Trying to fix power supply on Soundcraft GB 8 40 channel mixing board

Thread Starter

nbtone

Joined Oct 14, 2016
65
What is your reasoning to believe there is not a SMPS in your mixer? The power supply schematic clearly shows one.
It is a switch mode power supply in my mixer. It’s not a switch mode power supply in my bass tube preamps.
I’m going to leave the power supply alone for a while. I need to figure a way to bench test each channel strip to make sure that when the power supply goes back in it doesn’t fry again. I’ve been watching some YouTube videos about it, and it doesn’t look like it would be too hard. All I need is a power supply and a way to connect it to get started.
 

JoeJester

Joined Apr 26, 2005
4,390
I did buy a 200khz battery powered oscilloscope. Should I keep it or send it back? Here is what I have: https://www.arrow.com/en/products/1...sEGwepjKjD0hz-HFwWX3QAZ1kYQQuYbBoClAEQAvD_BwE
What I immediately don't like about it is there is no BNC connection for standard probes. That being said, it is suited for audio work. the SMPS switching frequency is probably lower than 200 kHz. My only concern is even at 10V per division, you can only read a peak of 40 volts, so you may need a x10 probe, IF the one you get with it isn't switchable between x1 and x10.

Here is the datasheet
 

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Thread Starter

nbtone

Joined Oct 14, 2016
65
There is an adapter available from 3.5mm mono to BNC, and I would need a 10x probe. It’s only rated for 200 kHz. What would happen if you were to measure something over that?
 

JoeJester

Joined Apr 26, 2005
4,390
The scope would not give you the true signal. It can be lower in amplitude as well as distorted from the true value. The depends on how much above the 200 kHz the signal is.

Most probes these days are 100 MHz probes, so the true signal would likely pass through them (attenuated) with no problem.
 
The scope would not give you the true signal. It can be lower in amplitude as well as distorted from the true value. The depends on how much above the 200 kHz the signal is.

Most probes these days are 100 MHz probes, so the true signal would likely pass through them (attenuated) with no problem.
Nominal frequency for that switcher is 500KHz. You won't see any signal with a 200KHz scope.
 
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JoeJester

Joined Apr 26, 2005
4,390
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Thread Starter

nbtone

Joined Oct 14, 2016
65
upload_2019-6-3_19-43-19.jpeg

I took the whole thing apart this weekend to clean contacts and study the situation. Now it’s back together most of the way:
upload_2019-6-3_19-45-5.jpeg

If I fix the power supply I’d be half afraid to hook it up for fear that it would fry again. I saw a guy on YouTube that built his own power supply for a Soundcraft mixing board. He has all the instructions for doing it, too. I’d like to build one for testing the individual channel strips. I wonder if there is a way to build it so that if something is shorted it won’t break the power supply?
 

dendad

Joined Feb 20, 2016
3,924
Check R24, 150K 2W resistor. it may be open. If so, replace it with 3 x 47K 1W resistors in series. This lowers the voltage across each resistor. Some resistors seem to give up with high volts across them.
Another good old trick is to place a mains lamp in series with the power input to limit the current in fault conditions.
 
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Thread Starter

nbtone

Joined Oct 14, 2016
65
Check R24, 150K 2W resistor. it may be open. If so, replace it with 3 x 47K 1W resistors in series. This lowers the voltage across each resistor. Some resistors seem to give up with high volts across them.
Another good old trick is to place a mains lamp in series with the power input to limit the current in fault conditions.
I measure 130K in the circuit, so it is okay.
 

dendad

Joined Feb 20, 2016
3,924
It’s not, so I’ll replace it with 3 47k 1 watt resistors with 5% tolerance. Do you think that that might fix the power supply?
If the resistor is the value you say, it will be ok. All it does is charge the capacitor with enough volts to start the supply, then the control power is supplied through the circuit feeding D11.
Wire your meter onto C23 so you can power it up safely, then see if it gets to about 12V.
 
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