[sdiy] SCI Pro One 15+ power rail

[rsdio at sounds.wa.com]

On Aug 27, 2013, at 13:25, Stewart Pye wrote:
> On 8/28/2013 4:57 AM, rsdio at sounds.wa.com wrote:
>> If you suspect a short on one of the ICs, then the quickest test  
>> would be to disconnect the power transformer and see if the  
>> voltage returns to +15V.
>
> I'm pretty sure the voltage won't return to +15V with the power  
> transformer disconnected!


Sorry! That was very poorly worded. I should have said power supply,  
not power transformer.

I did mention that it matters whether the regulator is on its own  
board, e.g. a power supply board, or on the main board. Obviously, if  
the regulator is on the main board then disconnecting the power means  
it won't work.

Looking at the schematic, it does seem that the J1/P101 molex that I  
remember is AC, not DC, and the power supply is on the main board.  
Yikes! Even though the schematic claims that pin 5 is keyed, my Pro  
One is not keyed at all, meaning it's far too easy to connect the AC  
power incorrectly!

You can still test for high versus low resistance from the +15 V rail  
to ground with power disconnected, but that's not a very conclusive  
test - it just means you have something to check without making  
irreversible changes.

Brian

p.s. Here's a question about international line voltages. The Pro One  
schematic shows a switch for 115 VAC versus 230 VAC operation. For  
230 VAC use, the two primary windings are connected in series, 1:2 
+3:4, with presumably half the line voltage across each winding. For  
115 VAC use, only one primary winding is connected: 1:2. I briefly  
thought that it might be wired to use the two primary windings in  
parallel instead of series, but on second thought I think that would  
have the same end result - wouldn't using two primaries in parallel  
create the same internal magnetic flux as two in series, thus  
producing the same output? I'm guessing it's actually a bad idea to  
connect transformer windings in parallel, because even though they  
"should" have the same voltage, it's easily possible that component  
variations would make the voltage different, and that would cause a  
problem. Anyway, can someone tell me whether adding the second  
primary winding in parallel would increase the output current, output  
voltage, or just blow up?