Concept HD1200 intermittent protection mode

[joselorenzo]

Hello
I received an HD1200 model concept amplifier.
It has an intermittent fault. It goes into protection mode on some occasions when applying remote tension.
It does not always enter protection mode, sometimes it works normally.

I Compare voltages of TL494 and LM393. When in protection mode and normal. In those measurements I do not have considerable variations.

Where if I have variable voltages measurements in IC 4080.
Here I put them:

POWER ON normally
1:41.7
2:11.97
3: .017
4:.000
5:5.98
6:5.85
7:5.89
8:5.19
9:5.24
10:41.7
11:35.8
12:30.55
13:5.66
14: .001
15:11.97
16:11.97
17: .002
18:5.68
19:30.51
20:35.7

IN PROTECT MODE :
1:11.43
2:11.97
3:11.46
4: .000
5: .004
6:5.83
7:10.94
8:5.19
9:11.70
10:11.44
11:9.00
12:9.00
13: .000
14: .000
15:11.97
16:11.97
17: .000
18: .000
19:9.01
20:9.00

I do not understand the operation of the 4080 well so I ask for help or a suggestion
Thanks for attention

 

[Perry Babin]

Does U8 drive pin 7 of the 4080 through a 10k resistor?

If so, solder a bridge between pins 1 and 2 of U8 and see if it will power up every time.

 

[joselorenzo]

The connection mentioned by pin 7 and the resistance of 10K goes to IC U7

 

[Perry Babin]

OK, solder a bridge on that IC.

 

[joselorenzo]

Continue entering protective mode with the bridge between pin 1 and 2 of U7

 

[Perry Babin]

Does pin 7 still go to 10v?

Does every group of outputs have a 470k resistor in parallel with a diode to pull the gates of the outputs down.

 

[joselorenzo]

Pin 7 gives me a 4.59v measurement with bridge between pin1 and 2 U7.

And if it has a one pair of resistance of 470k and a diode each group of gates in the outputs

 

[Perry Babin]

If it has the pulldown components, it's safe to pull the 4080. Does it go into protect without the 4080?

 

[joselorenzo]

Without the IC 4080 out of the circuit it always turns on normally.

I have already exchanged the 4080 for a new one from the mouser store.

With the original 4080 and the new 4080 it repeats the intermittent failure of the protection mode

 

[Perry Babin]

Is there a zener diode connected to pin 3 of U7?

 

[joselorenzo]

I did not find zener connected to pin 3 of U7

Pin 3 of U7 is connected to pin 6 of U5 and 1C11 while connected to C34

 

[Perry Babin]

Drive a relatively strong signal into the amp.

With the scope set to 5us and 2v/div (dc coupling) what is the waveform on pin 6 of the 4080?

Same 2v/div but 2ms. what is the signal on pin 7?

 

[joselorenzo]

I show the following measurements .



With the test tip of the oscilloscope connected to pin 6 the amplifier does not enter protection mode

 

[Perry Babin]

For the second image, if you had the scope set to 2ms, adjust the trigger level to see if you can get it to lock. You may have to remove the solder bridge to get an audio signal to pass. If the op-amp doesn't have a resistor connected between pins 1 and 2, you may need to connect a 10k resistor between those two pins to give it some local feedback.

This is all to be done with the 4080 out of the amp.

 

[Perry Babin]

Read #14 first.

Have you checked the output transistors for broken legs? Could be broken internally.

 

[joselorenzo]

Take the 4080 out of the circuit
Install a 10k resistor between pin 1 and 2 of U7
I checked each output fet 3710 and the 2 pairs of 7815 and 7915 with small lugs to verify that they are not broken from their terminals.

The measurements is with a tone of 110hz

 

[Perry Babin]

Are the output FETs original?

If so, did you lift them off of the heatsink so you could move the body of the FET side to side (watching carefully where the leg enters the body for movement)?

If you power up repeatedly, does the DC level always go back to where it is in the two photos?

If you push on the board with a non-conductive probe, does the DC level of either signal ever change?

 

[joselorenzo]

Are the output FETs original?

If so, did you lift them off of the heatsink so you could move the body of the FET side to side (watching carefully where the leg enters the body for movement)?

If you power up repeatedly, does the DC level always go back to where it is in the two photos?

If you push on the board with a non-conductive probe, does the DC level of either signal ever change?

# 1 Yes, the fets are original. The only thing I change is the 4080

# 2 Yes, I also took off the heatskin fets and made movements to know if they have any problems in their terminals.

# 3 I managed to capture a picture of the measurement when starting the amplifier

# 4 I push the PCB board hard and I do not get rare measurements.

The client tells me to have more amplifiers of the same model with the same fault symptom.
It will be a factory defect .....?

 

[Perry Babin]

Set all of the center 'cal' controls fully clockwise. They should click. They are moving and that will change the position on the display.

Are you referring to the distortion in the waveform?

 

[joselorenzo]

The gain control made a bit of noise. I already change it.
But the failure persist



What is the purpose or function of the small pot of 2 k near the 4080?