Heres one I have not encountered before.
Got a JL 1000/1 across my bench, and it had a pair of output FETS blown along with almost all the DC filter caps dried out/popped.
I changed all the FETs and caps, including the 4420 driver ICs I just do on standard practice. The amp tries to work, but the same pair that were blown originally which is Q502/503, the gate drive waveform is about half the voltage it should be. all other transistors are seeing around 10V p-p gate drive at 70Khz. these two only see about 5V so they get really hot really fast, and it consumes too much current so the amp wont start/idle with my 5A power supply.
Thoughts?
Got a JL 1000/1 across my bench, and it had a pair of output FETS blown along with almost all the DC filter caps dried out/popped.
I changed all the FETs and caps, including the 4420 driver ICs I just do on standard practice. The amp tries to work, but the same pair that were blown originally which is Q502/503, the gate drive waveform is about half the voltage it should be. all other transistors are seeing around 10V p-p gate drive at 70Khz. these two only see about 5V so they get really hot really fast, and it consumes too much current so the amp wont start/idle with my 5A power supply.
Thoughts?
On the driver board for those outputs, is the DC voltage across the power supply pins for the TC4420 the same as on the other driver board?
I found it. one of the two big black diodes on the driver card was open circuit, Heating the diode would cause it to intermittently conduct. So i replaced it.
That section now has the proper gate drive voltage, but theres a ton of ringing that slowly creeps up as the amp powers up. That is the only section with this amount of ringing. None of the other transistor groups have the ringing:
Are the supply voltage for the optocoupler and the 4420 clean? You'll need to go to differential mode unless you have a battery powered scope.
Its the only signal out of the 4 signals that are producing this.
EDIT: Actually im wrong. Q502/3 and 4/5 both are producing gate drives that look like this.
Not sure if there is a common point where I would test for this voltage, and ive never used differential mode on a scope before.
A differential input uses two inputs to produce a single waveform. The simplest way to get a differential input is to use a differential probe. A differential probe has two signal leads and a mixer amplifier built into it. It feeds the scope a normal signal (a composite of the two signals input into the differential probe). The problem with differential probes is that they're expensive.
The alternative is to use two scope probes and and both inputs of your oscilloscope. This is how you have to set up your scope:
Two probes
Both scope inputs used
Input set to add
Both channels set to DC coupling
Both channels set to 'cal'.
Both vertical amps set to the same voltage
Ch2 input set to invert
Bandwidth limited (works best for most measurements in car amps)
Trace aligned to the reference line on the scope's display
Ground leads for both probes connected together (not always necessary)
After setting up the scope, you need to confirm that it's working as it should. With the vertical amp set to 5v/div, touching the probe that's connected to Ch1 to the positive terminal of your 12v power supply should make the trace deflect about 2.5 divisions up from the reference (like it always does, seen below). Doing the same with the probe connected to Ch2 should make the trace deflect down about 2.5 divisions. Touching both probes to the positive terminal of the 12v power supply should cause no deflection. If it does, something isn't right.
I know that this may not be as simple as the isolated scope but if you take the time to learn it one time (even if it takes an hour or more of your time), you have that knowledge and this tool to use for the rest of the time you need to use a scope. Using the analog scope will give you much larger and cleaner waveforms.
The alternative is to use two scope probes and and both inputs of your oscilloscope. This is how you have to set up your scope:
Two probes
Both scope inputs used
Input set to add
Both channels set to DC coupling
Both channels set to 'cal'.
Both vertical amps set to the same voltage
Ch2 input set to invert
Bandwidth limited (works best for most measurements in car amps)
Trace aligned to the reference line on the scope's display
Ground leads for both probes connected together (not always necessary)
After setting up the scope, you need to confirm that it's working as it should. With the vertical amp set to 5v/div, touching the probe that's connected to Ch1 to the positive terminal of your 12v power supply should make the trace deflect about 2.5 divisions up from the reference (like it always does, seen below). Doing the same with the probe connected to Ch2 should make the trace deflect down about 2.5 divisions. Touching both probes to the positive terminal of the 12v power supply should cause no deflection. If it does, something isn't right.
I know that this may not be as simple as the isolated scope but if you take the time to learn it one time (even if it takes an hour or more of your time), you have that knowledge and this tool to use for the rest of the time you need to use a scope. Using the analog scope will give you much larger and cleaner waveforms.
Well currently, I have the scope itself isolated from ground. I had to do that when I was working on tube equipment. The ground is connected to the right most pin of the MOSFET which is the Source, and with the probe itself on the Left where the Gate is. Maybe thats why, Not sure.
Here is a quick little video that is showing the issue at hand:
https://drive.google.com/file/d/13Qz_x6fMDgbM3RbXb1HI-5FP8D1iUyTM/view?usp=sharing
This might be normal, I don't remember what a good JL 1000 waveform looks like, its been too long.
Here is a quick little video that is showing the issue at hand:
https://drive.google.com/file/d/13Qz_x6fMDgbM3RbXb1HI-5FP8D1iUyTM/view?usp=sharing
This might be normal, I don't remember what a good JL 1000 waveform looks like, its been too long.