Sony xm2000r - Guzzling power, need to trace the problem.

[agithegreat]

Well i'm truly stumped with this.

This amp is in 2 halves. Left channel and right channel. They are separate.
They have smps power supplies and are class A/B by the looks of things.

Now, I power up the left hand side it works like a dream. No problems at all!

Then feed power to the right, clunk, the circuit breaker (built in!) trips.
The cables I have going to the amp get warm! Now first thing to hit me around the back of the head was, meh, blown power supply.

However upon initial powerup, there is around 2 seconds before current inrushes in, so about 0.5a flows for a short while then suddenly it guzzles away.

So i pulled it apart, nothing visibly blown, Popped the dmm across the power and gnd planes on the ps section and get perfect conductivity one way (protection diode I guess?), then the other way I get 0 conductivity.

Hmmm... So where shall I try now?
Am I missing something blaringly obvious?

[Perry Babin]

Can you post a photo of one of the boards?

[agithegreat]

Here you are:

It looks like the PS is in 2 parts, there are IC' each end of the board. With common rails though?


Supply:

[Perry Babin]

HAve you checked the output transistors?

[agithegreat]

IRFP140's,

Yeh, they're shorted. Well one side is (i'm not talking about the left channel here, i'm talking about on this one board).
It's split into 2 sides, the one side shows 12Mohm and 8Mohn, the other shows perfect conductivity both ways.

I only looked at one side earlier

Thanks PB, i'll get them removed and report back

[agithegreat]

Ok,
Fet's removed. I'm now getting 5Mohm across the drain and source pads, There was one FET with a short, but I shall replace them all.
I get 10r2 across the gate resistors.

Does the 5Mohm sound ok? It drops down over time, so when the leads go on, it started at 5Mohm and was dropping to around 4.5Mohm?

Should I try and connect up the 3 good fets, and try it again, with a little load, on low volumes, to try it?
As then I could for sure, point out that it was the defective FET.

[Perry Babin]

5M is probably OK.

You can power it up with only 3 of the IRFP140s to see if there are any other problems. Be sure that the remaining FETs are tightly clamped to the heatsink before applying power.

[agithegreat]

Right, thanks.

I powered it up with the 3 fets and it's scratching away? Like you can hear the sound, but no treble at all, (I checked crossover settings). Some of the midrange was coming through intermittently.
Could it be like this as it's missing a fet. (My knowledge isn't great, but as a push pull amp, is the fact that is has 3 doing the top half of the waveform and 4 doing the bottom affecting it, or visa-versa?) The rest of the amp is fine now. It doesn't trip the breaker and current draw is minimal now. So it's definately on track to the mend.

Shall I get the scope on it?

[Perry Babin]

You may have a defective driver. The missing IRFP140 shouldn't cause a problem.

This amp is a bit more complex than most. The four paralleled IRFP140s are in series with the FS70. These are used to reproduce the top half of the waveform. The four paralleled IRFP9140s are in series with the FX50 and these reproduce the bottom half of the waveform. It's a cascode configuration.

It's possible that a driver failed.

Drive a sine wave into it and post the waveform.

[agithegreat]

Ok, will do.
I did notice the additional device.