fail. None feel any warmer than the other.
I don't see any deflection probing around the area.
With the amp fully re-assembled and all circuits restored, here is what I get for measurements on the three opamps.
U4
1: 160.8mV
2: 17.4mV
3: 17.3mV
4: -14.66V
5: amp draws a lot of current probing this pin
6: 17.8mV
7: 165.5mV
8: 14.77V
U5
1: 147.8mV
2: amp draws a lot of current probing this pin
3: 15.6mV
4: -14.66V
5: 15.7mV
6: 16.0mV
7: 147.8mV
8: 14.77V
U6
1: 149.3mV
2: amp draws a lot of current probing this pin, *see note below
3: 16.0mV
4: -14.66V
5: 16.2mV
6: 16.3mV
7: 150.2mV
8: 14.77V
* Pin 2 of U6, before I re-inserted the resistors to the J108 drains, I was able to measure 16.1mv here without issue.
Also interesting is the result of probing of pin 5 of U4 vs pin 2 of the other two opamps.
Anything to make of this?
I don't see any deflection probing around the area.
With the amp fully re-assembled and all circuits restored, here is what I get for measurements on the three opamps.
U4
1: 160.8mV
2: 17.4mV
3: 17.3mV
4: -14.66V
5: amp draws a lot of current probing this pin
6: 17.8mV
7: 165.5mV
8: 14.77V
U5
1: 147.8mV
2: amp draws a lot of current probing this pin
3: 15.6mV
4: -14.66V
5: 15.7mV
6: 16.0mV
7: 147.8mV
8: 14.77V
U6
1: 149.3mV
2: amp draws a lot of current probing this pin, *see note below
3: 16.0mV
4: -14.66V
5: 16.2mV
6: 16.3mV
7: 150.2mV
8: 14.77V
* Pin 2 of U6, before I re-inserted the resistors to the J108 drains, I was able to measure 16.1mv here without issue.
Also interesting is the result of probing of pin 5 of U4 vs pin 2 of the other two opamps.
Anything to make of this?
Is it causing the output of the corresponding channel to oscillate when the problem pins are probed?
I may have asked this but is the primary to secondary resistor within tolerance?
I may have asked this but is the primary to secondary resistor within tolerance?
Do you see a noise-inducing pulse on the outputs of all of these op-amps when the noise would be present on the output? Test with the resistors you removed, again out of the circuit.
Answer first question in post #44
With woofer connected to the output, probing pin 2 of both U5 and U6 cause loud pop then whine and high current draw.
Probing pin 5 of U4, just causes whine and high current draw. Amp returns to normal removing probe.
Answer to post #47
Using the scope to probe U6 output pins with resistors out of circuit, I don't see a pulse at power up, but I do see one at power down. The little bit of dc offset I measured above begins to fall then a quick spike happens and falls to zero. Now with those resistors removed, that set of outputs are muted but I think it corresponds to the pop.
With woofer connected to the output, probing pin 2 of both U5 and U6 cause loud pop then whine and high current draw.
Probing pin 5 of U4, just causes whine and high current draw. Amp returns to normal removing probe.
Answer to post #47
Using the scope to probe U6 output pins with resistors out of circuit, I don't see a pulse at power up, but I do see one at power down. The little bit of dc offset I measured above begins to fall then a quick spike happens and falls to zero. Now with those resistors removed, that set of outputs are muted but I think it corresponds to the pop.
Do you see a corresponding input pulse on the op-amps that could cause the noise at the output?
Check the inverting input at the input to the op-amp as well as at the resistor feeding that input (on the terminal not connected to the op-amp).
Check the inverting input at the input to the op-amp as well as at the resistor feeding that input (on the terminal not connected to the op-amp).
Yes, the spike I see at the output is on both input pins, both sides.
I also see it all the way back to the input header pin (without crossover board).
Probing the +/- 15V rails, they build smoothly and drop smoothly, no spiking. Same with the main rail voltage, no spiking there.
I also see it all the way back to the input header pin (without crossover board).
Probing the +/- 15V rails, they build smoothly and drop smoothly, no spiking. Same with the main rail voltage, no spiking there.
Where could it possibly coming from? Are there any components between the connector for the preamp board and those op-amps?
Could it be possible that, without the preamp board, that the op-amps there are not having their input stabilized?
Is there a corresponding group of op-amps on the 6800 diagram?
Could it be possible that, without the preamp board, that the op-amps there are not having their input stabilized?
Is there a corresponding group of op-amps on the 6800 diagram?
I could be wrong, but all I found between the header and the 2068 was a capacitor before the gain pot to another capacitor then resistor to the non-inverting input.
Probing those header pins with the crossover board installed yielded the same results with or without.
As to the 6800, if you are referring to the three 2068's I have, maybe the quad op amp right before the two j108, but the input pins are connected differently. I dont have a schematic for the crossover board to see whats in front of that quad op amp.
Probing those header pins with the crossover board installed yielded the same results with or without.
As to the 6800, if you are referring to the three 2068's I have, maybe the quad op amp right before the two j108, but the input pins are connected differently. I dont have a schematic for the crossover board to see whats in front of that quad op amp.
Can you find a ground and the pins on the xo board that go to the inputs of those op-amps?
If so, can you connect those inputs to that ground to see if the amp will be silent?
If so, can you connect those inputs to that ground to see if the amp will be silent?
Yes, have all the header pins identified and can test that.
Sorry, I wont be able to get to that now until tomorrow evening.
Thank you for the help thus far.
Sorry, I wont be able to get to that now until tomorrow evening.
Thank you for the help thus far.
Taking the input pins on the header with crossover board installed to secondary ground does not quiet the thump.
Do you have a different part number op-amp like a TL072 or similar that you could drop in to see if it made a difference?
I have some 072's, a bunch of 4558's and some 4560's and one 2068 that could be tried.
I am not able to determine which IC, if any, is more suspect than the other.
I'm thinking, if I remove any, I will remove all three 2068s and the LM837 (I see the spikes on its pins as well) one at a time, power it up to see if the spike goes away between each removal. If it does great, I will just order new ones. If it does not, I have more looking to do. I don't want to stress the vias any more than I have to on this board by inserting and removing ICs. Unfortunately, I do not have three sockets right now either.
Good or bad idea?
I am not able to determine which IC, if any, is more suspect than the other.
I'm thinking, if I remove any, I will remove all three 2068s and the LM837 (I see the spikes on its pins as well) one at a time, power it up to see if the spike goes away between each removal. If it does great, I will just order new ones. If it does not, I have more looking to do. I don't want to stress the vias any more than I have to on this board by inserting and removing ICs. Unfortunately, I do not have three sockets right now either.
Good or bad idea?
If you don't mind sacrificing the ICs, clip the legs off at the body of the IC with flush-cutters. Then remove one leg at a time. There will be no stress on the vias. If you're careful, the iron won't even touch them. Add additional solder, heat the leg and capture it against a jeweler's screwdriver and lift it out.
Can't you break the connection between the op-amps to try to determine which is the most likely suspect.
Can't you break the connection between the op-amps to try to determine which is the most likely suspect.
Attachments
I have not removed any ICs yet, just checking a few things out yet.
I disconnected the two resistors supplying the +/- 15V to the four ICs on the main board. I saw nothing on the input or output pins and there was no popping. I did see some small spiking on the -15V pins so I just replaced both regulators. The 7915 out of circuit measured leg to leg very differently from the new ones I have, the 7815 did not. Checking the -15V pins showed no spikes anymore but re-connecting the supply power showed changing those regulators made no difference amp still has a slight power up pop and two much louder at power down.
What I find at the moment is, the main rails +/-30V, rise at the same rate but do not fall exactly the same. The Negative rail falls with the Positive rail until a point, about when it reaches -3V, pauses then finishes after the Positive is done. At the point where the negative rail resumes going to zero, the outputs pop the second time. I also find it kind of weird that the positive rail actually falls to about -.350V instead of zero and then slowly goes to zero.
I'm not 100% convinced the op amps are the cause of the pops yet. It seems more power supply related.
I disconnected the two resistors supplying the +/- 15V to the four ICs on the main board. I saw nothing on the input or output pins and there was no popping. I did see some small spiking on the -15V pins so I just replaced both regulators. The 7915 out of circuit measured leg to leg very differently from the new ones I have, the 7815 did not. Checking the -15V pins showed no spikes anymore but re-connecting the supply power showed changing those regulators made no difference amp still has a slight power up pop and two much louder at power down.
What I find at the moment is, the main rails +/-30V, rise at the same rate but do not fall exactly the same. The Negative rail falls with the Positive rail until a point, about when it reaches -3V, pauses then finishes after the Positive is done. At the point where the negative rail resumes going to zero, the outputs pop the second time. I also find it kind of weird that the positive rail actually falls to about -.350V instead of zero and then slowly goes to zero.
I'm not 100% convinced the op amps are the cause of the pops yet. It seems more power supply related.
