Digital Designs A4

[mike49504]

This amp is stuck in protection mode I checked all of the outputs and they test fine.. I pulled the rectifiers out and amp is still stuck in protection mode..

With the rectifiers in the amp heres what i get on the 494

Pin 1:0.97
Pin 2:5.00
Pin 3:4.76
Pin 4:0.18
Pin 5:1.50
Pin 6:3.44
Pin 7:0
Pin 8:13.44
Pin 9:0
Pin 10:0
Pin 11:13.46
Pin 12:13.18
Pin 13:5.00
Pin 14:5.00
Pin 15:5.00
Pin 16:11.71

Any ideas on where to start looking??

 

[Perry Babin]

Pin 16 is being driven high which is shutting down the IC. What's driving pin 16?

 

[mike49504]

Theres a op-amp 4558 and a diode connected to pin 16

4558

Pin 1:12.36
Pin 2:5.00
Pin 3:13.18
Pin 4:0
Pin 5:6.09
Pin 6:11.56
Pin 7:0.59
Pin 8:13.19

 

[Perry Babin]

Pin 3 is being driven high which is driving the output of the 4558 high. What's driving pin 3?

 

[mike49504]

An A1266 transistor is driving pin 3

 

[Perry Babin]

Remove it, reinstall missing components and carefully power it up. Does it draw excessive current, have DC on the speaker outputs or does it produce clean audio?

 

[mike49504]

Once removed it powers up but draws excessive current

 

[Perry Babin]

Clamp all transistors, connect the current limiting resistor in series with the B+ line and measure the DC voltage across each of the emitter resistors. Which channel has significantly more DC voltage across the emitter resistors? Remember, all of the voltages will be well under 1v so measure as accurately as possible.

 

[mike49504]

Ok i found 1 emitter resistor with 6.22 volts across it

Which would be channel 1

 

[Perry Babin]

I have no information on that amp so I'm working in the dark.

If the transistor connected to that emitter resistor isn't shorted, check the driver for it. If it doesn't appear shorted, pull them to see if either is leaking.

 

[mike49504]

Ok it appears that someone worked on this amp before and have the outputs in the wrong spots ..

This amp uses C4467 and A1694's as outputs..

Does the A1694s control the positive or negative rail??

 

[Perry Babin]

The C is generally on the positive rail but that's not always the case.

Find the terminal of the output transistor on the rail for the other channels and install the same transistors on the corresponding rail for each of the outputs in the defective channel.

 

[mike49504]

Ok it powers up green now and idle's perfectly.. Someone had every channel messed up..

Both C's were in 1 channel Both A's were in another and so on..

 

[mike49504]

OK now channel 2 has 22.87 volts of DC on the outputs.. I will check that channel to see what i can find out