Spl dynamics d92

[Achim1409]

Received this amp. Repaired from the previous owner.. Bad repair, anyway it seems it works. The new owner mentioned, that the amp hasn't the power it should have. Found a damaged master /slave switch.
I have oscillation at each channel..
The amp is out of the heat sink, if I apply power, the r-ground resistor starts smoking.. What can cause this? Some leaking?

 

[Perry Babin]

What resistance do you read between the secondary ground and the B+ terminal?

 

[Don Hathcock]

Received this amp. Repaired from the previous owner.. Bad repair, anyway it seems it works. The new owner mentioned, that the amp hasn't the power it should have. Found a damaged master /slave switch.
I have oscillation at each channel..
The amp is out of the heat sink, if I apply power, the r-ground resistor starts smoking.. What can cause this? Some leaking?

Maybe a shorted input cap?

 

[Achim1409]

About 18k..but seems, I have problems with the power supply.. Positiv rail drops very fast. I get at 12 v input only 120v per rail.. Should be much higher I think.

 

[Perry Babin]

When that resistor burns, it's typically due to a transformer shorted primary to secondary. Does it burn with no remote applied?

The short may be intermittent. Twist the transformer to see if that makes a difference.

 

[Achim1409]

Without remote, it doesn't burn. But a damaged transformer makes sense..i will check later

 

[Achim1409]

I separated the ps from the audio board. Without that, the resistor stays cool. Rail at 12 v is - 127,+131. Both drops equal fast when switched off. So the problem will come from the audio board.

 

[Perry Babin]

Does twisting any of the transformers cause the secondary ground to go to 12v?

Did you measure the DC voltage across the resistor when it was smoking?

 

[Achim1409]

No, I haven't measured the voltage across. I twisted a lot, maybe that was the reason . For safety I removed the rectifiers, and used a dual supply. The amp looks at this stage fine to me.. Each bank at the audio stage has a good square wave.. No smoke, no current draw..

 

[Achim1409]

I have spoken with the customer. He claims, that the amp starts at 3kw allready heavy clipping.. That speaks also for a bad PS or transformer.

 

[Achim1409]

One question about the audio part. First picture is high side gate, black probe at sec. The amplitude is a little lifted to positive rail. Is this normal, or something leaking? Second picture is low side drain.. Is normal amplitude.
The last picture is ps gate. Frequency about 28khz..should be also okay

 

[Perry Babin]

The gate of the high-side drain has to drive 10v above the high-side source to efficiently drive the high side fully on.

If this is a battery powered scope with no connection to the mains, you can place the black probe (probe ground lead) on the high-side FET source and then you will see the high-side gate drive with none of the rail-rail waveform.

 

[Achim1409]

I have a battery powered scope, but if I connect it to source and gate, the amp goes into protect..

 

[Perry Babin]

Is it plugged into the charger?

 

[Achim1409]

Yes, you are right

 

[Perry Babin]

Unplug it and try again.