No. The resistors that drive the gate legs of Q405, 406, 408 and 422.
Did you power up the amp with the 900 series FETs in the board but with the transistors listed above out of the board?
Did you power up the amp with the 900 series FETs in the board but with the transistors listed above out of the board?
There are 2 banks of 7 smt resistors near the sub audio output transistors. Not sure which are the gate resistors. From left to right:
Left bank: 75.5 1k 1.8k .47k 10 100 103
Right bank: .47k 100 10 100 1k 1.8k 75.5
Left bank: 75.5 1k 1.8k .47k 10 100 103
Right bank: .47k 100 10 100 1k 1.8k 75.5
The gate resistors are the ones that read 0 ohms between one of the gate legs and one end of the resistor.
I found them. They read 100 ohms on each of the four. Should I have kept the output transistors out when first powering up? What else could be suspect?
Just to note, the fuse blows about 7 seconds after applying power.
Just to note, the fuse blows about 7 seconds after applying power.
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Were any of these 4 transistors defective initially?
When repairing complex amps, it's generally best to apply power to sections incrementally. That's why I wanted you to get the front channels completed, then move to the sub channel. There, the best thing would have been to install the 900 series FETs and power it up to see if there were no problems. If there were not, then you'd install the sub channel output FETs.
When repairing complex amps, it's generally best to apply power to sections incrementally. That's why I wanted you to get the front channels completed, then move to the sub channel. There, the best thing would have been to install the 900 series FETs and power it up to see if there were no problems. If there were not, then you'd install the sub channel output FETs.
You mention front channel, but the only channels that were not working, based on audio output, was the sub channel. Can you clarify?
I assume the appropriate next step would be to remove the sub channel FET's and power up? After dpi g so, what would I check for, other than a blown fuse?
I assume the appropriate next step would be to remove the sub channel FET's and power up? After dpi g so, what would I check for, other than a blown fuse?
Forgot to mention that, the 4 output transistors were initially ok, from what you said.
Ok, I thought the burned LED's were on the sub channel.
So after replacing the components that tested faulty, the amp draws more current than expected. More than the initial charge of the capacitors. So I will remove the sub channel FET's and power up, then post my findings.
Tonight I removed Q405, 406, 408 and 422 FET's. I re-clamped and powered the amp up. Everything appears stable, all card LED's are on, main power light stays on. There is some hissing from the power supply section but I remember that being there before.
No fuse has blown. So it seems that there is something wrong with the output section.
I tested each FET as shown here: Basic Amplifier Repair - Transistor Test Applet Link
All FET's are working according to the test.
No fuse has blown. So it seems that there is something wrong with the output section.
I tested each FET as shown here: Basic Amplifier Repair - Transistor Test Applet Link
All FET's are working according to the test.
With those 4 FETs out of the circuit, apply remote voltage to the amp for a few seconds and then remove the remote voltage. The driver stage for the class D section should remain active for about 5-10 seconds after removing remote voltage. During that time, measure the DC voltage on pads 1 and 3 (black probe on pin 3) for each of the 4 FETs. Post the voltage for all 4.
I followed your instructions. The voltage changed quite a bit while probing the pads on each FET's vias sequentially. I recorded the first number I saw, but they fluctuate several tens of millivolts every second:
Q406: 200 mV
Q405: 360 mV
Q422: 370 mV
Q408: 280 mV
I took the readings a second time, this time taking measurements but powering up again before taking the next FET's reading. Same fluctuation observed.
Q406: 28 mV
Q405: 118 mV
Q422: 88 mV
Q408: 41 mV
Q406: 200 mV
Q405: 360 mV
Q422: 370 mV
Q408: 280 mV
I took the readings a second time, this time taking measurements but powering up again before taking the next FET's reading. Same fluctuation observed.
Q406: 28 mV
Q405: 118 mV
Q422: 88 mV
Q408: 41 mV
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I would have expected higher voltage but your meter may not have enough bandwidth to read the high frequency signal. Do you have a scope? If not, can you borrow a meter like a Fluke 87v?
I am using an Amprobe DM9C.
I have access to a scope and potentially a Fluke 87v. I will check the voltages again tomorrow.
I have access to a scope and potentially a Fluke 87v. I will check the voltages again tomorrow.
If you're using a scope, you'll be looking for a square wave with an amplitude of about 10v. This will likely only be visible on the low-side FETs. To view the drive signal on the high-side FETs, you'll need to either use a differential probe setup or ground the 3rd leg of the high-side FETs. When you do this, make sure that there are no solder bridges between the pads for the FETs. Also use a 5 amp fuse if possible. The drive signal should essentially look the same as the low-side signal.
If you're using a fluke 87v (or similar), place the probes as you did previously.
If you're using a fluke 87v (or similar), place the probes as you did previously.
No luck finding a Fluke 87v today. I am curious, what is considered similar to the 87v?
In order to use a scope, I need to move this project to work and I'm a little hesitant to do so at the moment. I'd like to try and track down or rent a portable DVM, like the Fluke or similar.
In order to use a scope, I need to move this project to work and I'm a little hesitant to do so at the moment. I'd like to try and track down or rent a portable DVM, like the Fluke or similar.
To use a meter, you would need one that can measure high frequency signals. A usable meter would almost certainly be true RMS but not all true RMS meters can read higher frequencies. I've tested the 87v and it read accurately to well over 100kHz. That's why I recommended it.
I just wanted to follow up on this thread, have been meaning to do so for quite some time now.
I was laid off before Thanksgiving and this amp was boxed up and the project put on hold.
Last I checked the 87v meter was around $400. Rather than shelling out the cash for a meter, I picked up a JL 300/4 and JL 250/1 for $200 so I am going to sell the 500/5.
I absolutely appreciate your assistance Perry and wish I could complete the task at hand but I am still looking for gainful employment and therefore need to cut my loses.
I was laid off before Thanksgiving and this amp was boxed up and the project put on hold.
Last I checked the 87v meter was around $400. Rather than shelling out the cash for a meter, I picked up a JL 300/4 and JL 250/1 for $200 so I am going to sell the 500/5.
I absolutely appreciate your assistance Perry and wish I could complete the task at hand but I am still looking for gainful employment and therefore need to cut my loses.
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