Mustek R100A DVD Recorder power supply voltage high

[Guest]

Hello everyone,

Recently, I cheaply acquired a rebadged Mustek R100A DVD Recorder at auction, well aware of the risk. After lubricating a seized component inside the drive, the unit began working reliably (which is rare, search Google for this model to read horror stories) for a few days. Soon though, the unit began locking up, started taking longer to "boot up", would refuse to open the tray and fail to properly burn programs. I traced the problem to high voltages coming from the power supply which eventually dropped after it (over?)heated up. Once the voltages dropped a little, the unit began to respond again. The power supply is a Fairway VA3523 - A Rev 1.0. Interestingly, this very forum has a thread "-27volts from a PC power supply" about the subject, and a search on Google for "Fairway VA3523" turns up more.
The -27v rail supplies 31.6v when cold and 31v when hot, -12v is 15.3v cold and 14.8v hot, +5v is 4.1v cold and 4.4v hot and the 3.3v rail stays at 3.1. The unit uses a single opto-isolator for the feedback. Most of the large filter capacitors on the secondary side show bulging and run quite hot. Normally I would simply replace these, but I am still not convinced their damaged condition is the cause of the poor voltage regulation, rather the effect. In other words, I suspect they may have been damaged by another problem. I am convinced; the high failure rate of these DVD recorders is because of the power supply.
This brings me to my question: Do you know of a certain weakness or the cause of the power supply failures so I can correct them directly? I am a student and just don't have the time anymore for normal troubleshooting. Normally I would replace the opto-isolator and capacitors then continue with further tests, but I just can't afford to waste time and energy on false leads right now. Also, I am tempted to replace the capacitors with ones of similar (but not exact) capacitance from some PC PSUs I have lying around as I need to travel for 4-5 hours to buy new replacements. What is your experience with this? The capacitance is around 20% either way from the original.

Thank you in advance for any advice.

[gary]

CHANGE THE CAPACITORS ON THE +5V RAIL, DONT KNOW THE UNIT BUT THIS RAIL IS PROBABLY USED AS THE FEEDBACK REFERENCE, THEREFORE IF IT FALLS DUE TO POOR SMOOTHING THE OTHER RAILS WILL RISE. USE 105 DEGREE C TO IMPROVE RELIABILITY OF WHAT IS PROBABLY A BUDGET DESIGN.

[Guest]

For the record, to help anyone out in the future:

I replaced the 3 main (and largest) filter capacitors which sit between the two heatsinks. The 1200uf unit was replaced with a 2200uf cap. All sourced from faulty PC power supplies. They were tested first by applying a voltage, then connecting to a low wattage 12v light. The caps I took out had almost no capacitance but were 105 degree units. These caps are overstressed due to being near the heatsinks, having virtually no ventilation and appear to be under great stress electrically. This cost-saving screwup marred an otherwise decent DVD recorder model. If you do this repair for yourself, search Google for words like: switchmode power supply repair safety. Make sure it is unplugged and the capacitors on the primary side are discharged. This repair was quick and dirty but it suited me just fine as it took less than an hour.

[DougWoodrow]

[vtech]

As a rule, you can not have too much ripple floating around in any SMPS circuit as it will eventually self destruct. Unfortunately, it is rather common in most consumer designs especially from far east.

There are no headrooms allowed in chosing capacitor values & cutting corners
are more than apparent to cut costs.