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

Thread Starter

nbtone

Joined Oct 14, 2016
65
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.
I read the voltage from the plus side of the capacitor to the negative side and found 9.25 VDC.
 

Thread Starter

nbtone

Joined Oct 14, 2016
65
I do know the 1 MHz bandwidth dso is a little over $100 at Amazon. I haven't looked at higher bandwidth handhelds. Here is one ... https://www.amazon.com/Signstek-Por...nnel+handheld&qid=1559544326&s=gateway&sr=8-3

Here's a similar one with a built in signal generator https://www.amazon.com/SainSmart-Po...=1&refRID=Y1B6W94CJKSAKTXK2FEB#detail-bullets

The TS would have to decide what they want to do.

Here's another ET rating symbol Charles.

View attachment 178878
How does this one compare to these others? Will it work for what I need to do?
https://www.ebay.com/itm/Hantek-Han...631960?hash=item340d037d98:g:jH4AAOSwT8FdCT9F
 
How does this one compare to these others? Will it work for what I need to do?
https://www.ebay.com/itm/Hantek-Han...631960?hash=item340d037d98:g:jH4AAOSwT8FdCT9F
That appears to be a single channel oscilloscope, for line connected electronics it is better to have 2 channels. With 2 channels you can set the oscope in differential mode, use one probe as a reference and the other to make measurements. If you only have a single channel scope you will need to get an isolation transformer so you can set ground at a point inside the circuit.
 

dendad

Joined Feb 20, 2016
3,924
I would recomend getting an isolation transformer anyway.
For years I used a couple of valve TV transformers connected back to back, and they worked fine.
So any pair of transformers of reasonable power, like 100W or so, can be pressed into service if you cannot get an proper isolation transformer. You will loose a bit of power but that is ok.
An isolation transformer is a very handy tool, and I have a power board with an included incandescent lamp in the active lead to limit the current under fault conditions. That too is very handy.

A dual channel scope that you can switch to differential opens up more measurement possibilities. Go for one of those if you can.
 
Last edited:
Make sure you get an isolation transformer that can handle the power of the device under test.

To get into differential mode on an oscope, invert channel two and 'add' the channels. This will give you a single trace that is the difference between the two probes. If you are working with a high voltage keep the probes set to 10:1 and don't set the channel range too low. Also keep both channels to the same range setting.
 

Thread Starter

nbtone

Joined Oct 14, 2016
65
Make sure you get an isolation transformer that can handle the power of the device under test.

To get into differential mode on an oscope, invert channel two and 'add' the channels. This will give you a single trace that is the difference between the two probes. If you are working with a high voltage keep the probes set to 10:1 and don't set the channel range too low. Also keep both channels to the same range setting.
I just ordered the 2D72 Hantek battery powered oscilloscope. I looked at the features, and it says you can invert and add in the computer with the software. I don’t know if that means you can or cannot take a measurement, save it to the handheld device, and later load the data into the computer. That way the computer would never come close to the hazardous voltages. If not I’ll get an isolation transformer.
 

Thread Starter

nbtone

Joined Oct 14, 2016
65
Hi again everyone. I have a two channel battery powered oscilloscope, and I’ve been trying it with a signal generator. It works as well as could be expected. I have two 10x probes.
I’ve been reviewing all the info in this thread, and I’m not sure where to start with testing the power supply, which is disconnected from the rest of the mixing board and sitting on a table. I will plug it into a GFCI output to do the testing.
 
First set up the scope in differential mode. To do this invert channel 2 and "add" the channels. I'm not certain exactly how to do it on your scope but it should be relatively simple. Set probes to 10x.
Once you have it set to differential mode you can use the channel two probe as the reference, place it on the point labeled live ground. Do not connect the probe ground lines to the circuit, they should be attached to the chassis ground.
Now you can use the channel one probe to measure voltages and view waveforms. Remember to keep both channels set to the same range and don't go too low in the settings because the peak AC voltage is on the probe tips even though you won't be able to see it.
 

Thread Starter

nbtone

Joined Oct 14, 2016
65
I'm being very cautious about this, and I won't just dive into testing with an oscilloscope; battery powered or otherwise. I came across this article: http://www.giangrandi.ch/electronics/smpsfix/smpsfix.shtml It states this:

"No output, good fuse
SMPS can fail in many different ways, the most common being no output power at all. In this case, I start by checking the input fuse. If the fuse is good but there is no output, probably all the semiconductors are good and it could be easy to fix. Keep in mind that usually semiconductors blow up shorted and resistors (and often capacitors) blow up open.

A good candidate is the inrush current limiter (an NTC). Than I check for high power rating resistors, particularly on the primary side: I measure their resistance one by one, in circuit. If the value doesn't match what written (or colour coded) on the component, I unsolder one terminal and measure again: if the value is wrong, I replace it with a new one.

The first resistors to check are the one in series with the power transistors, usually less than one Ohm. Sometimes the regulator is powered by a high value high wattage resistor in series with a Zener diode: if the resistor is good, maybe the Zener is shorted, so I check all diode junctions with the diode function of the multimeter (most of the times, you can do this in circuit). Than I check the capacitors (see below). Faulty regulators IC can happen but it's not very common."
I'd like to test the inrush current limiter, but I don't know what it is on the schematic. I guess the high power rating resistors are the 1watt and above variety?
 

JoeJester

Joined Apr 26, 2005
4,390
Have you tested the components that dendad asked you to test? You can do them with your DMM.

The problem is you never saw it work, so you need to be methodical about testing things.

You know there is +163 V DC on C34, the output of the mains bridge. You got the oscilloscope to see pin 4 of IC 3. If there are no pulses, you will not have an output either.

With respect to the inrush current, the article stated just test the cold resistance of the thermistor. Its labeled TH on the schematic. Look up the data sheet to see what the cold resistance is. I don't think I would consider the inrush current this early in the game.
 
Last edited:

Thread Starter

nbtone

Joined Oct 14, 2016
65
How should I connect the oscilloscope? It is battery powered, so there should be no continuity between it and the power supply circuit grounding other than where I connect the ground. There will be no chance of the probe exploding, since I don’t have it connected to a computer.

Have you tested the components that dendad asked you to test? You can do them with your DMM.

The problem is you never saw it work, so you need to be methodical about testing things.

You know there is +163 V DC on C34, the output of the mains bridge. You got the oscilloscope to see pin 4 of IC 3. If there are no pulses, you will not have an output either.

With respect to the inrush current, the article stated just test the cold resistance of the thermistor. Its labeled TH on the schematic. Look up the data sheet to see what the cold resistance is. I don't think I would consider the inrush current this early in the game.
 

Thread Starter

nbtone

Joined Oct 14, 2016
65
Have you tested the components that dendad asked you to test? You can do them with your DMM.

The problem is you never saw it work, so you need to be methodical about testing things.

You know there is +163 V DC on C34, the output of the mains bridge. You got the oscilloscope to see pin 4 of IC 3. If there are no pulses, you will not have an output either.

With respect to the inrush current, the article stated just test the cold resistance of the thermistor. Its labeled TH on the schematic. Look up the data sheet to see what the cold resistance is. I don't think I would consider the inrush current this early in the game.
I don’t know how to test the parts dendad asked me to test with a DMM.
 

JoeJester

Joined Apr 26, 2005
4,390
You have three diodes, one transistor, and one capacitor to test.

Do a Google search to see how to test them with a DMM. Take copious notes so you can report back with the results.
 

JoeJester

Joined Apr 26, 2005
4,390
How should I connect the oscilloscope? It is battery powered, so there should be no continuity between it and the power supply circuit grounding other than where I connect the ground. There will be no chance of the probe exploding, since I don’t have it connected to a computer.
Now that I am back at my computer, you will connect the oscilloscope's probe ground to live gnd. Then you can measure the output of the chip as well as ALL the pins on the TIP122 (TR4). Measure the three pins of the FET as well, G, D, and S (TR2).
 

Thread Starter

nbtone

Joined Oct 14, 2016
65
I don’t know how to test the parts dendad asked me to test with a DMM.
Have you tested the components that dendad asked you to test? You can do them with your DMM.

The problem is you never saw it work, so you need to be methodical about testing things.

You know there is +163 V DC on C34, the output of the mains bridge. You got the oscilloscope to see pin 4 of IC 3. If there are no pulses, you will not have an output either.

With respect to the inrush current, the article stated just test the cold resistance of the thermistor. Its labeled TH on the schematic. Look up the data sheet to see what the cold resistance is. I don't think I would consider the inrush current this early in the game.
Doe
You have three diodes, one transistor, and one capacitor to test.

Do a Google search to see how to test them with a DMM. Take copious notes so you can report back with the results.
You have three diodes, one transistor, and one capacitor to test.

Do a Google search to see how to test them with a DMM. Take copious notes so you can report back with the results.
I’ve tested all the parts in the area that dendad pointed out. Here is what I found:
With the base of tr4 isolated from the circuit base to collector read .581v base to emitter .627v. With leads reversed both read OL.
D10 tests good (.443v one way, OL the other)
D11 tests good (.443v one way, OL the other)
ZD3 tests good ( .640v one way, OL the other)
Capacitor c22 tests bad. It reads .5 nf when it says 10 uf 100v on it. I took it out of the circuit to test it.
 

dendad

Joined Feb 20, 2016
3,924
It mostly is the capacitors.
There is a valid argument for testing all the caps, and even replacing them all if in doubt.
 

JoeJester

Joined Apr 26, 2005
4,390
Doe


I’ve tested all the parts in the area that dendad pointed out. Here is what I found:
With the base of tr4 isolated from the circuit base to collector read .581v base to emitter .627v. With leads reversed both read OL.
D10 tests good (.443v one way, OL the other)
D11 tests good (.443v one way, OL the other)
ZD3 tests good ( .640v one way, OL the other)
Capacitor c22 tests bad. It reads .5 nf when it says 10 uf 100v on it. I took it out of the circuit to test it.
Ok, replace the capacitor and then if all the voltages are present, you can start assembly. Dont put more than one channel in, so you can test all the functions. That would limit the fault to that channel or the common amplifier, that is common to all the channels.

Once the learning curve is done on one, the time from installation time between channels will be quicker as you will be more familiar with the channels as time moves forward.
 
Top