How does NAD protection circuit works?

[cs.dk]

Hi,
I've got a NAD 317 bridged with a NAD 214 - When powering on, the 317 sometimes turn the protection LED off as usual. Sometimes it can stay lid for 1-10 minuttes.
I have the service manual (attached), but they don't tell much about how it works.

Can someone help me out?

NAD 317 Service Manual

[pknoe3lh]

I would look for cooled solder points in the location attached ;-)

Also TH101 and TH102 could make problems (right upper corner in the Power Amplifier schematic)

be careful with the high voltage! 

[cs.dk]

Thanks a lot.
Just corious. How do you spot those two places as being the trouble?

High voltage? I suppose the amp runs under +/- 50VDC?

[mzacharias]

Check / replace C226, 1uF at 100V. This cap, along with diode D211, provides a sharp cut-off when the amp is powered down, so there are no pops through the speaker. NAD notoriously used cheap caps, and when this one dries out the amp may take a long time to come out of protection - or not come out of protect at all.

[cs.dk]

Thanks a lot.
Really much knowledge in here

For the cap, i don't think it's worth to check, i'll order a new.. I guess one of these will fit? tme.eu 1uF 100V electrolytics

[mzacharias]

Thanks a lot.
Really much knowledge in here

For the cap, i don't think it's worth to check, i'll order a new.. I guess one of these will fit? tme.eu 1uF 100V electrolytics

Yeah - radial (pc mount) is preferred.

Be sure to observe polarity when replacing.

Post back and let us know if this fixes your problem.

[cs.dk]

Sorry, english isn't my native language.. Radial is with a leg in both ends, right?

[mzacharias]

Radial is with both leads on the same side.

[cs.dk]

Bummer

The cap don't have any special means i guess... From the link tme.eu have Nichicon UVS, APW, UPV and UVR series. But what ever i choose should not do something performance vise?

[pknoe3lh]

I was going from the protection led backwards:

Q205 => Q206 => Q207 => Q208 => IC202

I was searching for something what mache the problem description.
for my understanding there must be one component responsible for the time till the protection will be turned off.

But actually I overlooked C222
and C226

What i marked is actually the relay ... it should work without a problem otherwise you will hear no sound after the protection turned off.
C223 can also play a role.

C502 and C501 can be charge wich 230V
and 55V can be also dangerous ...

[mzacharias]

Bummer

The cap don't have any special means i guess... From the link tme.eu have Nichicon UVS, APW, UPV and UVR series. But what ever i choose should not do something performance vise?

The cap isn't very special - axial or radial will work, polarized is best, and do observe + and -  polarity. A non-polarized cap would work also.

[mzacharias]

I was going from the protection led backwards:

Q205 => Q206 => Q207 => Q208 => IC202

I was searching for something what mache the problem description.
for my understanding there must be one component responsible for the time till the protection will be turned off.

But actually I overlooked C222
and C226

What i marked is actually the relay ... it should work without a problem otherwise you will hear no sound after the protection turned off.
C223 can also play a role.

C502 and C501 can be charge wich 230V
and 55V can be also dangerous ...

Yeah - I probably should have just mentioned that other caps in the area are also suspect - not a bad idea just to replace all the smaller electrolytics in the area.

[cs.dk]

Opened up the amplifiers today, just to see how which caps were fittet, and how much room there was.

When it was open, I took a reading at the "idle current", one channel was 4,8 mV, where the specs states 20 mV +/1,5. Adjustet all pots to within specs.. Wonder whats the idle current is doing. The amplifiers seems a lot more well behaving now, but not so loud as before.
One of them, the 317 gets extremely hot vs. the other. They are the same amps, and both in bridge mode. Anything to check before i order capacitors?

[mzacharias]

If the NAD 317 is running hot something is wrong and I would limit testing duration until the cause is found and corrected. A major failure may be imminent. Perhaps the amp bias is wrong despite your readings - perhaps there is an oscillation which is unseen with your DC milliamp measurements.

[tronde]

One of them, the 317 gets extremely hot vs. the other. They are the same amps, and both in bridge mode. Anything to check before i order capacitors?

Maybe test them as normal stereo amplifiers?

Get everything sorted out before you use the bridged mode. Be careful with an oscilloscope when they are bridged. If you connect the ground clip to any of the speaker wires and you have a commond ground to a signal source you will need a lot more new parts than just capacitors...

[IanMacdonald]

I'm curious as to what you mean by 'bridged with.'

Some of the NAD amps can use both channels (of the same amp) to drive a single speaker in bridge mode, but that doesn't seem to be what you mean here. Or, is it?

[gadget73]

NAD amps seem notorious for bad solder and bad capacitors.  The one I own was full of both.  Its not a bad amp, but the construction was not very good. 

[cs.dk]

It came to my mind that the 317 is placed on top of the 214. Maybe the convection heat it up. I'll try to run it as a normal stereo amp.

I'm curious as to what you mean by 'bridged with.'

Some of the NAD amps can use both channels (of the same amp) to drive a single speaker in bridge mode, but that doesn't seem to be what you mean here. Or, is it?

They are switched to bridge-mode, making them act as a mono amp. One amp pr speaker channel.

I have ordered some caps, will se how it works out.

[cs.dk]

Maybe test them as normal stereo amplifiers?

Get everything sorted out before you use the bridged mode. Be careful with an oscilloscope when they are bridged. If you connect the ground clip to any of the speaker wires and you have a commond ground to a signal source you will need a lot more new parts than just capacitors...

Hej min Norske Citroën ven

I'll try to run them as stereo amps to see what happens. I'll leave the scope alone for now.

[tronde]

 
You asked about the idling current. It is there so the amp is operating in class A at low levels. When the output level increases, the amp will operate as class B. The result is that you will get rid of cross-over distortion, but still maintain the efficiency of the class B amplifier. Make sure you have the correct voltages because they are a result of the idle current. Higher current means more idle power and heat. Check that the currents are stable over time, within the max-min values given by NAD.

[cs.dk]

You asked about the idling current. It is there so the amp is operating in class A at low levels. When the output level increases, the amp will operate as class B. The result is that you will get rid of cross-over distortion, but still maintain the efficiency of the class B amplifier. Make sure you have the correct voltages because they are a result of the idle current. Higher current means more idle power and heat. Check that the currents are stable over time, within the max-min values given by NAD.

I'll have a look again.. It took me over 2 hours pr. amp, for the reading to settle.
I don't get it - But a Sony, or similar amp don't have that adjustment.

[gadget73]

almost every solid state amp has a bias adjustment, and its very critical.  Makes all the difference between the transistors melting down and having extreme distortion.   Some amps are very touchy, 10 turn pots make a big difference on those.  Transistors are also thermally sensitive, so the current will change as it warms up.  Unfortunately they conduct more as they get warm, which makes more heat, etc.  Thermal runaway is not your friend.