Hi guys,
I'm working on an Allen & Heath qu16 digital audio mixer. The main issue was that the 48V phantom power needed for the condenser mics were very low. With no mic it measured around 26V and it went down as more mics were plugged in. Also there was sound of chirping coming from the I/O board.
Opened the unit and here is what I found out. The 48V is derived like this.
8V(coming from the main PS) --->+12V--->+53V--->Regulated 48V.
The 12V rail and 53V rail is made from TPS4021. When the unit is powered up the 12V output never reaches 12V it jumps around (sort of restarts) and settles around 5ishV, So the 53V doesn't reach 53V and nor does the 48V output. The 8V input voltage coming from the main PS remains stable.
The circuitry for 12V,53V looks very similar apart from few obvious change in component values.
From what I can deduce the problem is coming from the 12V output, and here is an something odd which I discovered. At the 12V (FB,Comp Pins) circuitry when I measured the resistance across R88 the value kept on increasing (the same as what happens when you see a capacitor charging). I didn't see this when I took the same measurement from the 53V side. So thinking that the resistor might have gone open or something I took it out of circuit and measured it. It showed dead on 47Kohms. I measured all the other components around there and noted the values. I still see the same sort of measurement when measuring across the open pads of where the resistor was.
I was hoping that I could get further tips on what to look for. I have a feeling that the TPS4021 IC maybe faulty but how can I confirm this? I have measured the outputs for shorts but didn't measure any.
Thanks in advance.
P.S I couldn't find how feedback resistors values are calculated to get the needed output voltage.
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