NAD 3155 outputs one channel only

[fiskmaas]

You should check for any signs of heat stress like discoloured resistors and faded lettering on
IC and transistors, candidates would be R628 Q604, R503,5, generally have a close look at the phono stage and around
IC601,Q602,4,7. Also check around the mute function, Q606,7.

Ok, I've started replacing caps and have replaced the voltage regulator IC902. It did help the voltage a bit at the red crosses. I can't see any discoloration on any of the resistors. There is however another discolored area on the board around R926 and R927 just like in the first picture around R907 but that's about it. I'll continue to replace caps for now.

Edit: replaced C916, 917, 707, 757, 615, 613, 614, 663, 664, 665, 912, 915, 910, 914 so far. I don't think any of them were bad, or that any of the ones left are bad. All looked solid and weren't bulging, but replacing anyway. Problem still persists.

 

[mbz]

Measure the volt drop across R907 on DBT, use minni-grabbers or similiar. It's 5W rated, 220ohms (150mA) so ballpark
25Vdc expected (common NAD design to burn volts on this scale). Since R907 is smoking the thinking is the current
is excessive possibly due to a downstream "near short". Would expect R905(10ohm fusible) to also be stressed. Measure
R905 resistance(powered off)

 

[fiskmaas]

Measure the volt drop across R907 on DBT, use minni-grabbers or similiar. It's 5W rated, 220ohms (150mA) so ballpark 25Vdc expected

R907 measured at 45V and 25V in/out respectively.

Would expect R905(10ohm fusible) to also be stressed. Measure R905 resistance(powered off)

Measured 10 Omh here.


I also noticed now when measuring that the soft clipping green light on the front now was lit, which it wasn't before.

 

[mbz]

I also noticed now when measuring that the soft clipping green light on the front now was lit, which it wasn't before.

From memory, indicates Soft Clipping switch on rear panel is in the ON position. Probably bumped it. Most leave it off unless
you play at party volumes, a clipping amp may damage your spkrs.

R907 measured at 45V and 25V in/out respectively.

R907 running at 1.8W (fine)

Measured 10 Omh here.

With R907 smoking, would have expected R905(FR) to have drifted a little high

Both above indicate no stress. Do you have a higher wattage bulb for the DBT? eg, 70-100W to wind up the voltage on the amp.
Will check your pix etc, R907 is supposed to be 5W maybe someone did a bad sub?

EDIT: R907 looks OEM/5W. Possibly something downstream is failing short under the higher (non-DBT) voltage, need to slowly wind up the voltage

 

[fiskmaas]

From memory, indicates Soft Clipping switch on rear panel is in the ON position. Probably bumped it. Most leave it off unless
you play at party volumes, a clipping amp may damage your spkrs.

Yes that might be it, lol.

Both above indicate no stress. Do you have a higher wattage bulb for the DBT? eg, 70-100W to wind up the voltage on the amp.
Will check your pix etc, R907 is supposed to be 5W maybe someone did a bad sub?

Yes, bought a 100W bulb with the 60w one.
I don't think anyone has ever worked on it so nothing should be replaced.

EDIT: R907 looks OEM/5W. Possibly something downstream is failing short under the higher (non-DBT) voltage, need to slowly wind up the voltage

Ok, I'll try with the 100w bulb then?

Edit: found something that looks like it has been worked on before. Attached images.
Edit2: nvm, comparing to other amps it looks like it is original.

Edit3: Ok, so I tested with the 100w bulb. R907 measured at 55V and 35V. Also smoking when playing music.

 

[Stepwise]

If you have positively ruled out that any of the switches are intermittent you can stop reading here.

I recently did a major overhaul of a NAD 3400 and almost every switch had gone bad
during a decade or so of sitting unused in storage. Spraying miracle stuff on the outside
had no effect, the contact elements ar hidden deep inside plastic parts. Some were
"excercised" a lot (> 50 times) and still were erratic.
https://www.audiokarma.org/forums/index.php?threads/nad-switches-inside-out.923174/

(disclaimer - I don't know if the swithches in my amp are exactly the same as in yours)

("fiskmås" verkar bekant)

 

[mbz]

I tested with the 100w bulb. R907 measured at 55V and 35V. Also smoking when playing music.

Still only 20V drop, R907 running under 2W. Suggest replace R907 (220/5W)
When playing music/smoking what were the voltages, 55V, 35V. Might be getting close to IC902(L78M24) max
input voltage(40Vdc), it might be going into protection.

 

[fiskmaas]

Still only 20V drop, R907 running under 2W. Suggest replace R907 (220/5W)
When playing music/smoking what were the voltages, 55V, 35V. Might be getting close to IC902(L78M24) max
input voltage(40Vdc), it might be going into protection.

Allright, replaced the resistor. No smoking now and runs much cooler. Measured 30V and 53.5V when playing music now. So progress I guess. Woho!

Edit: Tested without bulb, 35V and 58.8V. No smoke, still cool.

("fiskmås" verkar bekant)

 

[mbz]

So... back to orignal problem, no output on left channel. Use oscilloscope and confirm (again) no audio at left pre-out/main-in
jumper, will get some more test points...

 

[mbz]

Check for audio at following marked points(red crosses),
R618
R616
R610

 

[fiskmaas]

Check for audio at following marked points(red crosses),
R618
R616
R610

This is a 500hz signal from a tone generator on my phone.
R618:
R616:
R610:

Signal from pre-amp was good for both channels.

 

[mbz]

Can you describe the problem, is it still missing audio on one or both channels, or distortion or low volume...
I will check traces...
EIDT: R618 trace looks good, effectively pre-out. Noted audio on L&R pre-outs.

 

[fiskmaas]

Can you describe the problem, is it still missing audio on one or both channels, or distortion or low volume...

Left channel is still completely dead. Right channel is fine I believe, but will double check.
Edit: right channel is good, although input jacks might need some tending to (crackeling and distant sound in certain positions), but that's a different matter.

R618 trace looks good, effectively pre-out.

Yes, they look basically identical.

 

[mbz]

Use your oscilloscope to check for 500Hz at 3 jumpers below

 

[fiskmaas]

Use your oscilloscope to check for 500Hz at 3 jumpers below
View attachment 1983086

Left most jumpers:

Right most jumper:


Edit: retested left most jumpers.

 

[mbz]

Maybe a dirty speaker selector or protection relay.
Right most jumper is output from power amp stage, looks ok.
It then goes through the protection relay and then onto the speaker selector.

Try connecting speakers to Set "B"
Otherwise need to test the spkr switch, ie, oscilloscope at white wire on switch board
Audio at that point means dirty switch, ie, dismantle and deep clean, need to be careful...

 

[fiskmaas]

Try connecting speakers to Set "B"
Otherwise need to test the spkr switch, ie, oscilloscope at white wire on switch board
Audio at that point means dirty switch, ie, dismantle and deep clean, need to be careful...
View attachment 1983099

Same for B, only right channel. Will do some testing.
White cable showed signal:


Looks like Stepwise' intuition was right then. Will carefully dismantle and clean.

 

[mbz]

White cable showed signal:

Means it's the spkr selector switch = major PITA (not that bad if you are careful). Post a pic of the actual switch
mechanism, component side, appears to be just a slide switch.

 

[mbz]

This is the white cable you tested. Should also inspect the solder connections at speaker posts
and also check for dry solder connections on the selector switch.

 

[fiskmaas]

Post a pic of the actual switch mechanism, component side, appears to be just a slide switch.

Best I can manage atm as it's underneath the power switch and the power switch seems finicky to disassemble.
Edit: