Hi guys,
So, working on my first P6002.
The amp has some trouble with the output fet/ drive circuit. I'm a bit confused by the measurements.
I removed all the output cards/fets, and the amp powers up just fine.
In case of the output fets removed and +-41v rail voltage, there is approximatelly +28v and -28v on the gate of the output fets.
When driving a 40Hz sine wave into the RCA's, there comes a very clean square wave signal on all the output fets gates, with a DC voltage of the above +28v or -28v (high side and low side banks)
When soldering in new fets (1 per bank), the +28v and -28v on the high/low side gates changes into positive or negative rail on the output fets gates.
No square wave signal present anymore on the output fets gates, and full negative rail on the speaker terminal.
I don't get it. This should be a class AB amp right?
So, working on my first P6002.
The amp has some trouble with the output fet/ drive circuit. I'm a bit confused by the measurements.
I removed all the output cards/fets, and the amp powers up just fine.
In case of the output fets removed and +-41v rail voltage, there is approximatelly +28v and -28v on the gate of the output fets.
When driving a 40Hz sine wave into the RCA's, there comes a very clean square wave signal on all the output fets gates, with a DC voltage of the above +28v or -28v (high side and low side banks)
When soldering in new fets (1 per bank), the +28v and -28v on the high/low side gates changes into positive or negative rail on the output fets gates.
No square wave signal present anymore on the output fets gates, and full negative rail on the speaker terminal.
I don't get it. This should be a class AB amp right?
Attachments
With the outputs (or any component in the feedback loop) missing and breaking the loop, the servo/feedback circuit will drive as as possible to try to correct for the loss of feedback.
Does the amp produce feedback when you have outputs installed?
Does the amp produce feedback when you have outputs installed?
Without fets installed the DC on the gate rises to positive/negative rail when powering up, then 2 sec later it straightly shoots to +28 and -28v, is this what you mean?
What excactly do you mean with feedback in this amp? In terms of what is measurable as feedback?
What excactly do you mean with feedback in this amp? In terms of what is measurable as feedback?
Amplifiers (not including tube amps or the ultra-expensive home amps) have global negative feedback. The output of the amp is compared to the input (in the differential amplifier) and the drive is adjusted to prevent any error in the output.
Does the amp produce feedback when you have outputs installed?
^^My mistake.
Does the amp produce output when you have outputs installed?
Does the amp produce feedback when you have outputs installed?
^^My mistake.
Does the amp produce output when you have outputs installed?
Thats a very interesting working principle. How is the output compared to the input? Will it be compared for amplitude? Or for filtering any strange spikes in the output?
Since this is a P6002, I installed 1 fet per bank in only one channel of the amplifier (I assume this would be no problem, since this is a seperate channel).
There is no output produced. There is full negative rail on the speaker terminal.
Current draw was +-1.1A at 12.9v of PS voltage.
Since this is a P6002, I installed 1 fet per bank in only one channel of the amplifier (I assume this would be no problem, since this is a seperate channel).
There is no output produced. There is full negative rail on the speaker terminal.
Current draw was +-1.1A at 12.9v of PS voltage.
It corrects for any difference in the input and output signals.
The output, since amplified, is passed through a voltage divider. The voltage divider determines the overall gain of the output stage.
Do you see the same conditions (full negative rail) on both channels when outputs are in the other channel as well?
Are the MEHSA insulators in place?
Did any of the 0.1 ohm source resistors fail?
The output, since amplified, is passed through a voltage divider. The voltage divider determines the overall gain of the output stage.
Do you see the same conditions (full negative rail) on both channels when outputs are in the other channel as well?
Are the MEHSA insulators in place?
Did any of the 0.1 ohm source resistors fail?
So, back on this amp.
The insulators are not in place. I removed all of them to check the fets. Since none of the fets measured defect.
Or does this create problems with this amp? Only the fets installed.
The insulators are not in place. I removed all of them to check the fets. Since none of the fets measured defect.
Or does this create problems with this amp? Only the fets installed.
The bias transistors are on the MEHSA insulators.
If you have the equivalent through-hole transistor, you can install that in the place of the 3-pin header. Confirm that you have the correct pin configuration when you install the transistor.
If you have the equivalent through-hole transistor, you can install that in the place of the 3-pin header. Confirm that you have the correct pin configuration when you install the transistor.
Maybe this will say something? Insulators not in place.
Installed fets on channel 1 and channel 2. Channel one has negative rail on the speaker terminal (probably coming from Q1026).
Channel 2 has no DC on the speaker terminal
Installed fets on channel 1 and channel 2. Channel one has negative rail on the speaker terminal (probably coming from Q1026).
Channel 2 has no DC on the speaker terminal
I don't think you can make any judgements based on voltage without having the circuit complete.
If you don't have through-hole biasing transistors, run small wire jumpers from the MEHSA headers.
If you don't have through-hole biasing transistors, run small wire jumpers from the MEHSA headers.
Connected the MEHSA insulators.
Still a negative DC on the speaker terminal.
Connecting a load to the terminal does not change anything.
Tried to compare the vias from the MESHA insulator pins at J2000, J9 and J1000, but they all read different.
I measure no DC voltages on J1000.
Still a negative DC on the speaker terminal.
Connecting a load to the terminal does not change anything.
Tried to compare the vias from the MESHA insulator pins at J2000, J9 and J1000, but they all read different.
I measure no DC voltages on J1000.
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With the MEHSA connected, do you have audio on the channel with no DC?
When you touch a load to the DC channel, does it cause an increase in current or does it simply drag the DC to 0v?
I've attached the diagram if you don't have it.
When you touch a load to the DC channel, does it cause an increase in current or does it simply drag the DC to 0v?
I've attached the diagram if you don't have it.
Thanks for the diagram Perry 🙂
With the MESHA connected, there is no audio on channel 2 with no DC on the speaker terminal.
However, changing the amplitude of the RCA signal gives a different reading on the transistor of the MESHA insulator from that channel.
So the RCA signal should be coming through the pre-amp section.
Attaching a load to the channel 1 DC speaker terminal results in a very high current pull.
With the MESHA connected, there is no audio on channel 2 with no DC on the speaker terminal.
However, changing the amplitude of the RCA signal gives a different reading on the transistor of the MESHA insulator from that channel.
So the RCA signal should be coming through the pre-amp section.
Attaching a load to the channel 1 DC speaker terminal results in a very high current pull.
I'd suggest that, if you cannot find a defective transistor, that you install the outputs, and pull the drivers that drive the output transistors. Does that get rid of the DC voltage?
If not, reinstall those drivers and remove the next stage back. Does that get rid of the DC?
When you lose the DC, the problem is likely in the removed stage or farther back.
If not, reinstall those drivers and remove the next stage back. Does that get rid of the DC?
When you lose the DC, the problem is likely in the removed stage or farther back.
I got rid of the DC voltage at the speaker terminal. I've done a lot of measuring. Whoever tried to repair this before fitted wrong fets in place, that was the simple cause.
However, when removing the jumper wire at C9, the amplifier wants to pull a lot of amps.
With the jumper connected between both C9 pads, the current pull from this amplifier seems fine.
Does this jumper wire also causes the amp to not produce audio on the speaker terminal?
Since the pre-amp section seems to work perfectly, but there is no sound coming from the speaker terminal (clipping at 1v peak to peak).
However, when removing the jumper wire at C9, the amplifier wants to pull a lot of amps.
With the jumper connected between both C9 pads, the current pull from this amplifier seems fine.
Does this jumper wire also causes the amp to not produce audio on the speaker terminal?
Since the pre-amp section seems to work perfectly, but there is no sound coming from the speaker terminal (clipping at 1v peak to peak).
What solved the DC offset problem?
Yes. It shuts down the output section.
Set the bias pots fully CCW. Does it still draw excessive current?
Yes. It shuts down the output section.
Set the bias pots fully CCW. Does it still draw excessive current?
The amp came with wrong fets.
I installed the same fets as installed since they are used often in Rockford amps (28N15 and 36P15).
One new fet blew right away after installing, this caused the DC offset.
I removed lot of driver transistors, but this did not get rid of the DC. (There was one good working bank, and they had the same readings as the bad bank). So it did not make sense...
Ofcourse, this amp came with shitty bias potentiometers as well. It seemed like that they had glue inside as well, not possible to rotate it, even after removing the glue.... Sooo I fitted new 2K bias potentiometers which I took from a spare T600-4 donor board.
Removed the jumper wire at C9, rotated the potentiometers CCW. Amp seems to produce clean audio now.
Pulls +- 1.7A idle at 12.3v PS.
Fets do heat up.
I installed the same fets as installed since they are used often in Rockford amps (28N15 and 36P15).
One new fet blew right away after installing, this caused the DC offset.
I removed lot of driver transistors, but this did not get rid of the DC. (There was one good working bank, and they had the same readings as the bad bank). So it did not make sense...
Ofcourse, this amp came with shitty bias potentiometers as well. It seemed like that they had glue inside as well, not possible to rotate it, even after removing the glue.... Sooo I fitted new 2K bias potentiometers which I took from a spare T600-4 donor board.
Removed the jumper wire at C9, rotated the potentiometers CCW. Amp seems to produce clean audio now.
Pulls +- 1.7A idle at 12.3v PS.
Fets do heat up.