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Re: blown fuse/ power supply vr550
There is a small glass fuse there as well. These units have aswitch mode power supply and the transistor that is attached to the heatsink sometimes gets shorted. This is the most likely problem. If you feel comfortable working on these and can solder, let me know and I'll provide more detailed information.
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Well you are on the right course. I suspect you think the problem is in the power supply and it's probably not. I think that what's gone is either a transistor (or more) or an IC amp - if it uses one - on the main power amp stage. Look at the devices on the heat sink. I think that's where you will find your shorted device.
The likely cause of this is a dead short in the power supply. I don't receal the specifics of this unit, but there should be a set of four(4) diodes or a single bridge rectifier in the main power supply area. Check this/these for a direct short using a diode checker. Other than that, the output transistors being shorted can cause hte same symptom. Without seeing the actual unit, I can not tell for sure. If you know how, check the output transistors as well. Report your results here.
If fuse blows off repetedly, it shows that there is a direct short inside the power supply section. You just check the mains rectifier diodes for short. iIf a rectifier stack is used desolder it out and check for short. In some sansui sets, ther is a metal oxide variste connected across the supply line inside directly. It is for line spike suppression and an overvoltage protection. If any of the above appears on the power line, the varistor will get shorted, and the fuse will go out, to prevent further damage to the circuits. Check that varistor for short. Check the resistance across main filter capacitor for any direct short. If it seems shorted, check the switching transistor for collectror to emitter leak, Resolder it out and check. These are the main reasons for blowing fuse. If the switching transistor has got leaked, you must check the entire SMPS control section, especially in case of used zener diodes, before replaceing a new transistor. OK.
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When a fuse blows suspect an overload in the power supply or power amp section.
For culprits in the power supply check the transformer, large capacitors, rectifier, voltage control semi-conductors, diodes.
In the amp section any semi-conductor on the heat-sink.
Semi-conductors - transistors and IC's are the biggest cause of fuse blowing. They become short-circuit acting like a piece of wire!
Because of this transistors and diodes are easy to spot with an Ohm Meter, as it will be like you have just touched the probes together.
Very often they have holes, or damage, or have burnt up with this kind of fault.
If it is a fault on the main amp section, check your speaker wiring as the + & - wires could have touched to cause it.
Fuse blowing is due to excessive current drain and the the first clue is the blown fuse itself. If the fuse blows with a flash a bang and splatters copper across the inside of the glass you have a hard blow caused by a dead short on the power supply, check for power switch flashing over internaly. Short circuit main filter capacitors. Shorted turns in power transformer primary winding. If fuse just separates check for short on output transistors C to E. Try isolating fault by disconnecting LT pos/neg supplies to output stage. Also check for partial shorts on speakers and speaker wires using an analog ohm meter on low ohms setting.
If the fuse blows again after replacing it, there's a shorted diode, bridge rectifier, capacitor or transistor in the power supply.
I repair these power supplies. If you need furhter assistance, write to firstname.lastname@example.org
This often indicates either a power supply problem or shorted output transistors. In either case, the unit will require replacement parts. If you have a multi-meter that can check diodes, check the rectifier diodes or bridge rectifier in the power supply first. If those are OK, next check the output transistors. If you don't have access to the meter, expect a parts cost in the $15-$25 range plus the local labor rate for the repair. This should not be a difficult repair for any shop that does audio repair.