Blown AVD MP512 Pre

[LNS]

Hi,

A couple of months ago I built at TAC 11 slot 500 series rack and PSU, I turned it on and there were no puffs of magic smoke and the voltages all tested fine with a DMM, so I grabbed one of my AVD MP512 preamps to test with my MacBook a Focusrite Scarlett 2i2 and some test tones......... when I plugged everything in and powered it up something wasn't right and my MacBook immediately restarted. I unplugged everything and tried again and then the same thing happened. I took the MP512 out and tried it in one of my 500 series lunch boxes and it was dead, the 48v LED still lights up but no audio passes through it?????

I need to do some more tests on the TAC to get to the bottom of the fault but I want to get this preamp repaired first to use as a test unit as I don't want to risk blowing any other modules the only problem is I can't identify what has blown and why the MP512 is no longer working..........

Here's a list of what I have tested so far:

• The diodes test fine
• I’ve swapped the DOA out for another and still nothing, I’ve tried the DOA in another MP512 and it works fine.
• With the DOA in +/- V to ground measure just under 16v.
• I’ve checked the voltages on the DOA sockets when it’s powered up and they measure just under 16v as expected
• I’ve swapped out c12 as someone told me they thought that might be the problem and nothing

I’ve taken the following resistance measurements on the DOA sockets:
+ V & G = races up to around 18m before slowing
- V & G = races up to around 18m before slowing
+ V & O = races up to around 2.7m before slowing
- V & O = races up to around 2.7m before slowing
G & O = 108r
- in & G = 20k
+ V in & G = 1k
- in & O = 20k
+ V in & O = 1k

I'm really at a loss with the MP512 if anyone can point me in the right direction or what I should be looking at next I'd really appreciate it.


Thanks
Ow

 

[Walrus]

Have you checked all the windings on the transformers for continuity?

 

[LNS]

Have you checked all the windings on the transformers for continuity?

I've not done anything to test the transformers yet, can I test for continuity whilst they're in the PCB or would I need to remove them first?

 

[hoax]

Picture doesn't show it clearly, but perhaps check the pushbutton solder pads for shorts and/or cold joints?

 

[Walrus]

I've not done anything to test the transformers yet, can I test for continuity whilst they're in the PCB or would I need to remove them first?

Yes. Each winding of the 2503 should be about 20 Ohms. The secondary of a real 2622 is 1500 Ohms and the paralleled primary is 25 Ohms.
A copy should be similar.

 

[iprovlek]

Hi LNS you have very messy solder joints! You must desolder the board and solder component back again!
Then clean up after soldering with some clean flux....then you will see what happening...

 

[LNS]

Hi LNS you have very messy solder joints! You must desolder the board and solder component back again!
Then clean up after soldering with some clean flux....then you will see what happening...

The solder joints are fine, I just haven't cleaned the flux off the board as it went straight into a rack the test and then didn't leave (until 12 months later when I used it to test the TAC rack).

 

[iprovlek]

LNS try to check potentiometer area, under potentiometer!

On pics there isn't any sign of burnt component....so measure and check potentiometer!

 

[LNS]

Yes. Each winding of the 2503 should be about 20 Ohms. The secondary of a real 2622 is 1500 Ohms and the paralleled primary is 25 Ohms.
A copy should be similar.

I've revisited this tonight and I think I have discovered the issue and think the 2622 has gone bad.

Each winding of the 2503 measures 9.5ohms (I've spoken to AVD and he said that's correct), so that rules that out.

When I measure the 2622 the primary is 20 ohms (AVD said it should be around 21ohms) but the secondary is only measuring 3ohms (AVD said this should be 2.6k). I have a few of these AVD MP512 pres, so I measured the 2622 in a good unit, the primary measured 20ohms and the secondary measured 2.5k, so I'm pretty sure I just need to swap out the 2622 on my bad module and it should be back in business!

 

[Rob Flinn]

Did you have the phantom power on your Scarlett switched on ?

 

[LNS]

Did you have the phantom power on your Scarlett switched on?

Phantom power was off, I either hot plugged it as the PSU was on the floor or not paying attention plugged it into the edge connector upside down on my extender.