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mike49504 7th August 2011 06:27 PM

Digital Designs A4
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 7th August 2011 06:33 PM

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

mike49504 7th August 2011 07:13 PM

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


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 7th August 2011 07:17 PM

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

mike49504 7th August 2011 07:22 PM

An A1266 transistor is driving pin 3

Perry Babin 7th August 2011 07:44 PM

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 7th August 2011 07:58 PM

Once removed it powers up but draws excessive current

Perry Babin 7th August 2011 08:09 PM

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 7th August 2011 08:14 PM

Ok i found 1 emitter resistor with 6.22 volts across it

Which would be channel 1

Perry Babin 7th August 2011 08:19 PM

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.

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