Help repair NAD L76 audio receiver - fan not starting

[Mikaelin]

Hello,

This NAD L76 receiver is overheating and restarting (temperature protect is triggering) after about 6 hours of constant work. The audio output is OK. Nobody repaired it before.
It's overheating because the fan is never turned on. Normally, I suppose, when the temperature is OK, the fan motor is not spinning, but if the temperature is rising, it should start the fan for cooling.
Firstly I checked why the fan motor is never turning on. I ckecked the fan motor with a multimeter and with a battery - the fan is good, it's spinning when I apply voltage. Input voltages +5V and +15V are present.
Then I checked who is controlling the fan motor. TH71 PTC thermistor and two transistors, Q755 and Q751. TH71 is showing 278 ohm in normal state at 20C room temperature. When I apply heat, the resistance is increasing to about 380 ohm. I don't know if this component is OK (can't find the datasheet). But even without this thermistor, I can't start the motor. The transistor Q755 don't want to open. I can't figure out why the voltage is dropping and not reaching the gate. All resistance values are OK.
Somebody can help? Maybe I'm doing something wrong...

[David_AVD]

It looks like the fan comes on (at a lower speed) at a certain temperature (via Q753), then speeds up if the temperature reaches a higher value (via Q751).

Have you measured the voltage (with respect to 0V) at the bases of those two transistors when you think it should be hot enough for the fan to run?

[wasedadoc]

1. Are you measuring the thermistor when still connected or when at least one end is disconnected?  380 Ohms is too low to turn on Q755.

2.  If you disconnect the thermistor the fan should certainly run.  If not you need to find out why and fix before worrying about the thermistor.

[Mikaelin]

It looks like the fan comes on (at a lower speed) at a certain temperature (via Q753), then speeds up if the temperature reaches a higher value (via Q751).

Have you measured the voltage (with respect to 0V) at the bases of those two transistors when you think it should be hot enough for the fan to run?

Good idea. I have measured multiple points with reference to ground. Here are the results in the picture below.
The base of Q753 is 0.26V. But Q751 is closed because it is driven by Q755, which I can't open. I tried to simulate the opening of Q755 by connecting a jumper from R754 to the ground - the fan is spinning from 5V.

It looks like the 15v line is flowing through the transistor Q758 which has 2 internal resistances. If I desolder this transistor, will the voltage go up to Q755 ?

[Mikaelin]

1. Are you measuring the thermistor when still connected or when at least one end is disconnected?  380 Ohms is too low to turn on Q755.

2.  If you disconnect the thermistor the fan should certainly run.  If not you need to find out why and fix before worrying about the thermistor.

1. I measuring when one end is disconnected. But as David_AVD said, the fan should turn on at low speed even at some temperature. So it's not the thermistor problem.

2. Of course I tried to disconnect the thermistor - the motor is not spinning. Yesterday I have desoldered the thermistor and soldered a potentiometer to regulate different resistance levels, to simulate a high or low temperature from 278 ohm up to 2.2k - this test don't help. I soldered back the thermistor.

[wasedadoc]

You have no voltage coming in on DC_P (pin 4 of the connector).  Post the part of the schematic where that is coming from.  Better still, post a link to the complete schematic.

[fzabkar]

If you remove the thermistor, what is the voltage at the junction of R758 and R759? AISI, all 4 transistors should be turned on. The collectors of Q753 and Q755 should be at 0V.

[Mikaelin]

You have no voltage coming in on DC_P (pin 4 of the connector).  Post the part of the schematic where that is coming from.  Better still, post a link to the complete schematic.

I added a pdf service manual with all schematics in the first post
I think DC_P is "DC protect" over current protection, but not sure, and is coming from all PCBs, front amp, surround, speaker PCB

[Mikaelin]

If you remove the thermistor, what is the voltage at the junction of R758 and R759? AISI, all 4 transistors should be turned on. The collectors of Q753 and Q755 should be at 0V.

If I remove the thermistor, I have 10.30V at the junction of R758 and R759. But at the base of Q755 I have 0.6V. It's still closed. The fan is not spinning.

[wasedadoc]

Please avoid using the words open and closed.  Meanings are not clear.

1.  An open door is like a closed switch or closed relay contact.  Lets people or current through.

2.  A closed door is like an open switch or open relay contact or open circuit. Does not let through.

Transistors being on or off is clear to everyone.

[fzabkar]

If you remove the thermistor, what is the voltage at the junction of R758 and R759? AISI, all 4 transistors should be turned on. The collectors of Q753 and Q755 should be at 0V.

If I remove the thermistor, I have 10.30V at the junction of R758 and R759. But at the base of Q755 I have 0.6V. It's still closed. The fan is not spinning.

So, what is the voltage at the collector of Q755 (R754)?

What are the voltages on R755 and the 3 pins of Q751?

To me, it feels like the +5V supply may be missing. :-?

[wasedadoc]

If you remove the thermistor, what is the voltage at the junction of R758 and R759? AISI, all 4 transistors should be turned on. The collectors of Q753 and Q755 should be at 0V.

If I remove the thermistor, I have 10.30V at the junction of R758 and R759. But at the base of Q755 I have 0.6V. It's still closed. The fan is not spinning.

Apologies. I missed the link to the service manual in the opening post.  DC_P is not the problem

With the thermistor removed measure the voltage across R755.  If it is not zero, check capacitor C751 is not shorted.

[Mikaelin]

At the junction of D751 and R751 I have 9.75V. The fan is spinning now. 5V on the motor. Wow, that's crazy. I tested before and was not the same result.
Now the thermistor is desoldered and the fan it's spinning all the time. This is a progress
Thank you

Now, I need to figure out why with the thermistor in place it's not working (was not working, when it was hot, or when I heat it up with hot air)

[fzabkar]

Measure the resistance of the thermistor at room temperature and then again after heating it with a hair dryer. You don't need to resolder it to do this.

I suspect that there is either an intermittent 5V connection or an intermittent short in the ceramic bypass capacitor (C751).

[Mikaelin]

I tested one more time the resistance of the thermistor at room themperature - is 277 ohm. When I heat it up, the maximum resistance I saw was 800 ohm. In the manual, TH71 description is KRTP43T7D330B. Can't find any information about.

Can I solder a potentiometer in place of TH71 and find the right value of resistance, when the motor start and stop ?

Logically, if the motor is designed to turn on all the time, then we don't need a thermistor, just connect it directly. But here we have a thermistor, and I think it should start the motor only when the heat sink is very hot.

Yesterday I have desoldered C751 for testing. No short. 100nf capacity

[Mikaelin]

With a potentiometer in place of TH71 I found the threshold when the motor is stopping. It is 684 ohm. Lower 684 = stop; greater = start.
  But 684 ohm is between 277 and 800. I soldered the thermistor back. At room temperature the motor is stopped. OK. I applied heat and ... now it's turning on. It's incredible. I can't explain why now all works as expected. No component was changed. A mistery.
I will let the amp powered on all night to see if the cooling will trigger

[Mikaelin]

After 12 hours of constant work, the receiver is still working normally. No reboot, no temperature or current protection. All seems fine, for the moment.

I checked with IR camera. The heatsink at the thermistor TH71 is only 35°C. The hottest element on the system is R908, 101°C. I don't know if this is normal. I attached here some voltage measurements around R908

[David_AVD]

NAD stuff is notorious for cracked solder joints, so worth going over anything that looks suspect.