repair problem

[ulot]

Hi i've been away from the forum for a while and really missed everyone.

I have a subwoffer that suddenly stoped working after a few weeks: doesn't come on at all.
I tested it and voltage enters the transformer but the meter set to 200v AC maximum doesn't detect any voltage coming out of the transformer. it reads on the xformer that 8v and 12v AC is given out but i can't detect it from the meter. Is the 200V AC max. to high to read 12V AC? or is the transformer bad (it gives no physical sign of damage)
thanks

 

[zevon8]

Many consumer electronic devices use what are known as "Class 2" transformers that have built-in overload protection. In a Class-2 transformer, one of the windings will have a small fuseable link or thermal cut-out as part of the winding, often hidden under the insulating tape on the windings. It is usually connected near the end of the winding, and sometimes can be seen as a "bump" under the tape. Sometimes it is repairable. There is usually no outward sign of this protection having done its job, it is supposed to prevent an overheating condition, and possible fire from the transformer.

If there is no AC present on the output of the transformer, this protection could have opened circuit. There may have been an overload in the power supply causing this. You will need to check the power supply to see if it is still OK, before attempting to fix ( if possible ) the transformer.

The meter should be able to display the lower voltage when set to that range.

 

[Nigel Goodwin]

Yes, the transformer probably has a thermal fuse fitted internally, these are occasionally replaceable, but not very often!. If the unit has really been driven hard it may just have failed through getting too hot, the transformer is probably too low a rating for the amplifier (a common cost saving technique!) - however, if it's not been driven hard the transformer may have sorted turns - often you can connect directly to the primary winding, bypassing the fuse, BUT ONLY FOR TEMPORARY TEST PURPOSES!.

 

[Laplink]

As with any equipment, do as much testing with the equipment disconnected first!
Switch your meter to Ohms, test across the primary windings, is there a reading? With the equipment switch in the on position, test across the mains plug pins, is there a reading? Test across the secondary of the transformer, is the reading low high or open? Swap your meter leads over.

Open = fault
near zero = fault
reading = may be O.K.

Yes, you can measure for the presence of 12V on the 200Vrange, but it may not be accurate.