TA7317 protect circuit in NAD 319

[cviller]

Hi

I think I'm going crazy. I cannot figure out how I can make my amp get out of protection mode. I have attached the schematics for the psu. I hope someone will be able to help me (my girlfriend would be happy too, because a NAD319 can take up a lot of space, when disassembled)

To my understanding, the TA7317 chip drags PIN 6 to ground (PIN 4), when PIN 1 is less than 0.7V, PIN 2-3 are 0V and PIN 8 are >~ 2*0.7. Is that correct?

In the schematics NAD states that the voltages around the TA7317 should be:
P1: 0.3
P2-4: 0
P5: -0.77
P6: 0 (relay turned on)
P8: 1.3
P9: 3.13

When I measure the voltages in my amp I get:
P1: 0.120
P2-4: ~0 (0.x mV)
P5: -0.77
P6: 64 (relay turned off)
P8: 50mV
P9: 3.13

I have tried to replace most of the capacitors and I have not been able to find shorts or opens or anything like that. When I remove the cap (C224) on PIN8 the relay starts oscillating (switching on and off), but with a frequency greater than 1Hz. If I remove cap C223 on PIN 7, the frequency gets even higher. If I re-insert C224, the oscillations stops, but it stays in protect mode.
I have tried to replace the TA7317, but that didn't help.

Does anyone have an idea on how to fix this? Or just some hints on what I should investigate further.

Not sure if the attach worked:
Nad 319 psu schematics

/Christian

[Leolabs]

I think you need the datasheet of TA7317 before going further.

[cviller]

I have the datasheet already.

TA7317P datasheet

I have tried to simulate it in spice, but it worked nicely and I couldn't make it oscillate. I'm beginning to think that the low voltage on PIN 8 is not the problem itself, but rather a consequence of another failure in the circuit.

[Eva]

The circuit is being externally muted because pin 8 of the IC is not being pulled up. Check R233 (open) and C224 (shorted or leaky).

You may try to inject a very small current into pin 8 by connecting a very high value resistor (500k to 1 Meg) to one of the positive 60V rails. If C224 is not shorted, this should cause the relay to turn on (you may cause damage if the current is too high).

[cviller]

Thanks eva! It wasn't the problem, but just some fresh thinking made me approach the problem differently.
I think I'm quite close to the solution now. I have discovered that it's the voltage on p1 that mutes the system. I have tried to short it to ground (through my meter) several times, but then I suddenly realized that the time constant of the muting circuit is much larger than just a simple touch, so I was patient and after 2-3 seconds, the amp went out of protect. Now I just have to figure out why pin 1 is not tied to ground.

[cviller]

I finally found the broken component. It was a simple 1N4148 diode (D211) that was shorted. I'm not sure what I will do with that diode, but it'll have to suffer somehow. Maybe I should smash it with a big hammer. >

I have spend so much time on this amp that I could have made a complete diy power amp. I even made a new pcb for the psu - no silk screen, but I bet the quality is better than the original.

[Tekko]

Desolder it and throw it in the trashcan

[cviller]

Now that its working and disasembled, I might as well adjust the idle current. The output is four pairs of 2SC3519/2SA1386 connected through 0.22 Ohm resistors to the speaker. The two measure pins are connected between the transistor and resistor on one of the pairs.

I can adjust the voltage from 0V to approx 10mV. What should it be?

I have the schematics, but it doesn't say what it should be adjusted to. Is it a tradeof between power consumption and crossover distortion?

[Tekko]

Set it to whatever sounds best.

[twofake]

Hi Cviller,
Here are my TA 7317 voltages:
1. 0.3
2. 0
3. 0
4. 0
5 -0.7
6. 0
7. 0
8. 1.3
9. 3

Around my D211 diode I am reading around -29V rather than the supposed around 60V.

Can you also answer the following:
1.What does the relay of teh protection circuit actually control? (does it control teh speaker outputs or does it prevent an input into the main power amp boards?)
2.From th eoutput of Pin 6 of TA7317, where does that signal go (it goes to J107 but what next?)
3.Where does the J108 pin 2 connect to? It is connected to the Thermal Resistor (?) THR1?