RCA BA-31 Power Supply Confusion

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substitute

Well-known member
Joined
Jul 14, 2004
Messages
504
Location
PHILADELPHIA
Good morning everyone,
I'm working on racking an RCA BA-31 module, I opted not to use the built in power supply but instead used a Five Fish PSU. 
I Connected +30v to point B and -30v to point P.
When I fired up the unit it worked for a minute but then R17 started smoking and burned up.

Is that the proper hook up?  Looking at the schematic I don't really understand where ground is on the built in PSU.

Thanks!

http://www.waltzingbear.com/Schematics/RCA/BA-31A.htm
 
Did you use a 30V supply, or a +/- 30 V supply?  It's just a 30V supply.  I'm not sure why you wouldn't use the built in supply, it works great.  If I were to choose to ignore the built in supply on a 31, I'd sell it and buy a 71 or 72 instead, make some money back. 
 
EMRR!  So glad I've got your attention, is there like an RCA bat signal that activates on your computer?

The module was missing the built in PT when I bought it, seeing as how I wanted to add +48 I figured it'd be easier to start from scratch.

I did use a +/- 30v supply with +30v going to point B and -30v going to point P

So I should connect point P to +30v on the PSU and point B to ground, and get another 130r resistor.

Thanks!
 
You absolutely definitely never connect point P to +30v.  The terminology here is confusing, yeah, you put 60V into it.  You might have more problems than just R17.  Look at where ground is.  Look at where common is.  Look at the PSU's lack of direct ground reference, and  how it relates to ground through R17.
 
I have to admit I'm pretty lost, the lack of ground reference in the power supply is new to me.  Applying +30v to point B makes sense to me but I just don't know what to do with point P.

Looking over the circuit board, the new caps I put in look good, hopefully nothing else is fried. 

Where should I go from here?

Sorry I'm so dim on this one
 
If you follow the language of 'common', attach the common of your +/- supply to B, and -30 to P.  Don't use the +30 side at all, don't ground the common of the PSU, since the original PSU has no direct ground reference of it's own.  +30 on this drawing is a relative statement regarding the 'more positive' side of what is a single supply voltage, rather than a dual supply voltage. 
 
Ok, Thanks so much Doug.  That I can follow.  Hope it's not too hot down in your neck of the woods, up here in Philadelphia the weather is very uninspiring.

 
That totally fixed it, all I had to do was replace that resistor. Now it seems something got toasted on the 48v supply.

Thanks again!
 
Wow, you were one step ahead of me.  Somewhere in my journey I blew the LM317 on the +48 rail, so that's replaced now.  So, do I need to do anything other than usual when hooking the +48 up to the input xlr?

Thanks folks
 
Just check the transformer wiring like I mentioned.  You can also measure from pin one (ground) of your XLR to pins two and three.  If you have continuity, or a few ohms then the center tap is still connected and will short out your phantom supply.
 

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