mtx 1501d sawtooth waveform

[Perry Babin]

50mv on pin 1 of the 5532?

 

[Speeed3]

50mV on output.

But I tested to drive amp hard, without load. much before clipping amp drawed 6ams? And makes buzing noise again.

So changing resistors to 270ohm did only screw up output waveform.

 

[Perry Babin]

If it's drawing 6 amps with no load, the 100 ohm resistors that I mentioned earlier may need to be replaced with a higher value.

 

[Speeed3]

I did change them to 270ohm, it redused curret drow at idle. Before changing resistors the waveform was good, but amp drowed too much current at idle.

 

[Perry Babin]

You may need to go a bit higher in value than 270 ohms. In one amp I repaired, I had to go to 400 ohms. Try 300 or 330 ohms to see if you can get the current draw down when driving it with no load.

 

[Speeed3]

Even with 470ohm, I tried 330ohm too, current draw shuts amp down with current limiting resistor. I took pictuture of output waveform 20v/div and 10uS/div

 

[Perry Babin]

What's the current draw at idle without the current limiting resistor?

If the current draw is excessive, are you sure that the problem is due to the the output transistors? Could there be a problem with the power supply? Does the excessive current draw begin before LED238 lights?

When the value of the resistors needs to be increased, the amp will draw excessive current and LED238 will blink off when the amp is driven to near clipping. Was this happening with the 270 ohm resistors?

 

[Speeed3]

Sorry, I meant, that amp shut down when driven (not show) near clipping, without load. Yes LED238 blinks of.

 

[Perry Babin]

Did you replace the two HCPL3120s?

 

[Speeed3]

No I didn't. Should I just replase them, or can I meansure them?

 

[Perry Babin]

I've had several 3120s that seemed to work properly and checking them with a multimeter seemed to indicate that they were OK but the amp wouldn't work properly until they were replaced.

I'd also suggest that you use devices manufactured by agilent/avago. Someone sent me an amp with components from a different manufacturer and they would not work (even though they seemed to be OK when checked with a multimeter). I replaced them with agilent devices and the amp worked normally.

The following link shows approximately what you will read for a 3120 that is in good working order (device out of the board and meter set to diode check).

http://www.bcae1.com/temp/hcpl3120optocheck.swf

 

[Speeed3]

OK, since they could be defective even if they meansure ok, I will order new AVAGO parts.

 

[Speeed3]

Didn't change at all, with new HCPL3120s. Any more ideas?

 

[Perry Babin]

If you connect a speaker to it, does it produce clean audio at low levels?

With a speaker connected, do the output filter capacitors get warm/hot if the amp is left idling for 10 minutes?

Have the transistors tightly clamped to the sink when checking these.

 

[Speeed3]

Sound seems to be distortion free, exept the screaming noise comming from speaker when amp is on.

Nothing gets hot from idling.

 

[Perry Babin]

Is there any noise with no RCAs plugged into the amp?

With no RCAs plugged into the amp and the amp off, what is the resistance between the chassis ground terminal and the RCA input jack shield?

 

[Speeed3]

Same sound withor without rcas.

1,3Kohm increasing into 0,6Mohm and increasing very slowly from that? Fluke multimeter.

 

[Eva]

This looks like a control loop instability problem, as if the amplifier was oscillating.

 

[Perry Babin]

It appears that the 1k ohm resistor (shown in the photo below) is open.

 

[Speeed3]

I'm sorry I must have meansured it to B+ terminal.

It gives out 0,91K to ground terminal, that resistor is ok.