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

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).
NO, NO, NO!

Do not connect the probe ground to live ground! There is high voltage at that point. Please refer to my post #70 about how to set the scope. You can connect the probe grounds to the chassis but not to the circuit. In differential mode the channel 2 probe tip is the reference and channel 1 probe tip is for measurement.
 

Thread Starter

nbtone

Joined Oct 14, 2016
65
I wasn’t going to test it in differential mode. I was going to use only one probe. That’s how I understand is the correct way to test it with a battery powered oscilloscope. If the power supply works after I change the capacitor I might not need to test with an oscilloscope at all?
 
That is not how it works with any oscope, battery or otherwise. Single channel operation in this circumstance is NOT safe, despite what others on this forum will tell you. I have decades of experience in electronics, both as a technician and in engineering. One of my positions was as a Research and Development Engineer designing line connected high voltage power supplies. I would never do what you are attempting without an isolation transformer. Even then I would exercise extreme caution. Anyone who tells you that it is OK to use single channel is a fool! Don't listen to them.
 

Thread Starter

nbtone

Joined Oct 14, 2016
65
I’ve been away for a while, and now I’m getting back to this project. I bought an esr meter to check capacitors with. I found several that were out of spec, and I replaced all but one. I ordered a new 330uf 25 v nichicon audio capacitor, and the esr on the new one is too high. I sent for another of a different variety, and it’s too high, too. I’m not sure what to think of this.
 

Thread Starter

nbtone

Joined Oct 14, 2016
65
This is really remarkable! I got new capacitors to replace all the ones that the esr meter said were bad, and now my power supply works! Never had to use the oscilloscope. Now I’ll go through the rest of the sound board the same way. Thanks for all the advise, but in the end I got by with just dumb luck, once again.
 

dendad

Joined Feb 20, 2016
3,924
Good job!
The caps are the number one suspect in switch mode supplies. Even with popped FETs and things, often the root cause is the caps. The FETs are just a symptom.
Tossed out monitors and TVs can often be fixed that way too.
 

Marcellus

Joined Jul 13, 2021
1
Hi all,

Just wanted to let you all know that I have fixed this same Soundcraft power supply with an intermittent power-up fault. The main culprit was indeed C34, the 560uF 400V mains filter capacitor! (which had reduced / leaked to just 470uF over the years). It's interesting because all of the research that I've read in relation to SMPS repair seldom mentions the large mains filter capacitor(s), and, on the rare occasion that it is mentioned, it usually just says that this large cap is unlikely to be the culprit, and that you can rule it out - clearly not in the case of this Soundcraft SMPS!

I replaced this cap with a 680uF 400V electrolytic cap - the higher capacitance should help to prevent this fault from happening again anytime soon - even if this cap leaks down by 25%.

NB1. I also replaced a multitude of other parts (e.g. all of the other electrolytic caps) with higher spec components in order to ensure that this power supply will continue to run smoothly for many years to come.

NB2. Before I'd concluded that the issue was with C34, I also ran extensive tests on the transformer windings, in order to ensure that none of the seven windings had failed short (which would effectively reduce the number of turns in the failed winding). I did this by passing low voltage AC waveforms through each winding and observing the waveforms induced on the other windings. To be extra sure about the windings, I also ran a 4-wire winding resistance test using two multimeters: Run a low constant current (e.g. 1A DC) through the winding (monitor it carefully with multimeter i. in series with the winding in order to ensure that it is exactly 1A), and, with another multimeter (ii.), measure the DC voltage across the winding while the 1A DC current flows). Then, since V=IR (and so R=V/I), I divided my voltage reading by the current reading (the current is 1 amp so just dividing by 1 - i.e. no need to divide) meaning that the voltage reading on multimeter ii. is the same as the DC resistance (in ohms) of the winding. This method is useful since the DC resistances of the transformer windings are very low (less than 0.1 ohms) and trying to measure such low resistances using the usual resistance / ohms setting on most multimeteres is not sufficient. Using this 4-wire measurement method I was able to get very accurate results for the low resistances of each winding, and by comparing each to the others, I could tell that none of the windings were damaged / none had failed short.

I hope that this helps,

M.
 

Advtec

Joined Sep 10, 2021
2
Hey guys. I will initiate repairs to the Soundcraft-GB8 power supply. Can I access her power supply boards by opening it from below? Or do I need to disassemble it from the top panel? Thanks!
 

Thread Starter

nbtone

Joined Oct 14, 2016
65
It’s been a while since I took mine apart, so I don’t remember all the details. However, I believe the bottom is attached to the two sides. There are screws in the middle that go into a support that is attached to the top. I believe you can just remove the bottom and the sides in one piece. There may be a wire or two that may get in the way. Good luck!
 

Advtec

Joined Sep 10, 2021
2
It’s been a while since I took mine apart, so I don’t remember all the details. However, I believe the bottom is attached to the two sides. There are screws in the middle that go into a support that is attached to the top. I believe you can just remove the bottom and the sides in one piece. There may be a wire or two that may get in the way. Good luck!
I understood. Thank you very much!
 
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