Blown Power Supply: Soundstream Ref.200s

[gisewhcs]

As already said, I've got an Ref.200s lying here with a blown PS.

Q22 (MTP50n05e) was blown.
I soldered in a pair of FQP50n06, and Q22 blew again, without any load.
Now I'm suspecting the SG3524N to be dead.
Are there any other parts that should be checked, because I have to order the SG, so I could order them all together.

And are IRFZ44N usable substitutes for those P50n05e, because they are hard to get here in Germany.

Or does anybody even have schematics for this amplifier?

[Perry Babin]

I don't have the schematic diagram. Check with SoundStream.

Remove the FETs, power up the amp and measure the voltage on the gate pad for the FET. You should get the same for both (all) FETs and it should be ~5-6v DC (read with multimeter).

The Z44 should be a good sub.

[gisewhcs]

Hmmm, on the gate of Q22 I only have 3,79V, measured to prim. ground.
On the other gate it's 3,85V.
The gate resistors are also ok, both read 47Ohm
I think I will just change the controller! What else should be bad? The PS has no driver transistors or else!

[Perry Babin]

Shorted rectifiers, shorted outputs or a shorted power transformer could cause the FETs in the power supply to blow.

Do you have an oscilloscope?

[gisewhcs]

It's been only one of the two FETs that was blowing, quasi one halfwave (is this correct english?) of the PS.

I checked the outputs, none of the them reads bad (still in circuit), I also tested the transformer, no shorts from prim. to sec.,
I didn't check the rectifier diodes until now, I didn't want to desolder them, had only one amp with bad ones before... (That's been a Blaupunkt Velocity VR6000)

And yes, a scope is standing next to me...

Thanks in advantage for your help!

[Perry Babin]

If you have a scope, check the gate signals with it. You should see a square wave that goes up to approximately the same voltage you have on pins 13 and 15. It should also go down to ground.

There should be two pull down resistors connected from pins 11 and 14 to ground. Confirm that they're within tolerance.

If you solder the FETs in without connecting the drain, that will show you if the drive circuit is pulling the gate voltage down properly when loaded by the FET gate capacitance.

If you have a current limiter (resistor, headlamp, or a limiter built into your supply), you can limit the current to see if only the FET that blew twice is overheating (all FET legs soldered to the board).

A primary to primary short can cause the FETs to fail but you can't detect it with a standard multimeter.

I ALWAYS suspect the rectifiers in SS amps. I've seen them fail more in SS amps than in any other amp.

[gisewhcs]

Too late... I already desoldered the SG the hard way...
Clipping off the leads and removing one after another is so comfortable...

But if you say failing rectifiers are a common problem on SS amplifiers, I will desolder them tomorrow and check them out of circuit!

And when there's a new controller IC in circuit, I will perform the rest of your recommended tests!

[gisewhcs]

The amp is back in good working order!
It's been a defective rectifier, like Perry promised...

From now on I will test them before doing anything else!