Soundstream xxx4000d driver board

[maurosubito]

Hello, i'm repairing an xxx4000d driver board.

Now amp works fine but it has a bump when start sound and the small transistors (ktc3228-kta1277) become hot in 30 seconds after power on.

Is it right or I have a problem in the amp?

I think to replace small transistors with mje340-mje350

 

[Perry Babin]

Q43 sometimes goes leaky.

 

[maurosubito]

I have this board with this numbers.

I've tested all the transistors out of board and all seems ok.

 

[Perry Babin]

See attached.

 

[maurosubito]

Ok, I've tested all of its out of board and seems Ok
What do you think if I replace its with mje 340-350?
Rail is + and - 119 volts

 

[Perry Babin]

Class D amps can be sensitive to substitute parts. I don't know if those will work.

How much DC do you see at the speaker terminals before the amp switches the output stage on?

After switching on?

 

[maurosubito]

I have 0 volts at power on

I have a peak of 0.35 volts when the amp switch on the output stage

I have 0.025 volts when amp sounds.

 

[Perry Babin]

When the drivers are defective, there is generally DC between the time the power supply powers on and the SW terminal is driven to ground (when the red LED switches off, on most of the amps like this).

Does the amp produce clean power up to clipping when loaded?

 

[maurosubito]

I will test it tomorrow at clipping with load, I have to replace the fan and put the board in the chassis with cooled transistor.

 

[Perry Babin]

Most of the fans in amps like this can be cleaned and re-lubricated.

 

[maurosubito]

Fan is replaced with new fan, I pay it 4 euro.

Now I test the two drive signal referred to center tap.

Red is the gate of transistor with negative rail on source

Yellow is the gate of transistor with positive rail on drain.

In the driver board only the transistor that drives low side become hot.

Drivers and pre drivers, all are hot, but irfp264n on low side are cold as the high side

 

[maurosubito]

I cannot test amp with load.
With cooled mosfet I test the amp with music.
At idle it draws 3 amps
When sounds at low volume it draws 3-3.5 amps
After 30 seconds it increases the current and at 8-9 amps then I power off the amp.

Driver board is very hot only in the part that drives the output mosfet irfp264n with negative rail i on source

 

[Perry Babin]

Some of these amps draw a lot of current at idle. You can reduce it by increasing the value of the resistor in the constant current source. I think the resistor is connected to the emitter of the small transistor (Q112?) at the top corner of the board. That's a last resort if everything else appears to be OK.

The low side driver transistors run hot. You can see that one of the low-side drivers is brown in the photo. That's likely because the one in parallel with it has a broken solder connection and it's working alone (it could be defective from running too hot).

 

[maurosubito]

Tested with Toshiba 2sa1837 and 2sc4793
Mje340-Mje340

Amp working at the same conditions, same driving wave, low side mje340-mje340 are very hot, low side 1837-4793 are hot.

Irfp264n are ok, tested out of board, diode on gate are ok, resistors on driver board are ok

 

[Perry Babin]

The low side are going to run hot. It's due to the circuit design. The newer designs used in other amps use a dedicated supply that has the transistors operating at a lower voltage and they run cooler. There isn't much that you can do to make the ones in this amp run cooler.

If you leave the leads long so that the body of the transistor is very nearly touching the blades of the fan, that will help the transistors run somewhat cooler and get the heat away from the board to prevent damaging the board.

Is the main problem still the high idle current?

 

[maurosubito]

i note that this amp discharge the negative rail to fast

Without output transistor and without driver board the negative rail goes tu zero volts in e few time than positive rail.
When negative is at 0 positive is at 30 volts

Idle current is 2 amps without driver board and without low side Irfp264n

The problem is that pre-drivers become hot and amp starts draws a lot of current without control.
With mje and Toshiba I have the same problem

 

[Perry Babin]

Are R361 and R362 bleed-off resistors? If so, are both within tolerance?

 

[maurosubito]

Resistors are ok, changed, but old are ok, the inductor L305 is ok

 

[Perry Babin]

A couple of things are possible.

It could be that the rails aren't evenly loaded and that's causing one rail to discharge more quickly.

If the low-side drivers were defective, it could have caused the negative rail to see excessive voltage, damaging the capacitors. They could be leaking (electrically) or have reduced capacity.

I'm assuming that the rails have equal voltage (above and below 0v) during normal operation.

 

[maurosubito]

Rail has + 96 and - 96 during operations.

Capacitors seems ok, but I have to test without board.

Now I note that when amp works at low volume- idle, in driver board become hot all the pre drivers and drivers of low side, but low side most are cold

In the high side drivers are all cold, but I have only 2 irfp264n that are very hot. Only q131-q132.

At idle it draws 3 amps when power on, 4,5 amps when there is oscillation in output stage.