Thursday, August 14, 2014

 

Astron Power Supply Adjustment to use as a PC linear supply

Background:
I bought an Astron RS-12-A power supply recently to power a TPA3116 amp, but decided to try it to power my music PC.  I wanted to see if it would improve sound quality.  It was $53 shipped.

I am using a little Pico power supply to power the PC, and the pico requires 12VDC to power it.   I was using an old XBOX 12VDC power supply I had lying around.  With the 12V pico, 12VDC is used to power the pico and also provides 12V power to the motherboard.  With my DAC, I heard power supply noise coming from the music PC, and I suspected that the xbox PS was creating the noise.

PC draws around 25 watts during normal use, configured as a headless system.  Since the RS-12-A can put out 9 amps continuous, at 12VDC that is 108 watts, easily enough for my PC.

So I needed to adjust the Astron to put out 12VDC instead of 13.8.  It is supposed to be adjustable from 11-15VDC.

So what follows is how I adjusted my Astron.


Inside the Astron
To open it up, there are two screws on the top back, and four on the bottom.  The bottom screws go through the feet.  In my Astron, one of the bottom screws didn’t want to come out, I had to use pliers to break the screw to remove it.
Slide the cover off, and you can see its guts.

The Astron has this big transformer, most of the weight come from it.

Figure 1


Here you can see the a big cap on the left.


Figure 2


Close up of the cap.  It is rated at 25V, so not lethal voltage but a lot of energy stored in it.
Note the mounting bracket on the bottom of the cap.  To get to the voltage adjustment pot, I removed the three screws that hold the bracket to the chassis.


Figure 3


Another picture looking from the top.  Notice the two screws and copper pads on the left, which hold the large cap in place. 
I discharged the cap after every power down, because it did not appear to self discharge.  I used a low resistance, high wattage resistor from my junk drawer, 5-10 ohms at 10 watts.

To discharge, I had a couple wires soldered to the resistor, and applied the wires to the solder pads.

Figure 4


The adjustment pot is under the board, and kind of hard to reach.
To reach it, I removed the three mounting screws that hold the large cap to the bottom of the chassis, as shown in a pic above.
After that, you can carefully tilt the board up to get access to the pot.
The pot is shown below, it is the white thing that looks like a flower in the middle of the pic.  You can see the slot in the middle of it to use to turn it with a flat screwdriver.

Figure 5

Tools Required.
Philips screwdriver disassemble unit
Flat head screwdriver to adjust pot
DVM to measure voltage

Resistor to discharge the large cap, I used a 5-10 ohm 10 watt one from my junk drawer.


Adjustment method:
1.       Measure initial voltage, mine was 13.8VDC or something like that.
2.       Disassemble (remove cover) as described above.
3.        Discharge large cap as described above.
4.       Remove cap mounting screws so board can be tilted up as described above.
5.       Using a flat head screwdriver, turn counter clockwise to lower voltage.  I think I turned about ¾ of a turn.
6.       Put back board around it’s normal position.
7.       Turn on, and measure voltage, see how close you are.
8.       Repeat steps 3-7 until you get the voltage you want.  Since I didn’t want to measure voltage with the PS on, I went through a few iterations to get the voltage.
I wasn’t comfortable trying to adjust with power on, better to spend some extra time and be safe in my book.
9.       My final voltage was 12.2 volts.  Note that with a PC, I looked up the ATX spec, and it asked for 12 +/- 5%.  This means any voltage between 11.4 and 12.6 VDC is OK.  Since you get a little voltage drop, I set a little over 12V.

Mods:
I also made a minor mod to the Astron, since I had it opened.  At the output terminals, there was a 2000uf cap soldered across the inside of the terminals. The old cap is in right of Figure 2, marked ITT.   I looked in my cap box, and found a 1000uf elna silmic 2.  The silmic was much larger, and I went for quality over quantity and replaced the stock cap with the silmic. 
I didn’t compare, and have no idea what if any difference this made, just something I thought would be better, and was easy to try.


Results/Conclusion
I made an adapter to connect the Astron, then powered my PC with the Astron, and sound was MUCH better.  All noise was gone, and it just sounded more musical and detailed.
Easily worth the cost of the Astron.
Note that the first time I ran the Astron, I listened for a little while, and left it on for a couple hours.  When I came back, it had shut down.  I measured 0V coming out of it.
I disconnected it from the PC, turned it on and verified it put out 12V, reconnected and it worked fine.
I’ve used it for many days now, with no problems, seems to work fine, can leave it on all day if I want to.


So that’s it, pretty easy if you have a little ability and the tools required.



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