NAD 2155 amplifier fault

[Wozzaaah]

Morning electronics gurus.
I've got a mate's NAD power amp here to have a look at. His electronics know-how is zero, mine is just above.
It developed an intermittent static type noise in the right channel yesterday, nothing too major so he carried on using it. He turned it off then a bit later when he turned it on it was in protection mode.
He brought it here for me to see if there was anything obvious, blown cap, signs of heat, cracked solder joint etc. but I could see nothing obvious.
After a few minutes of having it powered on with nothing connected, the speaker protection relay clicked in and the indicator light went out.
I found this was repeatable so it makes me think it's a failing component which starts to work normally as it warms up.
I hooked it up to my system and it ran fine yesterday and continues to do so now, in fact, it fired straight up this morning.
I briefly heard the noise yesterday, it's akin to someone putting their hand lightly over a microphone.
Any ideas of a good starting point to try and identify the fault?

*A bit of history - may be relevant: I spotted the impedance switch on the back was set to 4 ohms, he said that's how it was when he bought it in the 80s and he's never touched it. It turns out his speakers are 8 ohms and yesterday morning he bi-wired them for the first time.

Here's the schematic.

 

[unimog]

hi
try y tube they are some y tube how to fix nad amps

unimog

 

[MattF]

If I recall correctly, it's worth checking joints on high wattage resistors and such on those by desoldering. I can recall some badly oxidised legs etc. My one lasting impression of NAD is that the reputation belies the build quality. Just because a joint looks good on those, don't assume it is.

 

[Wozzaaah]

I'm not a fan myself, the build quality isn't great and the sound quality is pretty lacklustre in my opinion.
Thanks Matt, will break out the soldering iron later.

 

[Seadog]

I'd clean or replace the bias trimmers, and then reset the bias points. They're only carbon pots and will have some oxidation.

 

[Wozzaaah]

Would that cause the speaker protection to kick in or just account for the intermittent noise?
For info, I've now had it running flawlessly for 2 1/2 hours.

 

[Seadog]

I'm not sure about the protection mode, but it certainly introduce noise.

 

[Morris]

It might be worth a mooch around on here https://www.diyaudio.com/forums/index.php in the solid state section.

 

[Wozzaaah]

I think I’ve found the fault!
It ran flawlessly under load for over 5 hours yesterday but after following Matt’s suggestion of resoldering the power resistors it then went straight into protection mode again on cold start-up. After about 10 mins or so the relay started clicking in then out and the gaps between the clicks were getting shorter until it finally clicked in and stayed in. That absolutely convinced me that it was a component that starts to work as it warms up.
I turned it off to let it cool, started it back up and used my soldering iron to warm random components.
Lo and behold, I found a resistor which, when heated, caused the relay to click in. A test (in circuit) showed it was reading around 400 ohms cold, it should be 1.6 k ohms.
When it was running I used an air duster to cool it and it caused the relay to click out.
New resistor ordered, will do another update when I’ve fitted it.

 

[MattH]

Well done. There's nothing worse than having faulty Nads .

 

[Wozzaaah]

Following this up... I was wrong.
I fitted a new resistor but as soon as I turned it back on it was in protection mode, this time permanently! I swapped the new resistor for a spare just in case I had a duffer but no joy.
Had another bash at it this afternoon, I resoldered many joints just to see if that made any difference, but it didn't.
While it was running (in protection mode) I started prodding various components and lo-and-behold, the relay clicked in and it worked. I'd only prodded two so out came the soldering iron, resoldered a transistor and a cap and fired it up.....woohoo!!
Every time I pushed the power button the relay clicked in after about 4 seconds so I thought I'd found it but....
I unplugged it, left it to sit for a few minutes, discharged the caps and... grrrrr.......
It seems fine if it's left plugged in and turned off at the power button but if you unplug it it doesn't seem to like it.
I feel like I'm narrowing it down but going round in circles at the same time.

 

[skotl]

In the olden days I used to use a tin of freeze to spray on components, which highlighted those that failed (or started working) when they were hot va cold.

I suspect that it was a tin of climate-changing CFCs so don’t know if you can still buy such a thing but sounds like it would help

Edit: Found them - not cheap! https://www.amazon.co.uk/s?k=electr...827851&tag=googhydr-21&ref=pd_sl_2d5lxhge83_e

 

[Wozzaaah]

I think I may have found it!.... again!
Unplugged it, left it off for a few mins, back on, fault present.
It's been sat for about 15 mins with the protection light on so I poked a bias pot (which I had suspicions about (and is actually right next to the resistor I originally thought was suspect! ) with a plastic pokey thing and again, the relay clicks in and the light goes off!
Testing the DC offset at the speaker terminals, it doesn't change as I rotate the pot but doing so causes the protection circuit to kick in.

 

[rikrobson]

If its not the pot. Then it likely to be a Cap on the protection circuit. Caps charging or discharging are used as timers. So there should be a few seconds before the relay kicks in. 8f you measure the voltage at the relay 801 and trace that back you'll find the problem

Edit.

Itas based on the ta7317p Dedicated ic 2hich is designed for protection circuits. If you look up the Data sheet this should further assist you as opposed to guessing.

 

[Wozzaaah]

There’s about 4 seconds before the relay clicks in when it’s working.
A faulty cap would fit in with it not liking being unplugged.

 

[Wozzaaah]

Just left it unplugged for about 90 mins, plugged it back in and it started up fine.
I’m going to replace both bias pots.

 

[rikrobson]

I'm not seeing any pots on the ta7317p circuit

 

[Seadog]

 

[Rig Pig]

I've got this joy to go through on my subwoofer, it randomly cuts out as it heats up. Your struggle has really given me hope that it's going to be a chore to fix.

 

[rikrobson]

R708 and r721 are not in the protection circuit though.