NAD Receiver Turns On then Off
I have a NAD receiver that will not stay on. It turns on for a couple of seconds and then clicks off. There is a small circuit board with a relay and a .0047 uf disc capacitor, I removed the cap and measured it with a capacitance meter and it tests .0043 uf.
The schematic indicates some voltages coming off of the power supply so I measured them with my DMM with the common lead attached to the chassis ground and the voltages read as indicated.
This is new territory for me (noob alert) so I'm looking for some guidance. 🙂
Here is a link to the hifiengine library with the service manual...
NAD 701 AM/FM Stereo Receiver Manual | HiFi Engine
.
I have a NAD receiver that will not stay on. It turns on for a couple of seconds and then clicks off. There is a small circuit board with a relay and a .0047 uf disc capacitor, I removed the cap and measured it with a capacitance meter and it tests .0043 uf.
The schematic indicates some voltages coming off of the power supply so I measured them with my DMM with the common lead attached to the chassis ground and the voltages read as indicated.
This is new territory for me (noob alert) so I'm looking for some guidance. 🙂
Here is a link to the hifiengine library with the service manual...
NAD 701 AM/FM Stereo Receiver Manual | HiFi Engine
.
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There are two sections to the power supply both sections are stabilized -IE they go through active devices to keep the supply constant .
You should have one section with 36Volts+/- and the other section 40 Volts + and 26 Volts +/- .
If you are not getting that then there is a fault in one of the power supplies , there is also a small third supply to the tuner.
Along with this is what looks like a thermistor .
Relay's are fitted at the mains input and there is a time constant (delay ) it therefore looks like a fault condition turning it off but you would need to test for those voltages first .
Thermal fuses are fitted but they are "once only " so if they blew it wouldn't turn on.
If it turns off too quick for testing then you will have to check the components in both stabilized power supplies.
You should have one section with 36Volts+/- and the other section 40 Volts + and 26 Volts +/- .
If you are not getting that then there is a fault in one of the power supplies , there is also a small third supply to the tuner.
Along with this is what looks like a thermistor .
Relay's are fitted at the mains input and there is a time constant (delay ) it therefore looks like a fault condition turning it off but you would need to test for those voltages first .
Thermal fuses are fitted but they are "once only " so if they blew it wouldn't turn on.
If it turns off too quick for testing then you will have to check the components in both stabilized power supplies.
Thank you for the quick response.
I was able to test the 36 v +/- and the 26 v +/- and they read good but quickly dropped off due to the unit powering off. I'll test the 40 volts this afternoon.
I was able to test the 36 v +/- and the 26 v +/- and they read good but quickly dropped off due to the unit powering off. I'll test the 40 volts this afternoon.
Also check the DC offset. Some protection circuits checks for DC on the output to the speakers to protect the speakers and may shut the amp off completely.
I took some readings at the speaker terminals with my DMM set to DC Volts...
Right Channel: -.36 volts when the amp is off, +.17 volts when powering on
Left Channel: -1.0 volts when the amp is off, -35.6 volts when powering on
OK, that left channel definitely looks bad. DC offset should ideally never be over (+/-)100mV. Up to (+/-)500mV I do not think it will be a big issue but -35.6V is bad and indicates that you probably have one or more output transistors on the left channel's negative phase that is a short-circuit. I would start by checking the MJ2955 transistor there.
Lucky for you the MJ2955G devices are readily available.
Lucky for you the MJ2955G devices are readily available.
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BTW, you should be able to get them locally in the UK at RS or Farnell. They are quite cheap, and while you are at it get more than one, and also some of the 2N3055 which is it's compliment, just in case you want to keep the amp in the long run.
These TO-3 package devices will soon be a thing of the past.
These TO-3 package devices will soon be a thing of the past.