My son acquired one of these but one channel was dud with terrible motorboating and noises, the other worked but with masses of mains hum.
Initially I assumed the TDA1514's were a goner, but there was no sign of burning, and the +-27V power rails were fine. I fed an audio signal direct into the input jumper of the power board and they sounded fine.
So I turned to the tone/input stage. The 15Vsupply rails, regulated by just zener diodes, were fine. I changed the TL072CN IC in the tone board; no better. Then I changed the same dual op amp in the input board and then it worked, but still with hum. I do not know what had caused the failure; could have been static damage from the input?
The hum was persistent. I changed all the electrolytics and the bridge rectifier; still the same. I wondered about earth loops. The circuit earth, which comes from the centre tap of the toroidal transformer is not actually connected to the chassis. When I connected them together the hum was significantly reduced, but not gone. This amp seems a bit bizarre in that it has an exposed metal case, but it only has a two core mains lead. The sticker on the back labels it as Class II (double insulated) electrical appliance. So lastly I replaced the mains lead with a three core and earthed the chassis, and hey presto the hum went away.
So effectively I have changed it to a Class 1 appliance. Not sure the rights and wrongs of all this, but it has restored the amp and seems safer to me to have a working earth with fuse protection.
Initially I assumed the TDA1514's were a goner, but there was no sign of burning, and the +-27V power rails were fine. I fed an audio signal direct into the input jumper of the power board and they sounded fine.
So I turned to the tone/input stage. The 15Vsupply rails, regulated by just zener diodes, were fine. I changed the TL072CN IC in the tone board; no better. Then I changed the same dual op amp in the input board and then it worked, but still with hum. I do not know what had caused the failure; could have been static damage from the input?
The hum was persistent. I changed all the electrolytics and the bridge rectifier; still the same. I wondered about earth loops. The circuit earth, which comes from the centre tap of the toroidal transformer is not actually connected to the chassis. When I connected them together the hum was significantly reduced, but not gone. This amp seems a bit bizarre in that it has an exposed metal case, but it only has a two core mains lead. The sticker on the back labels it as Class II (double insulated) electrical appliance. So lastly I replaced the mains lead with a three core and earthed the chassis, and hey presto the hum went away.
So effectively I have changed it to a Class 1 appliance. Not sure the rights and wrongs of all this, but it has restored the amp and seems safer to me to have a working earth with fuse protection.
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Hi Charles I have the same amp and the same mains hum. Thanks for posting this - I'll try your suggestion as it's a cheap and easy thing to do. It does seem odd that the chassis was not earthed on these. So essentially you just tied the centre tap of the transformer to the chassis and grounded the chassis by rewiring the mains cable as a 3 core?
They're not a great amplifier but right now it's all I have while I wait for a new amplifier to be delivered...
They're not a great amplifier but right now it's all I have while I wait for a new amplifier to be delivered...