Sound Skulptor forums

Build help and troubleshooting => MP512 Microphone preamplifier => Topic started by: powelles on March 29, 2023, 05:43:35 AM

Title: Phantom Power At About 4.5v
Post by: powelles on March 29, 2023, 05:43:35 AM

I'm a bit at a loss. Everything else about the pre works correctly. I can trace the 48v coming from the switch at roughly the correct voltage all the way to r1 and r2. On one side it reads 45v and the other it reads 4.5v. I can't find any obvious problems. 

Title: Re: Phantom Power At About 4.5v
Post by: JPK on March 29, 2023, 11:47:26 AM

Is it the same when the mic input is left unconnected?

Title: Re: Phantom Power At About 4.5v
Post by: powelles on March 29, 2023, 06:20:16 PM

I've only been testing disconnected. It drops to about 3.8 with a phantom powered mic connected.

Title: Re: Phantom Power At About 4.5v
Post by: JPK on March 30, 2023, 04:57:33 PM

Did you leave a small gap under the input transformer?
Are the relays inserted in the right direction?
Is C4 inserted in the right direction?
Is the value of R3 correct (150R)?
Is the 48V voltage correct when the 48V switch is off (measured at the connector, last bottom finger).

Title: Re: Phantom Power At About 4.5v
Post by: powelles on March 30, 2023, 06:18:06 PM

There is about 1mm between the transformer and the pcb. The relays as well as c4 are in the correct direction. The value of r3 is correct and the preamp is receiving 48v with the switch off. The pre otherwise works as expected.

Title: Re: Phantom Power At About 4.5v
Post by: JPK on March 31, 2023, 03:55:38 PM

Check that you don't have a component lead that touches the chassis plate.
Check that there is no solder bridge anywhere or forgotten solder joint.
If nothing of the above works I propose that you send us your unit for a check. Contact us here (https://www.soundskulptor.com/en/content/6-contact).