I just got this amp with a problem. It has mega DC out put on the speaker out puts. I saw that the pre-amp stage fails. If someone could give me the common parts that go wrong?
Does the DC on the output vary with the gain setting?
Do you have plus and minus 18 volts on the op-amps (pins 4 and 8)? Use the chassis ground for the reference point (black meter lead).
Do you have plus and minus 18 volts on the op-amps (pins 4 and 8)? Use the chassis ground for the reference point (black meter lead).
The op-amps will be either TL072s or NE5532s. They are the square, 8 legged ICs.
Remember to have the transistors clamped to the sink and have a 10 amp fuse in the B+ line while testing.
Remember to have the transistors clamped to the sink and have a 10 amp fuse in the B+ line while testing.
Did not get that, Here are my readings. Also I notice alot of hack job resistors and missing one's. Why did the previous repair shop do this?
The 3 TL072CP's were
pin1: -1.6
pin2: -91.7mv
pin3: -1.6
pin4: -2.59
pin5: -1.5
pin6: -1.5
pin7: -1.7
pin8: -2.4
One NR5532N was
pin1: -1.1
pin2: -0.88
pin3: -1.4
pin4: -2.58
pin5: -3.11
pin6: -3.42
pin7: -1.4
pin8: -2.37
Other one was
pin1: -1.41
pin2: -1.41
pin3: -1.66
pin4: -2.59
pin5: -1.66
pin6: -1.41
pin7: -1.41
pin8: -2.36
The 3 TL072CP's were
pin1: -1.6
pin2: -91.7mv
pin3: -1.6
pin4: -2.59
pin5: -1.5
pin6: -1.5
pin7: -1.7
pin8: -2.4
One NR5532N was
pin1: -1.1
pin2: -0.88
pin3: -1.4
pin4: -2.58
pin5: -3.11
pin6: -3.42
pin7: -1.4
pin8: -2.37
Other one was
pin1: -1.41
pin2: -1.41
pin3: -1.66
pin4: -2.59
pin5: -1.66
pin6: -1.41
pin7: -1.41
pin8: -2.36
Some of the components were left out by MTX. I've seen alot of amps like these (including the 250Ds) that had resistors jumped from point to point. I believe it was done by MTX.
Assuming that the power supply is producing rail voltage (you said you had DC on the outputs), it appears that the regulators have failed. Generally, when the regulators fail, they destroy virtually all of the op-amps. It's often easier to replace all of them than it is to try to find the defective ones. Sometimes, defective op-amps work but produce slightly distorted output. This type of problem is difficult to find without a scope.
If you want to confirm that the power supply is operating and producing rail voltage, measure the DC voltage on the legs of the output FETs (near the small green toroidal inductor). You should find both positive and negative voltage (~±50v?).
If the regs failed, this amp may not be worth repairing. You can buy the 500D used for very little on eBay. If you decide to repair it, it probably won't be easy.
Assuming that the power supply is producing rail voltage (you said you had DC on the outputs), it appears that the regulators have failed. Generally, when the regulators fail, they destroy virtually all of the op-amps. It's often easier to replace all of them than it is to try to find the defective ones. Sometimes, defective op-amps work but produce slightly distorted output. This type of problem is difficult to find without a scope.
If you want to confirm that the power supply is operating and producing rail voltage, measure the DC voltage on the legs of the output FETs (near the small green toroidal inductor). You should find both positive and negative voltage (~±50v?).
If the regs failed, this amp may not be worth repairing. You can buy the 500D used for very little on eBay. If you decide to repair it, it probably won't be easy.
The power supply is operating. The DC is being driven to the outputs because the low-side FETs are being driven on (or so it appears from the voltages you posted).
Near the center of the audio section of the board, you'll see an area that's been blackened. There are two 150 ohm resistors, two MPSW42s and two MPSW92s. The transistors will likely be a slightly different shade due to overheating. These four transistors need to be replaced. You can check them but they're almost certainly defective. Even if they're not, they've been run hot for a long time and should be replaced.
There are two orange/black zeners that are likely defective. They will be connected to the center leg of the transistors.
The filter capacitors connected to the transistors are likely defective and often vent or swell up.
The 150 ohm resistors often open.
Check these components.
Near the center of the audio section of the board, you'll see an area that's been blackened. There are two 150 ohm resistors, two MPSW42s and two MPSW92s. The transistors will likely be a slightly different shade due to overheating. These four transistors need to be replaced. You can check them but they're almost certainly defective. Even if they're not, they've been run hot for a long time and should be replaced.
There are two orange/black zeners that are likely defective. They will be connected to the center leg of the transistors.
The filter capacitors connected to the transistors are likely defective and often vent or swell up.
The 150 ohm resistors often open.
Check these components.
Some amplifiers have the W56s instead of the W92s. The W92 is a higher voltage device but otherwise essentially the same as the W56.
The designations are Q2, Q3, Q61 and Q62.
The designations are Q2, Q3, Q61 and Q62.
Now with these out of the board would it still create the DC voltage across the speaker outputs?
If the op-amps were driving the output low, it's unlikely. There could be other damage causing the outputs to be driven on.
If you're going to try to power it up (to see if it still has DC on the outputs), be sure that you don't have any solder bridges on/around the pads of the components you removed.
Are you going to try to repair the amp?
If you're going to try to power it up (to see if it still has DC on the outputs), be sure that you don't have any solder bridges on/around the pads of the components you removed.
Are you going to try to repair the amp?
I would like to repair it, if all the suspect parts are cheap I have no problem spending time on it...not doing for profit just learning and getting new stuff. I also have a MTX RT251D with DC voltage on the output. I'm going to guess its almost the same cause
The 500 D has problems related to the parts they chose to use. The 251D uses a completely different set of components. The 251D probably has blown outputs.
If you're going to repair this amp, you'll need to order the following (at least enough to replace all that may be defective):
TL072
NE5532
2155 >> 2180 VCA (mouser search for > 2180 that sip-8)
MPSW42
MPSW56 (or MPSW92)
Filter caps for the regulators
Zener diodes for the regs (18v? - part number will be printed on the diodes)
Resistors for the regulators
When repairing these amps, I generally use 390 ohm resistors in place of the 150 ohm components. It allows a bit more voltage drop across the resistors so there is less power dissipation in the regulator transistors. I install the resistors on the other side of the board (between the board and the sink). You'll need to use a resistor rated for more than 1/4 watt (the originals were 1/4w). You want something that will easily fit in the space between the board and the sink. You'll have to look up the dimensions of the resistors. Having the resistors on the other side of the board keeps the area of the regulators slightly cooler. These transistors are being run right at their rated power and need all of the help they can get.
I'd also recommend that you order sockets for the op-amps. It will make them easier to change in the future.
Order extras of everything. Nothing is really expensive (except the 2180).
If you didn't buy a good desoldering tool, this would be a good time to do so.
Are all of the legs of the op-amps bent over flat to the bottom of the board?
If you're going to repair this amp, you'll need to order the following (at least enough to replace all that may be defective):
TL072
NE5532
2155 >> 2180 VCA (mouser search for > 2180 that sip-8)
MPSW42
MPSW56 (or MPSW92)
Filter caps for the regulators
Zener diodes for the regs (18v? - part number will be printed on the diodes)
Resistors for the regulators
When repairing these amps, I generally use 390 ohm resistors in place of the 150 ohm components. It allows a bit more voltage drop across the resistors so there is less power dissipation in the regulator transistors. I install the resistors on the other side of the board (between the board and the sink). You'll need to use a resistor rated for more than 1/4 watt (the originals were 1/4w). You want something that will easily fit in the space between the board and the sink. You'll have to look up the dimensions of the resistors. Having the resistors on the other side of the board keeps the area of the regulators slightly cooler. These transistors are being run right at their rated power and need all of the help they can get.
I'd also recommend that you order sockets for the op-amps. It will make them easier to change in the future.
Order extras of everything. Nothing is really expensive (except the 2180).
If you didn't buy a good desoldering tool, this would be a good time to do so.
Are all of the legs of the op-amps bent over flat to the bottom of the board?
No the legs are not bent, but I know that because this amp was fixed atleast one time before I got it.
Is there something wrong with the capacitor(s) next to the green inductor? it looks like it may have spilled its guts on itself and the board.
Perry do these go bad? I took them all out and all my high voltage went away and the pads i had 0.5V as the highest reading. Did these go bad causing my problems?
http://i227.photobucket.com/albums/dd146/mxracer592/amp 2/vfdsgd.jpg
http://i227.photobucket.com/albums/dd146/mxracer592/amp 2/vfdsgd.jpg
Those sometimes fail but generally only when the outputs fail. I doubt that any are defective in this amp.
The voltage went away because you removed the drive circuit for the outputs. If something in the preamp section was the cause of the DC, it can no longer drive the outputs because you removed the link between them.
The voltage went away because you removed the drive circuit for the outputs. If something in the preamp section was the cause of the DC, it can no longer drive the outputs because you removed the link between them.
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