Amiga 4000 badly Distorted Clipped Saturated audio Repair.
The problem is quite simple to find. You have to make some measurements on the integrated SMD LF347 (Op AMP) with the computer running and no sound.
You need to measuring pin 1 and pin 7 and should have a voltage of about 2.5V if the voltage as in my case (see photo) is of 11.43v and 11.37v. Something is not working.
At this point you must also perform a new measurement on the pin 14 and pin 8, if also in this case it has a higher voltage of 2.5v evidently there is’ something that doesn’t work or with the LF347 or with the 750ohm resistor (R440 and R430).
The other three measurements to be done are the two input voltages, pin 4 should be about 12V and -12V on pin 11 approximately. In my case (see picture) are + 12.28v and -12.04v and are fine.
The third measurement is the Voltage Reference (Vref), this is very important, should not be less than 2v or greater than 3v. In my case (see photo) was 5.30v. This problem could be caused either by the circuit that provides the VRef (2 resistors and two capacitors) or just the same ic that creates problems with the Vref input.
In my case the failure was caused of the integrated SMD LF347 that i have replaced with a LM324. After replacing the voltages are back to normal, in the example photo you can see the Vref at 2.33v
The Amiga sound now works very well.
I have repaired several Commodore Amiga 4000 with the LF347 burned and from my personal experience can say that the acid leakage of capacitors C404, C433 and C443 make some short circuits on pins of the LF347 that obviously dies.
Audio before and after the repair:
Gallery:
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