Behringer KX1200 w/ output problem

[pontiacpete]

I've been working on this amp and getting nowhere. It powers up but the volume is covered with large popping static sound. My light limiter lights up bright when the static starts.
When turning the master volume the light limiter comes up bright and goes down.
I first thought that the master vol pot was the problem, so I changed it and nothing, the same problem.
Then monitoring the voltages on the regulators, when the popping static started the voltages dropped off. I changed out the LM3886 and the regulators and the filter caps 3300ufs and no positive results.
Not really sure which way to go.
Does anyone have any suggestions.??

also I'm not using the speaker in the cabinet to test this amp. I'm using a direct connection to the amp with a test speaker I have for my bench. There seems to be about 2vdc on the output which goes up and down .

[Steve L]

The typical problem is usually one of the TL074 op-amps. Many times you can actually see the bad one because it will have burned.
That is not to say that it could not be something else, however that is where I would look first.
You should be able to trace the popping noise with a scope.

Steve

[pontiacpete]

There are no visual signs of them being burned. I tried to see if they were getting hot by touching them but they don't seem to be. I'll try with scope. Thanks

[Enzo]

I would agree, most of the repairs to these amps, unless the powr amp IC is blown, have been TL074 chips, at least in my experience. They rarely are physically burnt or otherwise visually detected.

Scope would be my first choice too.

[pontiacpete]

Trying to use the scope to detect a problem with these op amp and I have a few questions. THis is how I'm doing it and I hope someone can correct me because the method I'm using is not working. I put a signal into the input of the first channel on a dummy load. I see the signal on the input jack when the volume on the amp is low enough. When move my probe to pin 9 or 10 of IC6c and there is nothing. I'm using the dual trace to look at the output as well as what the second is picking up. As I turn the volume up on the master which is to the point were all the noise starts the signal on the output gets crazy and distorted looking.
I changed this IC and still the same noise. If I turn the master up past 12 o'clock the noise disappears and so does the wave. Either my probe is picking up the noise or what....?

Could the problem be at the master volume and it drifts back to the input?
Should I be injecting the signal at different places on the board?
If so how or where?

[52 Bill]

I don't understand how you are inputting the signal with a dummy load.

Try and isolate the problem area by trying different things. The Monitor out bypasses the master volume control. If you take the signal from the monitor send is it clean or distorted? Do all of the channels have the distortion? Is there any dc voltage on the master volume control?

[pontiacpete]

Sorry about my post, I meant that I have a dummy load on the output and I'm monitoring that with on channel of the scope .
I'll try the send.
This, to me is a strange problem. With nothing plugged in, and on the light limiter, as the MV is turned up from about 8:00 to about 12:00 it sounds like it's arcing. After 12:00 nothing.

[pontiacpete]

The send jack has the same results.

[52 Bill]

The send jack has the same results.

So you can't get any clean signal from the Monitor out jack from any of the 4 input channels?

Does turning up any of the monitor send pots cause the popping or only the master?

Did you check for dc on the master pot?

[pontiacpete]

no clean signal out of the monitor jack out

No, the monitor pots do nothing to it

There is no DC on the MV

[52 Bill]

Tell us what the power supply is doing. Do you have both plus and minus supplies? How about the 15 volts supplies?

When you turn up the master control and the popping starts, what happens to the power supplies? Which ones are dropping out when the noise starts? What happens to the voltages when you turn the master all the way up?

[pontiacpete]

- +15v? I thought I had that but I don't. No -15 on the regulators. THere's - + 36 or so volts on the filter caps but no -15v on the 7915 reg. I checked 330R resistor betw the 3300uf cap and the regulator and that 's fine. On this amp it's a 220r resistor but it's still good. How could there not be a -15v on the regulator which is next in line?
The 4002 diodes check out OK in the circuit.
What could be dragging the negative voltage down?
Could that be cause by something further up stream?

[Enzo]

How? If the regulator is defective. The 7915 has three legs, if I recall, that would be ground, input, and output. NO output? Is there input voltage?

You got -36v at one end of the resistor, what is at the4 regulator end of that resistor? And whatever is there, it should be the same at the regulator input pin, so is it?

Regulators are easy to diagnose. Check input pin for voltage, if it is ther and no output, replace it. If the input pin is barren, then look for an open 220 ohm resistor or a broken circuit trace between rsistor and regulator.

[pontiacpete]

I changed the regulator already.
The funny thing is that on the filter cap end of the 330r resistor there's -36 v and on the other there's -7. It's dropping a lot of voltage.
On the regulator input is -7v and the output -6v

[Enzo]

Then isolate the load. After it has been on a bit, are any ICs getting hot? Any look cracked or discolored?