Small soundcraft power supply (for EPM12 mixer)

alistair · 30th January 2012

Hey guys,

I've got a EPM12 in for repair here- all of the preamp channels are dead (no gain) and have no phantom coming in at all. The monitoring/aux channels/master faders are all working fine, which I have traced back to only 2 out of 3 rails working.

The power supply is a pretty crappy switchmode, that supplies a V1+ (+18v), V1- (-18v) and a V2. I've tested the outputs of the power supply, and lo and behold V1+/- is working fine, and I'm getting nothing on V2.
The power supply board is showing some heat damage, but without a schem I'm not really sure where to start short of replacing all components.

Unfortunately, Soundcraft will give me technical documents for the mixer, but not the power supply. They want me to buy a new PS, at around $250 AUD, which is a bit ridiculous on a desk worth not much more than.

Has anyone worked on these before? I was assuming that V2 was the phantom supply, except it seems to also be powering all the preamp gain opamps, so I don't believe it is a 48v tap.
Any ideas? I'm thinking for a little desk like this that probably has a pretty short lifespan anyway, the smartest option might be just to buy a small external switchmode to provide the voltage needed.

Mixermend · 30th January 2012

This is going to be an easy one (I hope!). If the audio voltage rails are OK - check everything on the secondary side of the SMPS transformer in the Phantom section.

Especially check the high speed rectifier diodes - I have had them fail short circuit before.......

Normal power supply faulting techniques apply on the output side of the transformer.

Let us know what you find?

alistair · 31st January 2012

Thanks for your reply. I actually spent a little bit of time last night tracing out the PS of the output side. Now that I can follow the circuit through, it appears that the rail is coming out of the transformer correctly, and is shorting out right near the end of the circuit, at a ZTX651 transistor. The rail seems to be running at around 75v open (I'm assuming it will drop down to 48v due to the transistor operating correctly and being under load, as V1+/- is running at 30v open and 18v under load- there is no way to test the PS under load due to the stupid physical design of this mixer), and as the max voltage of a ZTX651 is 80v I think, I wonder if it might have crept up at some point and blown it. I'll try replacing it and see how I go.

alistair · 8th February 2012

I replaced the ZTX651 and phantom power and the preamp channels are working again. Whilst doing a quick grounding check I noticed that on the line input jack on channel 1, there is 48v on the Tip connection! Not the greatest place to have stray voltages.
The 1st channel is also distorted and has very little level compared to the other 12 channels.
I'm still trying to hunt down why this is happening, but at least I think I've found the culprit- I can't imagine it's a coincidence between there being 48v here and the 48v power line dying.

30th January 2012	#1
alistair Gear nut Joined: Mar 2007 Posts: 93 Thread Starter	Small soundcraft power supply (for EPM12 mixer) Hey guys, I've got a EPM12 in for repair here- all of the preamp channels are dead (no gain) and have no phantom coming in at all. The monitoring/aux channels/master faders are all working fine, which I have traced back to only 2 out of 3 rails working. The power supply is a pretty crappy switchmode, that supplies a V1+ (+18v), V1- (-18v) and a V2. I've tested the outputs of the power supply, and lo and behold V1+/- is working fine, and I'm getting nothing on V2. The power supply board is showing some heat damage, but without a schem I'm not really sure where to start short of replacing all components. Unfortunately, Soundcraft will give me technical documents for the mixer, but not the power supply. They want me to buy a new PS, at around $250 AUD, which is a bit ridiculous on a desk worth not much more than. Has anyone worked on these before? I was assuming that V2 was the phantom supply, except it seems to also be powering all the preamp gain opamps, so I don't believe it is a 48v tap. Any ideas? I'm thinking for a little desk like this that probably has a pretty short lifespan anyway, the smartest option might be just to buy a small external switchmode to provide the voltage needed.

30th January 2012	#2
Mixermend Gear maniac Joined: Jan 2006 Location: Exeter UK Posts: 166	This is going to be an easy one (I hope!). If the audio voltage rails are OK - check everything on the secondary side of the SMPS transformer in the Phantom section. Especially check the high speed rectifier diodes - I have had them fail short circuit before....... Normal power supply faulting techniques apply on the output side of the transformer. Let us know what you find? __________________ http://www.orchid-electronics.co.uk Audio Engineers Helping To Make Music!

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31st January 2012	#3
alistair Gear nut Joined: Mar 2007 Posts: 93 Thread Starter	Thanks for your reply. I actually spent a little bit of time last night tracing out the PS of the output side. Now that I can follow the circuit through, it appears that the rail is coming out of the transformer correctly, and is shorting out right near the end of the circuit, at a ZTX651 transistor. The rail seems to be running at around 75v open (I'm assuming it will drop down to 48v due to the transistor operating correctly and being under load, as V1+/- is running at 30v open and 18v under load- there is no way to test the PS under load due to the stupid physical design of this mixer), and as the max voltage of a ZTX651 is 80v I think, I wonder if it might have crept up at some point and blown it. I'll try replacing it and see how I go.

8th February 2012	#4
alistair Gear nut Joined: Mar 2007 Posts: 93 Thread Starter	I replaced the ZTX651 and phantom power and the preamp channels are working again. Whilst doing a quick grounding check I noticed that on the line input jack on channel 1, there is 48v on the Tip connection! Not the greatest place to have stray voltages. The 1st channel is also distorted and has very little level compared to the other 12 channels. I'm still trying to hunt down why this is happening, but at least I think I've found the culprit- I can't imagine it's a coincidence between there being 48v here and the 48v power line dying.