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Protection circuits respond to overloads. Unless the protection circuit is faulty there is probably something wrong with the power amp parts located on the heat sink. It would be a good idea to have the top cover off (on power) and listen for that noise to try and pin it down what is making it.
Why was it on for over a year :D
Has anyone replaced the fuse? In the service manual I found it has to be only 2 Amps!
Okay let's solve the problem. The power transformer was overloaded which caused the broken winding. It is still overloaded what can be heard by the hum sound.
First thought was to check main capacitors or better just replace them! If the hum gets louder, they might also become warm.
Check voltages after bridge rectifier and smoothing circuit. They should be +/-42 V (TP3, TP4) and +/- 16 V (TP5, TP6)
Let me know which of them is faulty
Hello Check the connection to speaker with this amp, for any short or any other fault. disconnect the outputs, ie, speakers from the amp, and switch it On, and see the protect display comes or not. If it comes, the fault is internal and the set need service. Your M3 dual Mono integrated amp, is of high output power and its outputs are protected against any overload or short circuit. If the system detect any among this condition specified above, it will show protect mode display. Never load this amp, as protection display is there, as it may damage the speaker or its internal power amplifier section circuits, may cause more money to get it repaired. OK.
The red light is a power protection circuit designed to shut the amplifier down due to an overload or wiring problem. If the amp was playing then shut off, my guess is a problem with the speakers or the speaker wiring. Some amplifiers are only stable to a certain load. For example, most car audio speakers are 4 ohms which is the resistance placed upon your amplifier to produce a certain volume of sound. If you bridge two 4 ohm speakers together in parallel, you cut the resistance in half, thus increasing the amount of power (and volume) to your speakers. This also creates more heat and could, in effect, shut your amplifier down. I would find out how the speakers are wired to determine the resistance of the load your amplifier is pushing.
You need to determine if the power is getting to the audio amp section. For instance with the volume full up is it humming etc? As a 'rule' it's unlikely that both left and right channels will fail at the same time (unless you have done something silly with the amp) you are looking for something that is common to both channels. Which is why the power section is first suspect. The exception to the rule (above) is where amps use say an IC power amp for both channels. You will find this IC on the big heat sink. Check whatever is on that anyway for damage or burning. The only other area could be the pre-amp part. Again this could be a single IC controling both channels, so it's failure could cause a loss of sound. By the way if you can hear music/sound coming through when you put it up to full volume the pre-amp will be generaly good.
I had the same problem on my VTX65. I noticed it would oscillate with different effects so I removed the effects module. Problem solved. Now it's dead quiet. I use a Digitech RP500 so removing the effects module was a non issue.
The amplifier may be overloaded, check to see if it can handle 4 OHM loads and 2 OHM loads. if it turns off at high volume the amp may be overloaded. if so try a 8 OHM load. if it still turns off it may be an internal problem.