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

Thread Starter

nbtone

Joined Oct 14, 2016
65
Hi,
I just bought a mixing board that doesnt power up. The fuse is missing, and when a fuse is put in nothing happens. I’m not able to find a schematic for it. What should I do? I have basic knowledge of analog electronic circuits, and I figure that all I will have to do is replace the rectifiers and perhaps a capacitor or two in the power supply, after giving all the pots and switches in the board a good cleaning of course.
 

danadak

Joined Mar 10, 2018
4,057
Without a schematic its tedious.

Look for caps with bulging tops and/or leakage stains on PCB.



If they short they will typically take out components they are connected to,
like transistors. If you have a C ESR meter use that to examine caps.

Powered down examine diodes and transistors for junction capability, not conducting one
polarity, conducting in the other. This is tricky due to other attached circuitry, basically a forward
biased junction, bipolar transistor, will experience a .7 - 1.0 V drop across junction when conducting.

Look for burnt discolored components, and check the ones they are connected to.


Regards, Dana.
 

Thread Starter

nbtone

Joined Oct 14, 2016
65
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I was able to finally get it apart. It looks like there are two circuit boards that make up the power supply. I’m not sure where to start looking. My first guess would be blown rectifiers.
 

drc_567

Joined Dec 29, 2008
1,154
... Can you check the thermistor for continuity? ... Part no. SG220?
I had one or two beverage heaters with similar temperature control and they seem to fail after a certain time ... less than expected.
 

Thread Starter

nbtone

Joined Oct 14, 2016
65
I found one thermistor, and it shows continuity. I don’t see a part no. Sg220, though. Where do you see that please?
... Can you check the thermistor for continuity? ... Part no. SG220?
I had one or two beverage heaters with similar temperature control and they seem to fail after a certain time ... less than expected.
 

Thread Starter

nbtone

Joined Oct 14, 2016
65
Without a schematic its tedious.

Look for caps with bulging tops and/or leakage stains on PCB.



If they short they will typically take out components they are connected to,
like transistors. If you have a C ESR meter use that to examine caps.

Powered down examine diodes and transistors for junction capability, not conducting one
polarity, conducting in the other. This is tricky due to other attached circuitry, basically a forward
biased junction, bipolar transistor, will experience a .7 - 1.0 V drop across junction when conducting.

Look for burnt discolored components, and check the ones they are connected to.


Regards, Dana.
Someone just posted a schematic. I haven’t had time to study it yet, and I am waiting for a replacement fuse holder, before I can try to power up the unit. Thanks for the advice, and I’ll try what you’ve suggested when I find some time.
 

drc_567

Joined Dec 29, 2008
1,154
... part SG220 is located on the schematic above in the very top section at the left, after the choke, and before the diode bridge. It seems to have a thermistor like symbol with a C, denoting temperature. It may be a thermistor or a thermally related component. It is marked TH1.
... It is that black lozenge shaped part next to a blue SP5 connector.
... There should be continuity between the board terminal for SP5 and either one end or the other of the empty fuse holder.
... There could be some kind of opening or discontinuity in the metal board traces, so if nothing else turns up, grab a magnifier and see if there is anything of that nature.
 
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Thread Starter

nbtone

Joined Oct 14, 2016
65
... part SG220 is located on the schematic above in the very top section at the left, after the choke, and before the diode bridge. It seems to have a thermistor like symbol with a C, denoting temperature. It may be a thermistor or a thermally related component. It is marked TH1.
... It is that black lozenge shaped part next to a blue SP5 connector.
... There should be continuity between the board terminal for SP5 and either one end or the other of the empty fuse holder.
... There could be some kind of opening or discontinuity in the metal board traces, so if nothing else turns up, grab a magnifier and see if there is anything of that nature.
I just tested for continuity, and there is continuity back to one side of the switch. There is no continuity to either side of the fuse, since it is connected to the other side of the switch.




Someone just posted a schematic. I haven’t had time to study it yet, and I am waiting for a replacement fuse holder, before I can try to power up the unit. Thanks for the advice, and I’ll try what you’ve suggested when I find some time.
 

drc_567

Joined Dec 29, 2008
1,154
... understand.
When you get the new fuse holder, see if it powers up without any problem. Try to trace the DC voltage from the diode bridge, to where it terminates. That schematic should help.
 

Thread Starter

nbtone

Joined Oct 14, 2016
65
I just got my fuse holder, and I was able to plug it in to try it. There is 0 v at the output pins. I wonder where I should start to look.
At the bridge the center two pins read around 50 VAC with the black lead of the VOM on ground and the red lead o the pin. I’ve attached the safety ground to the ground on the circuit board.
 

drc_567

Joined Dec 29, 2008
1,154
... at least nothing bad happened.
See if you can find capacitor C34. Be extremely careful with the fingers.
... That looks like several hundred volts DC. Some caps can retain a charge, even after the switch is turned off.
... The thing is to keep one hand in the pocket, so that a path to ground through the body is not completed.
... That between being said, it would be useful to know the DC voltage between the positive capacitor terminal and circuit ground. See if you can find a label on the board that says +V1. This terminal connects with the positive terminal of capacitor C34.
 

drc_567

Joined Dec 29, 2008
1,154
... What does it say on the large black cylindrical capacitor, the one next to the two blue jumper wires? That looks like an electrolytic capacitor, but I can't read the markings. Is it C34?
 
I just got my fuse holder, and I was able to plug it in to try it. There is 0 v at the output pins. I wonder where I should start to look.
At the bridge the center two pins read around 50 VAC with the black lead of the VOM on ground and the red lead o the pin. I’ve attached the safety ground to the ground on the circuit board.
Which ground did you attach to? There are multiple grounds. If you attached to the one marked "live" ground then you have a serious safety issue. This is a line connected power supply and the "live" ground is actually hot! It pulses to a negative peak of the input voltage. In order to safely "ground' the live ground you need to use an isolation transformer on the line inputs. This circuit does not have an actual ground, it is very dangerous.

Please disconnect this equipment until you are prepare to operate it safely.
 

JoeJester

Joined Apr 26, 2005
4,390
I recommend you get familiar with the schematic, specifically the live ground, the DC Ground, Line, Neutral, and earth.

Where are you so we have an idea of what your line voltage and frequency is.

State your testing plan to check the voltages, ie, where are you placing the probes for each of the voltages listed on that first schematic. Publish your list so if we have questions, you can answer them before probing around, potentially connecting to the wrong ground reference.

I don't know your abilities, so bear with me.
 

Thread Starter

nbtone

Joined Oct 14, 2016
65
I am Self trained in electronics. I studied through several electronics manuals, but I never got to figure out how to use a logic probe to test chips. I don’t have an oscilloscope. I know that you have to remove transistors and diodes from the circuit to test them with a transistor tester or a VOM. Actually, I manufacture bass tube preamps on the side, too, so I have a good solder station with solder suckers, etc.
I took the green wire from the plug to the wall and attached it to the circuit board where it is screwed to the chassis. I measured for voltage at the pins from the ground pins and from where the green wire is attached. Is the green wire the live ground?
C34 is the large electrolytic cap. It’s marked 560uf 400v. I’m in the US, and I measure 116vac at the wall plug. I’ll have time later to do more exploring.
 
The green wire attaches to chassis ground. Live ground is not actually a ground, it is a floating reference point for the line connected supply. Transformer TX1 separates the line (hot) side of the unit from the low voltage side on the right of the schematic. Everything to the left of TX1 is deadly, use extreme caution. The right side is safer but the power supply puts out 48 volts and this is still in the range of deadly voltages. (i.e. voltages that can supply sufficient current to kill you. It is the current that kills and it only takes 100 to 200 milliamps.)

Without an oscilloscope you will have a had time troubleshooting this power supply. This is a switch mode power supply and a meter will not give much information. However, there are a couple of things you can check if you do so safely.

Here is a brief description of how the circuit works and what can be tested with a voltmeter:

Of course you know the purpose of the bridge rectifier and that you can measure the high voltage DC across C34. Since you have 116 volts input, the voltage across C34 should be 116 x 1.41 = 164 volts. (RMS x the square root of 2) The negative side of C34 is "live" ground. If you had a scope you would see it with a negative half cycle waveform at the line frequency. So "live" ground is very hot! The positive side of C34 is labeled +V1 and is also very hot.
If you have a good DC voltage of about 160 to 170 volts across C34 then you can proceed to the next step. IC3 is the heart of the power supply, it was manufactured by a company called Unitrode. Unitrode was bought by Texas Instruments and this particular part is no longer made. Here is how it works:
At start up a small amount of current flows through R24 to supply IC3. When IC3 gets sufficient power it will start oscillation on pin 4 which connects to the gate of TR2. TR2 supplies a little current through the primary winding of TX1. The secondary winding on terminals 3 and 14 supplies some small current through R30, D10 to TR4. TR4 is a linear regulator, the base is set to 13 volts (relative to live gnd) by R31 and zener diode ZD3. Due to the voltage drops across the base-emitter of TR4 and D11, the voltage supplied to pin 3 of IC3 will be about 11 volts. (referenced to live gnd) So the result is that R24 gives it a little kick but the actual voltage supply to IC3 is coming from the switching of the power circuit.
With a multimeter you can measure some of the DC voltages but you won't be able to see the actual switching. If the voltages on the primary side appear OK then check the secondaries, there are four outputs; +17, -17, 14 and 48 volts. In fact, check these first, it is a lot safer.
 

Thread Starter

nbtone

Joined Oct 14, 2016
65
I see where it says live ground with a white arrow head. I see where it says DGND with a white arrow head. I see several black arrow heads , and I see several slashes under a line that looks like a ground of some kind.
Are the voltages past the bridge rectifier DC? I measure 163 VDC across C34.
 
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