I have a DBX 100 boom box that has had a burn up on mains input stages, mains transformer primary is open circuit, it was switched to 110 volt operation and someone plugged into 250 volt mains in the UK without looking to adjust the input switch, there appears to be no other damage except 1 resistor burnt out, we don't know the value, so a service manual or schenatic diagram and layout is required to go any further with this repair.
Hope you can suggest an answer, many thanks.
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Re: DBX 100 Boombox
I don't recommend this, but the resistor you speak of is probably a small fuse. Try jumping out the fuse, then turn on unit for brief moment. If it powers up, fine. Replace jumper wire with 1 amp fuse. If not, take to prof. tech.
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The main fuse has blown, or the power transformer has a open primary, one or too audio outputs semiconductors have shorted. To narrow it down the top needs to be removed and unplugged, set digital meter to ohms and check fuses, next! the too wires going into the transformer should have a low reading, but if it reads infinity or open the primary side of the transformer is open.
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.
Suspect an overload in the power supply or audio output stage.
Power supply culprits include transformer, the big capacitors, diodes, bridge rectifier, voltage regulator etc.
Audio culprits are anything on the heat sink(s).
Look for burnt, damaged parts too, anything running very hot!
Follow the circuit from the fuse see where it takes you too start.
Chek transfomer with a multi meter set on continuity,check primary input across wires AMPLIFIER MUST NOT BE PLUGGED INTO WALL SOCKET...check outputs low readings in ohms are good no readings mean open transformer(buggered
I suspect outpuit stage short as this probably buggered relay in first place
I have to agree with you that that component is a thermal fuse. In my country, they are readily available in most electronics/hobbyist parts store. These components are also used for small motors and therefore would also be available in some (but not all) kitchen appliances parts stores. A temporary workaround is to install an ordinary fast blow fuse.
Just a start, do postback how things turned up or should you need additional information. Good luck and Thank you for using FixYa.
The amp sounds like it has a primary power supply failure and/or may be in protection mode. First, I would check for any blown fuses, replace any you fint with exactly the same type and rating. I can advise you here if you are not sure. If the unit blows a fuse after replace any failed ones, then there is a short circuit that you will have to trace out in the unit. DONT KEEP REPLACEING A BLOWING FUSE You generally are just doing more damage to the amp.
If all the fuses are OK and you are savvy with electronics and soldering, you could open it right inspect for these problems., and you have a multimeter, do some powersupply voltage checks, also check for that the main transformer is ok. Resolder any dry joints you find.
It may also (and or) have a problem in the output stages. Running it hard may have produced some dry solder joints, or blown a transistor in the output array. Happy to advise you further, just ask :)