Audio Interface Repaired
A customer brought me this unit to repair. It is an audio interface used for recording.
It was an easy fix, but I thought I would share some ways of finding a shorted component that is dragging the supply rails down. This was the case with this unit.
There are 3 ways I use to find shorted IC’s when doing these type of repairs. First I use my sense of touch, and feel around the board for hot areas, while the unit is powered on. I found an area with two op-amps and a Multiplexer IC that was feeling hot.
I then used some freeze spray to confirm my suspicion. The larger 14-pin IC was “un-freezing” first, long before the other two 8-pin op-amp IC’s. This means it is dissapating a lot more watts due to a short, and is drawing more current.
Another method you can use is by using a DMM set to ohms range, with the unit powered off. Measure across each IC’s supply pins. The one with lowest ohms reading is faulty. Usually a good IC should read a couple of Kilo ohms.
From the above pictures you can see the 14-pin IC (74HC4066) has the lowest reading of the three. Definitely shorted. Another method I use is to measure voltage across supply pins of each IC. There can still be a small voltage present even if one is shorted. You may have to put your multimeter in mV range if the voltage is too low. Here you can see the 14-pin chip again measures lowest of the lot.
This is the method I use most, as it is the quickest test. I then went about replacing this IC. First to make things easier I apply some flux to all the pins, and solder link all of them. The solder blob helps the hot air reach all pins faster.
I then remove with my hot air station and tweezers. You could use a normal heat gun.
I then clear all the pads of solder using flux and solder braid, and then clean with some Isopropyl Alcohol and a brush.
Without the faulty chip, I now measure full supply rails.
So this again confirms that this was in fact the faulty IC. Using some more flux and solder I then solder new chip in place.
After the replacement all voltages checked out fine. I hope this helps someone, as it’s just some of the techniques I use in my workshop.
This article was prepared for you by Riaan Diedericks. He runs his own electronics repair shop in Pretoria, South Africa. He specializes in Pro Audio repairs.
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You can also check his previous repair article below: