The AD-500W Power Supply
This is the Power Supply PCB. Note the yellow line. Any parts above that line are at mains voltage potential and TOUCHING THEM WITH THE POWER PLUG IN THE WALL MAY KILL YOU so beware!
The power supply is a fairly conventional switchmode design based around the UC3842 controller chip. Most regulation of the Low Voltage side of the power supply is actually done on the High Voltage side by this controller chip. There is a regulation feedback loop by means of an opto-coupler and TL431 IC monitoring the +5V rail and feeding back regulation information to the UC3842 controller chip. If the +5V rail creeps up the UC3842 is instructed to "throttle back" a little and if the +5V drops down the UC3842 is instructed to "boost" a little. All this happens within a split second.
Most of the problems with the power supply are due to high-voltage surges through the mains house supply either by other appliances, or lightning. The main casualties are the four diodes at the top (D1-4), the fuse and the main chopper FET (Q1). Luckily these parts can be replaced easily with common variety substitutes.
If you would like to get a copy of the power supply circuit you can download mine which I drew up for the 120V American model. These circuits are a guide only and part values may differ for other countries.
Click HERE to get the circuits
COMMON PROBLEMS
If you have any contributions or questions with your power supply problems please email me and I shall include your findings in the list.
CAUTION
Before you proceed any further, please remember that working on power supplies can be dangerous and the voltages present CAN KILL YOU. So unless you have a sound knowledge of repairing switchmode power supplies please leave it to an expert. I will not be held responsible for your actions.
By far the most common problem is that you have recently had a close lightning strike and/or power surge and now your player is dead. The reason for this is that there are many parts on the PSU board that are directly connected to the mains supply and are only rated at maybe 50 volts higher than their usual operating condition. These parts are above that yellow line on the picture at the top of this page. When you get a power surge or spike these parts exceed their maximum working voltage and therefore they fail.
You will find that more often than not you have a blown fuse. Vary rare, but sometimes just replacing this fuse will fix the problem. BUT BE CAREFUL! Do not just stick any old fuse in, or bridge the fuse out with a piece of wire. Doing this is asking for trouble and will cause more serious problems like the board catching on fire and possibly burning your house down. The fuse plays an important safety function and should be replaced with EXACTLY the same type as the one which has blown.
IF THERE IS A PROBLEM AND THE FUSE IS BLOWN:
More often than not the fuse has blown because of something else wrong. Your first replacement fuse will probably blow straight away which means you have a problem elsewhere on the psu board. A good trick while testing is NOT to use a fuse at all, but instead place a 60W light globe across where the fuse should be. If there is anything wrong the globe lights up and absorbs the energy. I have actually had a power supply running normally and playing a DVD with a 60W globe connected and the globe did not even start to glow. The most common problem if the fuse blows instantly or the globe lights up is a shorted component and you can easily check this by placing an ohms meter across C5 (the large filter capacitor). If you read a low ohms (or a dead short) then check D1 to D4, D5, D6 and Q1. These are the most likely causes for a fuse to keep blowing. Other parts which may cause this (but less likely) are C5, C9, C33 and Q3.
THE MOST COMMON FAULT WITH THE 500W:
In the experience I have had with both my own 500W and comments from many other people there is a common problem with the 500W power supply in that one of the diodes at location D1 to D4 go short circuit for no apparent reason and the fuse blows. In this case the solution is easy. Go to your local electronics store and purchase four new diodes and replace the existing diodes in locations D1 to D4. Don't even worry about testing them as replacement standard silicon rectifier diodes are very cheap. The ones used in the 500W are type 1N4007 which are rated at 1 Amp/1000V PIV, however if these are not readily available just about any other standard silicon rectifier type diode will work just as well. The diodes in question are shown in this photo in the pink box:
IF THERE IS A PROBLEM BUT THE FUSE DOES NOT BLOW:
If however a new fuse does not blow but you still have no sign of life on any of the low voltage connectors on the psu board then you have something which is open circuit. If all the low voltage outputs are completely dead then the fault is most likely on the high voltage side of the transformer. Common parts to go open circuit are R10, R11, R12, R9, D6 and Q1. Just a word of caution here: When checking high-ohms resistors like R10 and R12 it is often best to de-solder one leg clear of the board so you are checking that part only. Many times you will get a false reading if the part is left in circuit from another part "dragging down" the measurement. In my experience, high-ohms resistors like R10 and R12 in high-voltage applications do not mix and often go open circuit for no reason. You may also have U1 shutting down the power supply because it is sensing a short circuit on the Low Voltage side. This may be caused by a shorted part in any of the voltage rails. More on this below.
On the Low-Voltage side of faults, you can have many different problems which I will talk about separately. For starters, you may have all but one voltage rail. This is where you must "sectionalise" the low-voltage circuit and work out what parts are related to particular voltage sections. OK, for example, you have all voltages but -24V. Working back from CN7 you can trace that the only parts related to -24V rail are ZD2, C25, R27, C24 and D11. As you can see only approx 10% of the whole low voltage parts are related to -24V.
Another fault may be the power supply is "hunting". This is where the voltages slide up and down and are not constant. This is a problem with the feedback circuit so check all parts around the opto-coupler (IC2) on both the high voltage and low voltage side of the power supply.
You may strike a situation where all the voltage rails are low. Ruling out a regulation fault on the High Voltage side of the psu, we can assume that there must be a partial short somewhere on the Low Voltage side. This may take some finding, but you can be assisted by feeling if any parts are getting very hot. This is a dead giveaway that something is wrong.
It may also have something to do with the feedback loop not sending back proper regulating information so also check R13, R14, R15, R16, R17, C15, and IC3.
Generally though a shorted part on the Low Voltage side will cause the UC3842 chip on the High Voltage side to sense a high-current drag and shut down the power supply. The only way to find this is to start checking all voltage rails with an ohm meter, looking for a low ohms (or short circuit). check between each voltage rail and GND in turn at the output connectors. If you find one rail which is shorted then work backwards towards T1 (the transformer) until you find the offending part.
That is all for now. If you have any further faults you wish me to discuss here please feel free to email me with the details.
