These are a few things that will help you when have to get a piece of electronic equipment repaired.
Find a repair center that is factory authorized to service the equipment that needs repairing. Call the manufacturer and they will be able to tell you the closest service center.
If you can't find an authorized service center or don't want to ship it off, call the Better Business Bureau in your area to see if the perspective shops have any unresolved complaints. If they have a lot of unresolved complaints, find another shop.
Ask for an estimate. If they charge for estimates, it is usually applied to the repair charge when the estimate is approved. Remember that they will have to pay a technician to look at your equipment. I don't know any techs that work for free.
To speed up service, you can tell them that they can go ahead and repair the unit if the total charge is less than $50, 60, or any amount that you are willing to spend. This will allow them to start the repair work without having to get in touch with you. Now there are some repair centers that will charge you the maximum that you tell them that you are willing to spend but I believe that most are honest and will charge you a fair price.
In Shop Testing After Repair:
When I repair a piece of equipment and the customer (the same customer that previously murdered the unit) reinstalls it without checking any of the wiring or speakers or anything else and the piece of equipment is again damaged, it's extremely annoying when they return with the unit claiming that I didn't repair it properly. Since every piece is checked a minimum of 3 times (after replacing parts, when mounted back in case/heat sink and after all covers have been replaced - all amps are tested for a minimum of 10 minutes at full power) I know that the unit was working when it left. Most techs thoroughly test equipment so they don't have any problems. If you do something wrong when reinstalling the equipment and damage the unit again, tell the technician what happened (take responsibility). Good service centers can be hard to find. If you damage the repaired unit again when reinstalling it and try to blame it on the service center, you may get it repaired for free the second time but the next time you need something serviced, you'll likely be told to go somewhere else. If you go in and tell them what you did wrong, they will likely appreciate your honesty and may repair it again for a minimum charge (maybe just the cost of parts).
If at all possible, allow a qualified technician to reinstall the equipment. If that's not possible follow the following guidelines.
If the internal amplifier of a head unit was damaged, you need to check to make sure that none of the speaker wires are shorted to ground. If the speaker wires are allowed to contact ground or a source of power (12 volts), the internal amplifier WILL be damaged again. If you don't check the speakers/wiring and the unit is again damaged, it is NOT the fault of the repair tech (it's your fault).
When reinstalling your amplifier, remove the fuse from the power line (removing the fuses from the amp itself will do little good†). When all of the connections have been made, reinstall a fuse rated at approximately 1/2 of what it needs to be for full power. Virtually all amplifiers will play at low to moderate volume when fused in this way. Listen for any low level distortion (which could be indicative of a defective speaker or a shorted speaker wire). If it sounds fine and all of the speakers are playing, then you can reinstall the proper fuse for full power operation. If the sound is even slightly distorted or the fuse blows at low volume, double and even triple check everything. If one channel is distorted, remove the speakers from that channel and try a known good speaker on it to confirm whether or not the amp is the culprit.
†If you don't remove the main power fuse and you allow the screwdriver/allen wrench to contact the metal frame of the amplifier, it will very possibly damage the amp again. Since the technician will immediately know what you did (there's only one way to cause this type of damage), the warranty will be void (warranties do not cover stupidity or laziness).
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