Weird NAD 208 THX protection loop

[SchLikA]

Hi All,

I have a strange issue with my beloved NAD 208 THX, it kinda loops into protection mode with bad bangs to the speakers ... here is a video of it :

nad 208 thx problem - YouTube

It happened from time to time but now does all the time. I checked a few caps, found they were out of specs so recapped the whole power supply board & input amplifier unfortunately to no effect.

The protection IC (IC 601), a TA7317P, seems to have commonly been a source of problem. Might it be the culprit ? Have some of you ever seen that ?

Thanks for helping !!

 

[jaycee]

If you are hearing bangs in the speaker, the amplifier is the problem, not the protection circuit

 

[rayma]

Is there DC voltage at the output?

 

[SchLikA]

Thanks for helping !

there is DC voltage at the output, and it goes all over the place, from minus 20 mv to +15, when the protection kicks, it goes back to zero, then starts again.

When shutting it down, I see mv going gradually down from ~120 to 0.

I must say that I am lost :-(

 

[SchLikA]

Is there DC voltage at the output?

Hi ! Yes, there is strange DC stuff, as written in my previous post.

 

[Wazzadj69]

SchLikA I can help you with this amp if you are still needing help
I have just fixed a NAD208
Cheers
Wayne

 

[nily74]

Hello Wazzadj69! I need som help with my nad 208thx ,if you can help me it would be great.
Cheers.

 

[BSST]

Hi nily74,

Welcome to the forum!

Were you able to contact Wazzadj69?

If you get stuck, I enjoy troubleshooting. I prefer probing live circuitry, as I believe it's usually the quickest path to finding a problem. If you are comfortable working with powered equipment and know how probe safely without putting yourself and your equipment in jeopardy, I'd be happy to offer my help.

 

[nily74]

Hello BSST! I would like to get help from you, please. I am sending the schematic so you can see what I mean.
I have a problem with the resistor 358 on page 16. It gets very hot.

 

[BSST]

I assume you’ve already checked supply voltages to see if they are reasonabl.

Let‘s try to develop a sense of how and from where the excessive current is flowing. Would you measure with respect to ground voltages at ends of:

R353
R358
D319
D325

These voltages may be only symptoms of a more global problem. For a quick Look, would you also measure Feedback 1 and 2, R326, and the Input.

Please report any negative voltages as signed numbers.


Thanks!

 

[BSST]

I didn’t look carefully enough when I ask for “Input” voltage, as that’s on the wrong side of blocking caps C306 and C307. To measure the input bias, would you measure three voltages at the junctions of:

R313,314
R314,315
R315,316

These voltages should tell us the base bias on Q304 and Q305.

Thanks again.

 

[nily74]

Here is some result i got. I will get back to you on the other resistors 🙂
R353 24,9VDC
R358 74,4VDC
D319 10,1VDC
D325 75,2VDC
Feedback 1 0VDC
Feedback 2 0VDC
R326 -0.004VDC
Input 0,01VDC

 

[nily74]

R313,R314 0,826VDC
R314,R315 -0,004VDC
R315,316 -0,824VDC

 

[BSST]

Thanks, nily74.

I wasn't as detailed as I should have been in specifying the voltage measurements I sought. When I asked for voltages with respect to ground, I assumed you would give me two measurements, i.e. each end of the resistor with respect to ground. They would have given me insights to the behavior of nearby circuit nodes. I surmise you've measured voltages across the resistors.

The picture is very incomplete at present, but I don't see anything amiss so far. Do you observe any misbehavior? The amp output offset looks quite reasonable, especially re the measurements in post 13. About 74V across R358 looks about right, but you are correct to note it being very hot. Dissipation is about 3.6W, so a 3W resistor is undersized.

Any additional details?

 

[nily74]

Thanks BSST for the help so far. The last measurements I made with the -measuring probe on the signal ground and I moved the +probe around between the different resistances. I have the first measurement point with the + probe between R313 and R314 and the second measurement point between R314 and R315 and so on. Sincerely.

 

[BSST]

Do you have any known performance issues? Is your only concern the hot resistor? I assume the other channel has the same issue? Do you have any equipment to more thoroughly test the amp at higher power? This seems like a nice amp with lots of available power.

I'll contemplate the Wattage of R358 and make a suggestion for something more conservative.

 

[nily74]

It started with the rectifier BR501 breaking. After I changed it, I haven't been able to use it because some components get very hot. I read that you usually replace the capacitors, so I have done that as well. Have turned down the VR301 gain as much as possible. But as you say, it is both channel that get hot. I think something is wrong with VH because BR 501 broke.

 

[BSST]

Learning that BR501 failed is good to know. So we need to expand the scope of troubleshooting.

Am I correct in surmising that the heatsinks in both channels are hot?

It's good to minimize bias current via VR301. As a sanity check with power off, use an ohmmeter to confirm VR301 is at maximum resistance; if it shows minimum resistance, the instructions for presetting VR301 Idle current in page 6 are backwards. In similar fashion with power on, note voltage drop across R393 for insight about actual bias current. Likewise, for the other channel.

 

[nily74]

Yes, it is true that both channels get hot but only the R358 that i can see with thermography. R502 on page 17 get hot also.

 

[nily74]

Measured vr301 and it was 20k so seems ok. Doesn't seem to be any voltage across R393 likewise the other channel.