NAD 208 arbitrarily into protect mode - more commen when hot

[coolnose]

Hi all,

I am currently working on a NAD 208 that arbitrarily goes into protect mode, more often when being driven hard and hot.

I have downloaded that manual, but must admit that I would highly appreciate any insight from someone either having worked on or simply having more knowledge than me.

Strange thing is, even in protect mode I can measure around 1,6 V offset on the binding posts (both speaker A & B).

Offset when randomly getting out of protect mode is 1 or 2 mV.

I can attach the service manual if needed.

Thanks in advance for your help.

Cheers,

Max

 

[Ian Finch]

OMG! Dual rail mosfet output stages with multiple protection circuits This will not be fun.
NAD 208 | Owners Manual, Service Manual, Schematics, Free Download | HiFi Engine

 

[tauro0221]

Hi,
Do you have scope? I do not know how old it is this amplifier but check to see if you have ripple in your PS. If you do not have an scope check the rails voltage using the voltmeter in AC. It should read very close to zero.

 

[coolnose]

Hi,

thanks for linking the manual, I was trying to attach it and it was way to big...

Amp is from 1993, will check on the ripple and have a closer look to the protection circuit, as I think there's nothing wrong with the amp itself - as it's working fine when the protection does not engage.

I found several threads a bout various NAD protection circuit going bad over time, will pop it open and check caps for ESR and let you know.

Thanks you,

Max

 

[coolnose]

Finally some results...

Hi All,

intially I assumed tha amp was working fine and a fault in the protection circuitry would cause the amp to go into protection mode erraticly .

After some playing around with my ESR-meter hoping to find a leaking or shorted cap (without any result) I decided to do things right (at least I think so) and measure my DC offset ahead of the speaker relais....... AND haha.. here we have it between -25 V and -70 V on the output of both amplifiers on several switch-ons...when turning the amp off, the voltage drops rapidly, goes positive (roughly 25 V) and then goes towards zero.

I have checked the PSU, I have correct rail voltages, despite being quite high as the unit is wire jumperd for 230V and I measured 241 V AC on my mains... I will jumper it for 240 before going any further.

Not blowing fuses, but no way to get it out of protection mode any way as I could before.

Any hints on this one ?

Max

 

[coolnose]

Hi,
Do you have scope? I do not know how old it is this amplifier but check to see if you have ripple in your PS. If you do not have an scope check the rails voltage using the voltmeter in AC. It should read very close to zero.

Hi Tauro0221,

I do have a scope, I do lack some preactive using it though...

I quickly checked with my Fluke on AC and got something like 20mV, not what I would call significant ripple - doesn't seem to be the problem to me.

Cheers,

Max

 

[coolnose]

Hi,
Do you have scope? I do not know how old it is this amplifier but check to see if you have ripple in your PS. If you do not have an scope check the rails voltage using the voltmeter in AC. It should read very close to zero.

Hi Tauro0221,

I do have a scope, I do lack some preactive using it though...

I quickly checked with my Fluke on AC and got something like 20mV, not what I would call significant ripple - doesn't seem to be the problem to me.

Cheers,

Max

 

[tauro0221]

Hi,
If you read 20mv the rails power supplies capacitors are good. Now you mentioned that both channels read -25 volts when powered On the amplifier. So that means that it is something common to both channels. Did you check the +/- 18 volts? . They are used to power the op amps in the protection circuit and in the preamp for the amplifier.

 

[coolnose]

Hi,
If you read 20mv the rails power supplies capacitors are good. Now you mentioned that both channels read -25 volts when powered On the amplifier. So that means that it is something common to both channels. Did you check the +/- 18 volts? . They are used to power the op amps in the protection circuit and in the preamp for the amplifier.

Hi,

thanks for getting back to me!

I mentionned the -25V on the speaker output (AHEAD of the relais) as this is exactly the fault I had initially (measuring roughly 1.6 V AFTER ther relais)... thing is the fault is now permanent!

I only took of amp board out and checked for bad caps as mentionned before.

I forgot to mention that I have both boards blackened by heat in the same area, around a zener switching between HI/LOW rail voltage, I could send you the service manual, as I'ts protected and I cannot extract the relevant pages with the schematics.

I have not checked the output of the LM317/LM337 pair so far, but will proceed to do so and post the results here!

Thanks for you kind help!

Max

 

[coolnose]

SChematics and circuit description

Hi,

to make it easier for everybody to follow I have extracted the relevant pages of the service manual and attached them.

In the Schematics PDF I have a red circle in the area where both amp boards have burning marks around the 15V zener diode.

After looking at the schematics and reading the circuit description I think the reason for having negative voltage on the output might come from there, but I am not competent enough to confirm that.

After checking I can confrim that I do have +/- 18 V, values are a bit off though, I measured the resistors around both LM 317 and LM 337 and the values are slightly off (far more than 1%) which explains the former, will swap them as well.

I would highly appreciate your comments on the annotated schematics.

I'll post pics of the boards later.

Cheers,

Max