Who needs those pesky protection circuits anyway.
Lifted diodes, and powered it on 10v, the amplifier is out of protection mode, the current draw is about 1.5amp with no input signal.
Nothing is heating up excessively.
I will look into power and output oscillation and report back.
Lifted diodes, and powered it on 10v, the amplifier is out of protection mode, the current draw is about 1.5amp with no input signal.
Nothing is heating up excessively.
I will look into power and output oscillation and report back.
It's VERY common to defeat the protection circuit when doing repairs. You're not going to leave them defeated.
This amplifier is definitely not feeling good.
With 12V power applied, before the relay engages, there is 67v on both main power rails.
Measured on the terminals of the main smoothing capacitors.
But after it clicks the negative rail goes to 15v and the positive rail jitters between 4-7v dc.
The power mosfets heat up unevenly with one of them becoming hot and other 7 staying warm.
Output fets stay relatively cold.
With 12V power applied, before the relay engages, there is 67v on both main power rails.
Measured on the terminals of the main smoothing capacitors.
But after it clicks the negative rail goes to 15v and the positive rail jitters between 4-7v dc.
The power mosfets heat up unevenly with one of them becoming hot and other 7 staying warm.
Output fets stay relatively cold.
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Do you see oscillation on the output transistors before the relay clicks?
If not, try driving a strong signal into the amp. Some class D amps need a signal to start oscillating.
Look at the gate signal on the hot PS FET to see if it differs from the other PS FETs.
If not, try driving a strong signal into the amp. Some class D amps need a signal to start oscillating.
Look at the gate signal on the hot PS FET to see if it differs from the other PS FETs.
I wasn't sure where the reference point should be for this measurement, as I only vaguely understand how all the output transistors are connected.
I've tried to measure on an output transistor gate with a ground clip connected to the rail ground and I don't see any oscillation neither before relay click, nor after.
Also I measured aux +-9v voltage and before relay clicks it's a proper 9v, but after the click it dips down to 2.5v
All power supply transistors have an identical gate drive signal, so maybe that uneven heating is due to a badly matched transistors.
I've tried to measure on an output transistor gate with a ground clip connected to the rail ground and I don't see any oscillation neither before relay click, nor after.
Also I measured aux +-9v voltage and before relay clicks it's a proper 9v, but after the click it dips down to 2.5v
All power supply transistors have an identical gate drive signal, so maybe that uneven heating is due to a badly matched transistors.
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I mean the ground pin of the smoothing capacitor on a given rail.
With no input signal on 12v the amplifier draws around 3amp and I'm not sure where all this power goes as I can't find anything heating up in the output section.
With no input signal on 12v the amplifier draws around 3amp and I'm not sure where all this power goes as I can't find anything heating up in the output section.
Unless you have an isolated scope, you cannot ground to the negative terminals of rail caps since the negative terminal of the negative rail cap is equal to negative rail voltage. Use the secondary center tap for the scope ground.
Do you see input signal on the driver board?
You may have to remove the output transistors for further testing.
Are you 100% sure that none of the outputs are shorted/leaking?
What was the result of checking the drive on the heating PS FET?
Do you see input signal on the driver board?
You may have to remove the output transistors for further testing.
Are you 100% sure that none of the outputs are shorted/leaking?
What was the result of checking the drive on the heating PS FET?
"the negative terminal of the negative rail cap is equal to negative rail voltage"
Yes, I understand that, for negative rail I was grounding to the positive capacitor pin.
I will remeasure the drive signal with ground on the transformer secondary and will report back.
All power supply transistors have an identical gate drive signal, so maybe that uneven heating is due to a badly matched transistors.
If output transistors were shorted they would be heating up and they are cold.
Yes, I understand that, for negative rail I was grounding to the positive capacitor pin.
I will remeasure the drive signal with ground on the transformer secondary and will report back.
All power supply transistors have an identical gate drive signal, so maybe that uneven heating is due to a badly matched transistors.
If output transistors were shorted they would be heating up and they are cold.
Two shorted outputs on opposing rails will not heat up. Shorted = no resistance and the power dissipation across 0 ohms is 0 watts (no heat).
Pull the outputs. If you have a good iron, it wall take only 3-4 minutes. You can then check them definitively and it may allow the amp to power up without having various voltages dragged down.
Pull the outputs. If you have a good iron, it wall take only 3-4 minutes. You can then check them definitively and it may allow the amp to power up without having various voltages dragged down.
I pulled the output mosfets.
All dc voltages are back to where they should be, ±68v rails, ±9v aux.
Power supply fets stopped heating up and stay cold, aux voltage regulators are relatively hot.
All output mosfets testing as good
all four 640 nfets - good
all five 9640 pfets - good
My tester doesn't show Rdson value but I doubt it does matter here.
There is no oscillation in the output section, with or without input sine wave, only dc voltages everywhere.
On the positive rail (9640 fets) there is 58v dc on the gate pin, 68v on the source pin.
On the negative rail (640 fets) there is -67v dc on the gate pin, -68v on the source pin
All dc voltages are back to where they should be, ±68v rails, ±9v aux.
Power supply fets stopped heating up and stay cold, aux voltage regulators are relatively hot.
All output mosfets testing as good
all four 640 nfets - good
all five 9640 pfets - good
My tester doesn't show Rdson value but I doubt it does matter here.
There is no oscillation in the output section, with or without input sine wave, only dc voltages everywhere.
On the positive rail (9640 fets) there is 58v dc on the gate pin, 68v on the source pin.
On the negative rail (640 fets) there is -67v dc on the gate pin, -68v on the source pin
Input signal is present ok, it's a drive square wave that is absent.
Input signal is coming to the driver board on the "IN" terminal, goes into add-on board to a tl072 opamp, and at least that half of opamp is working as i can see an amplified signal on it's output pin (according to jacampb2's schematic)
I'm going to trace other voltages and signals on the driver board to determine if there is something wrong with it.
Input signal is coming to the driver board on the "IN" terminal, goes into add-on board to a tl072 opamp, and at least that half of opamp is working as i can see an amplified signal on it's output pin (according to jacampb2's schematic)
I'm going to trace other voltages and signals on the driver board to determine if there is something wrong with it.
I'm not sure if this driver could work without the feedback loop and this complicate things even more.
Maybe, just maybe, it's a good moment to stop here as I don't believe that I will be able to fully understand this self-oscillating driver thing any soon.
I could try to replace an opamp/comparator board blindly, and if that will not help just go and replace everything on the driver board.
Maybe, just maybe, it's a good moment to stop here as I don't believe that I will be able to fully understand this self-oscillating driver thing any soon.
I could try to replace an opamp/comparator board blindly, and if that will not help just go and replace everything on the driver board.
For most self oscillating circuits, if you drive a strong enough signal into the input, the pulses will be driven to the outputs.
You'll have to decide whether the amp is worth the effort to replace the small board on the driver board.
You'll have to decide whether the amp is worth the effort to replace the small board on the driver board.
I'm going to spend more time to experiment and learn how does this specific modulator work.
Will this amp be repaired or not I just want to say thank you, Perry, your guidance was invaluable to proceed with this repair.
Will this amp be repaired or not I just want to say thank you, Perry, your guidance was invaluable to proceed with this repair.
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Well, no luck with this.
I've replaced the board and it does "something" as I can see the oscillation on the output fet gates with no fets soldered, but the moment I solder in the output fets it all goes to ****, power rails all over the place, power fets heating up, the amp no longer in protection mode, but it definitely doesn't work as it should.
May be those "1kw1ch" boards are wired differently, I don't know.
I think I will wrap up here, unless I'll get my hands on a working dlm board to compare.
I've replaced the board and it does "something" as I can see the oscillation on the output fet gates with no fets soldered, but the moment I solder in the output fets it all goes to ****, power rails all over the place, power fets heating up, the amp no longer in protection mode, but it definitely doesn't work as it should.
May be those "1kw1ch" boards are wired differently, I don't know.
I think I will wrap up here, unless I'll get my hands on a working dlm board to compare.
If you run out of things to do, pull the FETs and load one FET location in each bank (gate to source) with a 0.01uF or similar value capacitor. Do you still have a good drive signal on both banks?
I returned to this amplifier after a break.
I've spent some time to learn how the dlm4500 work. It turned out that this is not a self oscillating modulator, but a more traditional pwm with a fixed carrier freq.
It has a triangular generator circuit which consists of a schmitt trigger (one half of lm319) and an integrator (one half of tl072). This generator should work either with or without input signal.
My problem is this generator doesn't produce a triangular wave.
Instead of 100kHz triangular wave with amplitude of 9v it produces a tiny amplitude square wave biased to -9v
I checked all related passive elements and they measuring ok.
I will remeasure them and if this will not reveal the problem, then I tempted to replace ICs. Again. There is a small chance that I killed the ICs during soldering.
At least "fixing a standard triangular generator" instead of "fixing a proprietary modulator board" sounds much more manageable to me.
I've spent some time to learn how the dlm4500 work. It turned out that this is not a self oscillating modulator, but a more traditional pwm with a fixed carrier freq.
It has a triangular generator circuit which consists of a schmitt trigger (one half of lm319) and an integrator (one half of tl072). This generator should work either with or without input signal.
My problem is this generator doesn't produce a triangular wave.
Instead of 100kHz triangular wave with amplitude of 9v it produces a tiny amplitude square wave biased to -9v
I checked all related passive elements and they measuring ok.
I will remeasure them and if this will not reveal the problem, then I tempted to replace ICs. Again. There is a small chance that I killed the ICs during soldering.
At least "fixing a standard triangular generator" instead of "fixing a proprietary modulator board" sounds much more manageable to me.
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