Mackie 1640i with explosive PSU

[wub]

I hope this is the right place to post about this because I could do with some advice!

I have a Mackie firewire mixing desk that has occasionally made a bit of a bang when switched on. One time I even got a mains shock as I flipped the switch, so I took it apart and bypassed the switch thinking it must be faulty.

I've been plugging it into a switched extension and using that as the "on" switch but yesterday it did it again. I decided to investigate and this is what I found:

therm1.jpg

I removed this thermistor and the scorched TVR 14471 varistor behind it.

I've never seen anything like this before, so I took a couple more pictures:

therm2.jpg

therm3.jpg

I can get a replacement easily enough but I wondered what could cause this? I found an old post on another forum where someone has the same problem:

https://groupdiy.com/threads/mackie-164 ... st-1060217

There's some general advice there but nothing that really helps. Can anyone offer any advice about what I should look for as a possible cause? I have the schematic for the psu if that would help.

[exxos]

Is there a schematic ?

[wub]

Hey Exxos! Yes, there is an official schematic, Mackie are very helpful in providing these on their webpage:

0027370-00-A00-SCH.pdf

Thanks for taking a look, I really appreciate it! :)

[exxos]

Hey Exxos! Yes, there is an official schematic, Mackie are very helpful in providing these on their webpage:

0027370-00-A00-SCH.pdf

Thanks for taking a look, I really appreciate it! :)

Thanks, which part is it R1, R2 etc ?

[wub]

It's R1, sorry for not being more clear about that.

The yellow varistor behind it is R2. I cleaned the scorch mark of it but I'm guessing it would be a good idea to replace it too while I'm at it.

[exxos]

It's R1, sorry for not being more clear about that.

Inrush current limiter. Those things are evil and should be banned. They don't age well and do tend to explode like that.

I would check the rectifier isn't shorted out to be on the safe side. I'm assuming no other failures in the circuit as well.

Generally what I used to do, was run a 100watt mains bulb in series in the mains lead. Normally enough to power up a PSU to test it. Though if something has shorted out, the 100watt lamp lights up bright and limits the current. So the PSU's don't tend to explode then.

[wub]

I would check the rectifier isn't shorted out to be on the safe side. I'm assuming no other failures in the circuit as well.

Is that D1? But I'm guessing everything is ok because, despite the loud bang and damage to R1, I continued to use the desk all day yesterday with no problems. It just seems to be that initial moment of powering up that isn't 100%. Still, it makes life interesting! :)

Many thanks for your help!

[exxos]

The PSU would not function without R1. It would function without R2. But that is a transient suppressor. If R2 is blown then C6 could also be damaged,

[wub]

Thanks for the extra info! R2 seems to be ok, and I couldn't find a direct replacement so I'm going to reuse it. I'm still lacking a bit of confidence when it comes to choosing components!

All the caps look ok so I'm not going to risk desoldering anything to test, just in case I damage the PCB. R1 was incredibly stubborn coming out and I was getting a bit nervous about burning the board.

I've ordered a new R1 thermistor and I'll post again after I've replaced it to let you know what happens. Thanks again for taking the time to help! :)

[exxos]

That does not make sense...

R1 is in series with the mains rectifier.. With no R1.. the PSU has ZERO power. It will not function without it. Basically has the same effect of the mains fuse blowing.

R2 is a voltage suppressor where the power supply can run without it.

So either you have the wrong part number or the wrong schematic...Or you have placed a wire link over the part or something else ??

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