Dead A2000 power supply

[Castellen]

The fuse in the 230V model is normally 5A, not sure about the 110V model.

But the fact it's gone suggests that it's blown and someone has removed it.  Possibly a short circuit in the main bridge rectifier or a shorted chopper transistor.

Switchmode power supplies are hardly rocket science to repair, providing you understand how they work.  But be wary of the high voltages inside them when they're operating!  Even after you've switched off the supply, the main DC bus capacitors will remain charged to over 300V for a few minutes.

I've written some notes on A3000 power supply repair here, under the "A3000 power supply common faults" heading.  The same theory applies to A2000 supplies.  And depending on what model of Liteon supply you have, I might have a schematic for it somewhere if that helps.

A good technique for working on switchmode power supplies is to connect a 100W incandescent light bulb in series with the mains input to the power supply.  If there is a short circuit, the lamp will simply glow full brightness.

Otherwise there should be a brief flash of light from the lamp when the supply starts as the main bus capacitors charge, then the lamp should glow very dimly as only the fan runs.  Obviously the power supply will not run any loads in this state, but the series lamp makes power supply repair so much easier when you're dealing with potential short circuit problems.

[Castellen]

@DoogUK
Some of the A2000 power supplies have an internal link which selects the supply voltage, though it depends on the model of the supply.  I recall the Pkihong PSM-2000 has this option, where the LiteOn PA-4201-5B doesn't??

[Castellen]

It's not stated if the power supply was measured under load or not.  It needs to be measured under load if you want to test load regulation.

The +5V outputs are only +3.5% which is fine considering the supply is probably rated at +/-5%.  This supply is well within the specification for both TTL and CMOS logic, how can this possibly cause damage??

Granted that the -5V supply is outside normal limits.  That may or may not indicate a problem, depending if this was measured under load or not.  As I recall, -5V is routed to a rarely used pin on the serial port.  It's not required to run any of the computer logic, which runs entirely from +5V.

Given the measured results, the power supply should not be the cause of the computer not booting.  Of course this needs to be measured under load to have any relevance.


@orb85750
Non-booting problems may be caused by the failure of a single IC anywhere on the board, a single bad connection in an IC socket, a single PCB track failure (corrosion), etc.  So while the non-booting symptoms are the same, there can be many different causes for it.


The best approach to booting problems is to check for visual problems first; physical damage you can see.  For example, if there has been battery corrosion, check PCB tracks and vias are not open circuit.  Check socket contacts for corrosion and replace accordingly.

Secondly, most people are limited to swapping out socketed devices for known good parts.  It can work well so long as ICs don't get damaged in the process.  I've seen many instances where people try this and end up causing more damage due to putting ICs in backwards, bent/broken pins.

Lastly, you can go a bit further with an oscilloscope or logic analyser.  Checking bus activity, clock sources, reset lines, etc.  It's time consuming and requires equipment and skill that many people won't have.