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Astro City PSU not fully outputting 5V AND 12V (400-5621)

bgwelistyl

bgwelistyl

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Hi everyone, I'm having issues with my Astro City power supply, its not outputting the necessary 5V to run anything.

It seems to max out at 3V. and adjusting the 5v potentiometer doesnt really do anything.

I recapped and checked every solder joint and everything seems to be in order. No cold solder joints or any traces.

The weird part is that when I physically press down on the potentiometer area, the PSU/game seems to briefly light up and work to its necessary voltage, but once I let go it stops working.


I was wondering what area would you guys recommend I check out?



image0.jpeg
 
Could be the trim pot itself, could be a bad solder joint, coul be lots of stuff. Your probably going to have to narrow it down with a multimeter
 
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bgwelistyl said:
The weird part is that when I physically press down on the potentiometer area, the PSU/game seems to briefly light up and work to its necessary voltage, but once I let go it stops working.
Click to expand...

Sounds to me like the solder joints need reflowing.
 
Whoops.. I completely didn't read that part.

Yeah I agree, it does sound like the pot either needs reflowing or replacing.
 
I did change the pot (1k) but that didnt change anything. I'm pretty careful in checking my traces, cant really see any cold joints, Im guessing it might be a regulator
 
I've attached a schematic I've found on the web. Assuming it is correct.

If the 3v DC measured is on the 5v output, this would indicate that the power supply is oscillating and that the feedback circuit through the optocoupler IC1 is working to an extent.

With the power turned off I would start measuring the resistors R19 and R20. This form part of a voltage divider network for the 5v regulation.

I would also confirm that slowly turning the Variable Resistor VR1 is allows for a range of resistance between pins 1 and 3 of VR1. 0ohms to 1kOhms.

I would also check R21 (47ohms). I would assume that this resistor provides some minimum load to allow for voltage regulation.

Be careful as having the power supply turned on and probing around with a multi meter you can short out traces and make the supply unrepairable.
 

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So I sent off my PSU to get it repaired via Sharp Image. They replaced one of the transistors mounted on the heat sink (as well as checking solder reflow and the usual), and apparently when they tested, it was outputting 5V and 12V

Unfortunately, when I got it back today and replugged it into my Astro, I'm still getting the same problem (still had to physically press down on the potentiometer to get a 5V AND 12V voltage).

Actually I just noticed when I was switching between readings on my jamma voltmeter that I wasn't getting a full 12V as well.

Would you guys suspect at this point the issue is still the power supply? I mean once the PSU gets to the 5V/12V range (physically pressed), everything boots up the way it should.
 
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I am thinking if the PSU has been checked and you are still having the same issue check the wires going in to the PSU. They are AC but it may be a short or bad connection on the PSU input voltage. This is likely outside the actual PSU box (or maybe on the connector).
 
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