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

#### 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.

#### CharlesWMcDonald

Joined May 16, 2019
233
I checked your source and they are out of stock but Digikey has 49 in stock: https://www.digikey.com/products/en?mpart=UCC35702N&v=296
If you are located in the U.S. you can get it easily, it not you will have to find a TI distributor.
I also found it on the TI website and they have the specification sheet. (attached)

Lack of an oscilloscope still hinders your ability to test this. Ask around locally, you may find someone to assist.

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#### JoeJester

Joined Apr 26, 2005
4,390
You have the live ground in the primary side of the transformer. This is a floating return to the transformer.

You have earth ground at the AC input which is tied to neutral back at the electrical distribution.

You have Dgnd (white triangle) and the black triangle tied together via Lk1 And Lk2 to a point I assume is the chassis grounds ... As the angles are more than the normal chassis ground symbol.

This could confuse the person working on a system, especially if their skills are less than optimal.

#### nbtone

Joined Oct 14, 2016
65
I have connected the earth ground to the correct screw connection point on the circuit board. Now there is continuity from earth ground to dgnd and the black arrow.
I measured the voltage at pin 3 of the chip and found about 9.5 VDC, using the live ground at the negative terminal of c34 for ground reference.
I measured the voltages at ch2 and found 0 bolts where there is supposed to be + or - 17v, and I found 365mv where the 48 v is supposed to be. I used the earth ground, which is connected to the black and white arrows, as the ground reference for this test.
If I find someone with an oscilloscope what would I need to do to find which parts need replaced?

#### nbtone

Joined Oct 14, 2016
65
I have an extension cord plugged into the gfci receptacle in the bathroom. Thank you!

#### drc_567

Joined Dec 29, 2008
1,156
It may be that the TX1 transformer is not receiving a pulse waveform on its primary side for one reason or another. With a scope, you would be able to examine pin 4 of IC3, the UCC37502 chip, in order to see if things are working up to that point.
... Again, there are big problems with connecting a scope to a live AC circuit. Usually, plugging the scope into an isolation transformer is required. A GFCI outlet may work, but get another opinion.
... There are 3 or 4 parts besides IC3 that may be where the problem is.

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#### JoeJester

Joined Apr 26, 2005
4,390
There are only three ways to isolate an oscilliscope circuit. The easiest is a DC operated oscilloscope and they are pretty cheap for a single channel hobbyist model with limited bandwidth. Those scopes do not come with x10 probes so you would end up spending a little more money for that item. The few I saw at Amazon for under \$50 were 200 kHz bandwidth.

Most scopes are earth grounded via the green wire. The best way is to use an isolation transformer. I've heard of people just removing the earth ground connection. I've never tried using that because I didn't want to do the paperwork if someone killed themselves.

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#### mlsirkis

Joined Aug 11, 2010
32
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.
You may want to consider disconnecting the wires that connect the output of the supply to the rest of the console and adding resistors as a dummy load. This serves two purposes. First, it will prevent damaging the rest of the console if you accidentally slip while probing the power supply. Second, it’s possible that there is a short somewhere in the console that is pulling the supply down.

#### nbtone

Joined Oct 14, 2016
65
You may want to consider disconnecting the wires that connect the output of the supply to the rest of the console and adding resistors as a dummy load. This serves two purposes. First, it will prevent damaging the rest of the console if you accidentally slip while probing the power supply. Second, it’s possible that there is a short somewhere in the console that is pulling the supply down.
I’m sorry. I never said whether I had disconnected the rest of the console before I tested the power supply. Would that make a difference? I have the power supply all the way out of the console and sitting on a table. All the testing was done on the table with the earth ground screwed to one of the circuit board mounting holes.

#### nbtone

Joined Oct 14, 2016
65
It may be that the TX1 transformer is not receiving a pulse waveform on its primary side for one reason or another. With a scope, you would be able to examine pin 4 of IC3, the UCC37502 chip, in order to see if things are working up to that point.
... Again, there are big problems with connecting a scope to a live AC circuit. Usually, plugging the scope into an isolation transformer is required. A GFCI outlet may work, but get another opinion.
... There are 3 or 4 parts besides IC3 that may be where the problem is.
I see the need for an isolation transformer. Thank you! Electrocution and damage to the oscilloscope are avoided with the use of one. Here is the YouTube video about it is hat I watched : https://youtu.be/LbkCU-LBqKk

#### CharlesWMcDonald

Joined May 16, 2019
233
I've been on vacation and am just catching up.
An isolation transformer will permit you to connect live ground to chassis ground and make direct measurements. However, using the transformer removes the protection of the GFCI and is less safe. You can still use an oscilloscope to make measurements without the transformer if it is dual channel. Set channel 2 to inverted and "add" the channels. When making measurement place the channel 2 probe on live ground and use the channel 1 probe for the point to be measured. This is called differential mode operation, here is a video explaining how to use it:

A 200 KHz oscilloscope is inadequate for this circuit. Note on the IC spec sheet that it can operate up to 700 KHz. You will need at least 1MHz bandwidth.

#### nbtone

Joined Oct 14, 2016
65
I've been on vacation and am just catching up.
An isolation transformer will permit you to connect live ground to chassis ground and make direct measurements. However, using the transformer removes the protection of the GFCI and is less safe. You can still use an oscilloscope to make measurements without the transformer if it is dual channel. Set channel 2 to inverted and "add" the channels. When making measurement place the channel 2 probe on live ground and use the channel 1 probe for the point to be measured. This is called differential mode operation, here is a video explaining how to use it:

A 200 KHz oscilloscope is inadequate for this circuit. Note on the IC spec sheet that it can operate up to 700 KHz. You will need at least 1MHz bandwidth.
I'm looking at this one: https://www.picotech.com/oscilloscope/2000/picoscope-2000-overview
Do you think it would work? I'm thinking that it might help with testing the rest of the board after the power supply is fixed with all the other functions that it has.

#### CharlesWMcDonald

Joined May 16, 2019
233
I used a PicoScope several years ago and was generally satisfied with the performance. But I used it strictly for low voltages. It has a +-20 volt input limit. Even with a 10 to 1 probe it would barely be adequate for your high voltage needs. It would probably also have some distortion in the waveform at elevated voltages. Also be aware that it will be connected to your computer and could damage it if you connect it incorrectly.
So, good for low voltage, general purpose electronics but unknown for high voltage. Check online, see if any reviews or other information will help your decision.

#### nbtone

Joined Oct 14, 2016
65
I’m looking at two, and I’m not sure which would be better:

Bk Precision 2520 Digital Storage 20mhz Variable Sweep Oscilloscope
Or
Hantech DSO 5102P

The first one says 400v max, and I don’t see a voltage rating on the Hantech

#### CharlesWMcDonald

Joined May 16, 2019
233
I have two Hantek oscopes, a 6022BL USB scope and a DSO8060. I have never used the 6022 to observe high voltage but I have looked at line voltage using the DSO8060. The 8060 has the vertical range but the waveform was a little distorted but is usable.

Looks like the BK is only available used and has a real CRT instead of an LCD screen. Personally, I like the Hantek better, it is lighter and more portable. The input can stand 300V at low frequency with a 10:1 probe. Above 100KHz the max level is derated at 20dB per decade down to 13volts at 3MHz and stays at that level to full bandwidth. Still not a bad scope but good for the price.

#### nbtone

Joined Oct 14, 2016
65
I have two Hantek oscopes, a 6022BL USB scope and a DSO8060. I have never used the 6022 to observe high voltage but I have looked at line voltage using the DSO8060. The 8060 has the vertical range but the waveform was a little distorted but is usable.

Looks like the BK is only available used and has a real CRT instead of an LCD screen. Personally, I like the Hantek better, it is lighter and more portable. The input can stand 300V at low frequency with a 10:1 probe. Above 100KHz the max level is derated at 20dB per decade down to 13volts at 3MHz and stays at that level to full bandwidth. Still not a bad scope but good for the price.

I found a BK that has never been used and still in the box. I wouldn’t need 10x probes to use it, would I? The Hantech has more features, but I don’t know if I will need them.

#### CharlesWMcDonald

Joined May 16, 2019
233
Yes, you will need 10x probes but any probe you get should have a switch to set 10x. You at least need the ability to set differential mode on the scope. Differential mode is achieved by inverting one channel and adding the channels. This enables you to view signals when the reference point is floating with respect to ground. You use channel 2 as the reference and make the measurement with channel 1. You will need two probes.

I would really like to discourage you from attempting to repair the sound board. It is too dangerous for a beginner to work on line connected devices. Despite my earlier warnings you appear intent to continue. That power supply is death waiting to happen. I won't post anymore on this topic. If you have questions on other topics, I'll be glad to help.