JPK

Lots of broken links there...
Corrected! Thank you for reporting.


Spiritworks

I applied 48v phantom power switch yesterday to two of my MP573 and blew R21, as well as the 48v fuse in the power supply.  Setup of the pres went fine on my bench. Any idea where I should look?

JPK

Were the pre's working before or is it a first power up?
What PCB revision is it?

Spiritworks

Pres went through test and setup some time ago. They are Rev1.3, circa 2013. Pres were working fine with a dynamic mic and also the Hi-z input.  This is the first time I tried phantom power with them with microphones actually hooked up.  I wonder if the problem could be with my patch bays...…..

JPK

There is normally no connection between R21 and the 48V. But R21 is close to the 48V switch so I would check there is no unwanted connection like a solder bridge or a component lead touching the chassis plate around R21, C6, SW1.

Spiritworks

I erroneously identified R21 as the burnt resistor. The burnt resistor is actually R1, which is right beside R21 on the PCB and is indeed in line with the 48volt switch, if I read the schematic correctly. I disassembled the preamp and checked for any obvious build errors and solder bridges, etc. , but everything looks OK. The other four of my MP573 pass 48volts throughout my system without any problem. I checked all wiring to/from mic panels to patch bays, as well as the microphone cables I was using at the time. Any other thoughts where to look?

JPK

Does C1 look OK? Not bumped? Is it in the correct direction?
With an ohm-meter check you don't have a short circuit across C1.

Spiritworks

C1 is correct and not damaged

JPK

With an ohm-meter check you don't have a short circuit between the common point of R2-R3 and ground (or across C1).

Spiritworks

no shorts, all seems normal - strange.  I'll replace the burnt resistors, re-install in my rack and see what happens......
Thanks for your suggestions JPK