Mackie SRM450 fault - help!

[qubuntu]

Hi there,
Apologies if this turns out to be really obvious; my electronics knowledge isn't good enough to work this out. I have a Mackie SRM450 (original linear PS version) which pops loudly when I plug an XLR cable into it. Measuring the voltages across the input pins I am seeing 37V between hot (2) and ground, and 34V between cold and ground. I assume this is a power supply issue?

Thanks in advance for any help

Here's the schematic of the input:

[J M Fahey]

Looking at the sdchematic there is no way you can have such a high voltage there, maximum possible *could* be around +15V *if* for some reason D1-D3 protection diodes shorted *and* also did input capacitors C24 and C30 .

Thinking that ghosts do not exist but I met a few, just thinking aloud:

1) somebody tried to add phantom power to those inputs, in a very incompetent way.

2) somebody read too much DIY Audio Forum, tried to improve what's shown as "P1 ground post to chassis" , decided that star grounding is better and run a wire to PSU ... only problem is that he missed and "grounded" to -35V ... so now input pins read ~35V positive to "ground" .

Notice in all cases I suspect botched Human intervention, there is no "natural" failure which could show those voltages at the input.

[qubuntu]

Looking at the sdchematic there is no way you can have such a high voltage there, maximum possible *could* be around +15V *if* for some reason D1-D3 protection diodes shorted *and* also did input capacitors C24 and C30

Thanks for your reply - that's what I thought, it's weird.

BTW I've owned the unit from new and it's never had any modifications

[Mick Bailey]

Is that DC you're reading?

Is the voltage still there when something is plugged in?

Also, does the unit work normally aside from the pop?

[qubuntu]

Is that DC you're reading?

Is the voltage still there when something is plugged in?

Also, does the unit work normally aside from the pop?

Yep, it's DC.

I haven't tried reading the voltages with anything plugged in.

The unit is working normally apart from the stray voltages.

I forgot to mention also that the Signal Present LED lights up when I put the multimeter across pins 1 and 2, and stays off when across pins 1 and 3.