Problem with Phantom Power after swapping trafos in my Pre73

[nooel]

So I'm switching the mic input and output transformers on my Pre-73.  Got everything soldered up and when I tested it the phantom power wasn't working.  Checked the +48V rail and it's only at around 7 to 8V.  Anyone have any ideas what's up?

BTW, the power transformer is a 24V outside of the chassis.  I'm kind of a newbie to electronics, but I've done some work/troubleshooting, but I don't know how the 24V is doubled for the phantom.  Any suggestions on measurements or components to start with would be awesome.  Thanks!!

 

[gemini86]

1) do you have a schematic for the PSU? -step one for troubleshooting-
2) Have you double checked that you haven't shorted out your phantom input, maybe by a mis-wired transformer?
3) WHAT is you favorite color?

*blue, no YELLOOOOOOOOW!*

 

[Rybow]

1) do you have a schematic for the PSU? -step one for troubleshooting-
2) Have you double checked that you haven't shorted out your phantom input, maybe by a mis-wired transformer?
3) WHAT is you favorite color?

*blue, no YELLOOOOOOOOW!*

;D

 

[nooel]

I unfortunately do not have a schematic, and I did a brief googling for it and couldn't find it.

However, my guess is that you are right that bad soldering of the trafo is shorting the +48V.  Is that something that might take it down to about 7V/8V? 

Also, if anyone knew a little theory on phantom power and input transformers that would be of great help too.

Thanks guys!

my favorite color is red btw

 

[pucho812]

the input transformer has nothing to do with phantom power unless it has a center tap which is a whole other ball of wax. so I would suspect you shorted out either the resistors 6.8K going to pins 2 and 3 or you shorted out something related to phantom power in the psu. unlesss there was a real goof wiring the the transformer incorrectly will either have it out of phase or backwards.

 

[gemini86]

Well, let's get some more specifics. What transformer are you using? I'm assuming that it requires you to jumper some windings together to setup the appropriate ratio, so I would first look there... Maybe you mis-read the TX pinout. Check voltage from the PSU while disconnected from the pre, make sure it's correct. Then go from there.

 

[martini]

can you put photos of psu board, main board and your trafos and their wiring ?

 

[pucho812]

lets start with what we know. It worked before you put in the new transformer, now with the new transformer phantom doesn't work. 

I would disconnect the primaries of the new input transformer and try phantom. Measure the results with a dmm looking for 48vdc accross pins 2 and 3 on the input xlr. if you have it without the primaries connected then it's something to do with the new input transformer however I doubt it as the input transformer should be separate from phantom power.

 

[nooel]

Hey-

Sorry forgot I started this.  So were measuring +38V off the PSU.  Also, when measuring the phantom, we shorted it across the + and grd, is that a concern?