An ART Pro MPA II Tube Microphone Amplifier arrived for repair. This two channel unit uses two 12AX7 valves.
They are made in China. The fault was blowing mains fuses.
I dismantled the unit and was very surprised to see that the power switch does NOT isolate the 230volts AC mains. It only switches on and off the low voltage to the main board !!
The transformer is connected permanently to the mains no matter whether the power switch is on or off.
This means even if the unit is “off for months” power is running through the transformer. There is a half amp fuse which will blow if the unit develops a fault. This unit had blown the fuse.
I dis-connected the transformer from the main board so that only the transformer is connected. There are three outputs two at 19.5 volts AC and one at 11.5 volts AC. You can see the toroidal mains transformer in the picture below in the top left. The red and blue wires are going directly to the mains socket.
I replaced the fuse to find that with only the transformer connected the fuse was blowing. The transformer clearly has a short circuit and needs to be replaced. I now need to find a source for this strange Chinese transformer – not easy.
Being permanently connected to the mains has probably not been helpful in ensuring its long life and probbly contributed to a short life. T
Our safety recommendation for the ART Pro MPA II Tube Microphone Amplifier are as follows :
- Never leave the unit permanently plugged into the mains and disconnect when not in use
- NEVER open one of these units with the mains connected
- NEVER NEVER use a fuse rating higher than 0.5 Amp as you risk a transformer fire!!!
Electricity should always be treated with the greatest respect