Kenwood KAC-9104D drawing high current

[mild2wild]

Not fully versed on what is meant by "deflection" on an oscilloscope. I wasn't sure how I could touch the B+ terminal and monitor IC6 pin1 at the same time. I connected probe 2 and touched that one to B+and the waveform on IC6 pin1 started to shake like there was an earthquake but there was no interruption or change in amplitude, just shaking. The top and bottom of the waveform is still of the top and bottom of the screen at 5V/div.

When I turn the amplifier off, on 5V/div, the waveform shrinks down into the screen to approximately ±4V for a second just before disappearing as the amp shuts down. That's the only time pin1 of IC6 has a triangle waveform of approximately ±4V.

 

[Perry Babin]

Let's do this step by step.

Single probe, 5v/div

Trace aligned to the reference line.

Scope ground clip connected to the ground terminal of the 12v supply.

Probe tip on the positive terminal of the 12v supply.

How far up does the trace deflect?

Is it a straight line?

 

[mild2wild]

Ground clip to input GND and probe tip on B+ is a straight line on the screen.

 

[Perry Babin]

How far up does the trace deflect?

 

[mild2wild]

Set at 5V/10us the line goes off the top of the screen when the probe tip is touched to the B+ terminal. I have to do it with a solid touch to get a clean deflection because if it doesn't touch immediately and firmly I get all sorts of noise and squiggly lines on the screen before it goes back to a straight line. If I get a good solid touch, the straight line would just bump up off the top of the screen leaving the top edge red for a split second and then drop back down to just over the middle horizontal line.

 

[Perry Babin]

Are you on DC coupling?

 

[mild2wild]

Doh, I completely overlooked that, it was on AC. Now (DC coupling) when I touch the terminal the trace goes straight off the top of the screen and stays there until I lift the probe. I am not able to see the raised trace until I go to 50V/div.

 

[Perry Babin]

Set the probe to 10x.

 

[mild2wild]

Set the probe to 10x, DC coupling, 5v/10us/div

 

[Perry Babin]

Same settings, scope ground on IC6, pin 3 and the probe on pin 1.

 

[mild2wild]

Same settings, scope ground on IC6, pin 3 and the probe on pin 1.

 

[Perry Babin]

Align the trace to the reference line and recheck the waveform.

 

[mild2wild]

Same settings, trace aligned to the reference line

 

[mild2wild]

I guess IC6 pin1 output looks OK? What do I check next?

 

[Perry Babin]

I'd expect the triangle wave to be sharper and the amplitude is greater than expected but at least we're getting accurate information from the scope.

Go back to the speaker output (no input) and see what you have there. Use 2ms/div timebase.

 

[mild2wild]

Across the speaker terminals set for 5V/2ms/div

 

[Perry Babin]

If you connect a speaker to the amp, is there any sound from it (no input)?

 

[mild2wild]

With a speaker connected I don't hear anything audible except for a small pop when the amp powers up. I probed the outputs at 10us just to see what the waveform looked like. There is audio if I connect the RCAs and input music.

 

[Perry Babin]

That noise on the output is carrier leaking through the output filter. It's a bit excessive. Since you swapped the inductor, it could be due to a bad filter cap but could also be normal. Some amps are noisy (inaudible).

There is more DC offset than I'd expect. Measure the DC offset across the speaker terminals with your multimeter.

Connect a signal (sine wave preferred) and increase the output but not so much that the amp shuts down. Does the offset increase when the signal is driven into the amp? Check with and without a load.

 

[mild2wild]

Amp idles at 680-700mv of DC offset with no load and no input. Feeding it 100hz sine wave with no load and increasing the gain causes the DC offset to rise until it reaches about 720mv and the amp shuts down. Connecting a 2ohm dummy load to it causes the amp to idle at 670-680mv of DC offset. Driving a 100hz signal into the amp and increasing the gain causes the current draw to increase as well as the DC offset until it passes about 745mv and then jumps to 1.7mv and high current draw and just sits there even if the gain is turned back down, which is when I turn the amp off.