Nad t755 help needed

[kevinsonic]

One other thing that happened, only one time both speakers palyed when i suddenly advanced the volume, after about 10 seconds the left channel faded away and only one speaker was playing again. My immediate thought was perhaps its the volume pot that is worn, but usually you get a scratching sound as you turn the know when that happens.

I tried to reproduce the effect many times afterwards but it did not work,

 

[kevinsonic]

Can you test the preouts with your car amp (or the headphone out with cans)? Do the recording outputs work? That should help narrow down where a potential problem lies.

I would toggle through the listening modes to try and get the unit into analog bypass mode. Right now EARS seems to be active, which I would think would entail DSP action. If you can't get the DSP out of the picture, the number of possibilities increases exponentially. (At least the DSP shouldn't be of the known problematic variety yet from what I can tell.)

With some bad luck you might have a dead R2S15205FP, the symtoms certainly sound like a dead input selector IC. I guess checking its +/-7 V supplies at C6241/6242 (C6080/6078) couldn't hurt.

If you look at the service manual i attached, given what i tested and the results i that i just posted, (ie i do get sound but only through one channel and not as loud as it should be (this however might have to be set up in the osd menu as i know in the owners manual it says you must equalize the sound so both speakers sound the same then save the settings in the OSD menu i do not have access to)

Could it still be what you mentioned, the service manual is attached, if you could possibly point out what i could check next that could possibly help ?, i do have a multimeter.

nad t755 service manual:

Filebin | 6qlx4u372ftyd8z9

Convenient file sharing. Registration is not required. Large files are supported.

filebin.net

 

[kevinsonic]

sorry the link works now

 

[audio_tony]

My immediate thought was perhaps its the volume pot that is worn, but usually you get a scratching sound as you turn the know when that happens.

This receiver has a rotary encoder for the volume control. It's not a pot as such. (you can see this for yourself - the volume pot should just rotate continuously without any end stops).

A quick glance over the schematic suggests that volume control (plus many other audio functions) is controlled by U6001 (R2S15205FP) - see attached image.

Additionally, a Google search of R2S15205FP shows plenty for sale on Ebay etc. so that suggests to me that this is faulty (and probably a common fault). You will need an SMD rework station to replace this chip. You'll also need a 'scope to diagnose that the chip is indeed the problem.

I had a T751 about 20 years ago or so - and that developed a fault where one channel would randomly start crackling and then stop working altogether.

I'm a tech with 40 years experience, and I couldn't fix it. The boards are so crammed in, and you need extension cables to get the boards out on the bench for fault finding.

And then there's all the DSP stuff. This is not something you'll fix with a multimeter, and neither will you fix it without the remote.

Also, a factory reset would (should!) have taken care of any channel imbalance etc.

 

[kevinsonic]

This receiver has a rotary encoder for the volume control. It's not a pot as such. (you can see this for yourself - the volume pot should just rotate continuously without any end stops).

A quick glance over the schematic suggests that volume control (plus many other audio functions) is controlled by U6001 (R2S15205FP) - see attached image.

Additionally, a Google search of R2S15205FP shows plenty for sale on Ebay etc. so that suggests to me that this is faulty (and probably a common fault). You will need an SMD rework station to replace this chip. You'll also need a 'scope to diagnose that the chip is indeed the problem.

I had a T751 about 20 years ago or so - and that developed a fault where one channel would randomly start crackling and then stop working altogether.

I'm a tech with 40 years experience, and I couldn't fix it. The boards are so crammed in, and you need extension cables to get the boards out on the bench for fault finding.

And then there's all the DSP stuff. This is not something you'll fix with a multimeter, and neither will you fix it without the remote.

Also, a factory reset would (should!) have taken care of any channel imbalance etc.

View attachment 474489

Sorry for only now replying i've been so tired trying to get this amp to work...

I did not expect it to be that major, that's quite shocking... But i am glad i am being told the truth about likely the problem so i thank you.
I just hope it's not an endless rabit hole once what you mentioned is tested.

Since the most likely answer has been given and i thank you for that, i guess the only way to see is to pay a technician with the right equipment to test it. So i probably now i need to see how much i can get together to spend on a repair or sell it at a loss and get another amp. So much for trying to avoid a vintage 80s 90s amp to get a fresher component amp since this one is from 2007.

I should have rather gone with the sansui AU-D5 i had a chance on.

Side question, will the sound output use a different path when using a fiber connection (the previous owner of this amp said he only had fiber connections and had no problems, (unless it was a lie)

 

[audio_tony]

Side question, will the sound output use a different path when using a fiber connection (the previous owner of this amp said he only had fiber connections and had no problems, (unless it was a lie)

Both fiber and coaxial connections terminate to the same point - so no, this shouldn't make any difference.

To be brutally frank, the amp is probably not worth repairing.

If you only want a stereo amp, see if you can get a Pioneer A300 / A400, Sony (TA-F210, F220, F540 etc.) - all of these are good, reliable amps.

At the budget end, something from Sanyo (they did a couple of semi decent amps back in the 80's).

 

[AnalogSteph]

One other thing that happened, only one time both speakers palyed when i suddenly advanced the volume, after about 10 seconds the left channel faded away and only one speaker was playing again.

Congratulations, you've got some duff output relays in need of cleaning or swapping out. Probably, anyway. Find them and do the screwdriver handle tap test in operation.

Which reminds me that I've been meaning to do that on my old Onkyo TX-SV636 since 2006. Ahem. To be fair, I'm not really using it.

 

[kevinsonic]

Both fiber and coaxial connections terminate to the same point - so no, this shouldn't make any difference.

To be brutally frank, the amp is probably not worth repairing.

If you only want a stereo amp, see if you can get a Pioneer A300 / A400, Sony (TA-F210, F220, F540 etc.) - all of these are good, reliable amps.

At the budget end, something from Sanyo (they did a couple of semi decent amps back in the 80's).

I refuse to give up on this amp, i don't have a dime to even throw around for something else in it's class.
If what AnalogSteph is another probability, perhaps i could get it going at minimal cost

 

[kevinsonic]

Congratulations, you've got some duff output relays in need of cleaning or swapping out. Probably, anyway. Find them and do the screwdriver handle tap test in operation.

Which reminds me that I've been meaning to do that on my old Onkyo TX-SV636 since 2006. Ahem. To be fair, I'm not really using it.

I must try this thank you, could you spot them in the service manual attached, everytime i open this nad i get a headache from looking at all the components thats cramped in there.
What about a failed output transistor or cap

 

[AnalogSteph]

There's 3 on the L/C/R power amplifier board already, designations M5001 through M5003 all next to each other. Plus one more DPDT on the surround amplifier board, M5601.

It would also appear that the connection to the speaker terminal board is made via connectors that are plugged in, at connectors J5304A (L/C/R) and J5602A (SL/SR) respectively - if that's the case, I'd give those a whiff of DeOxit first, they'd be even hotter candidates than the relays. Study the wiring diagram (p.8) for their respective locations.

It's a modern(ish) AV receiver, some complexity is just par for the course unfortunately. Be glad it's only a 5.1. Receivers could get very involved even in the golden age of hi-fi.

 

[kevinsonic]

Thank you Analog Steph.

For the last 2 months the amp was at a technician, what his experience is i don't know, however, he got both channels to work without funny cracks or white noise, but now, its limited to playing at half volume.

I really hope that because of it working now, that hopefully the dsp chip and caps are ok....


If hopefully that is the case:

After looking into it i was told the amp (after a factory reset) reverts to limiting the volume output and the only way to disable the limit is to go into the advanced menu to disable it. That option is only available via using the Remote, RS232 as far as i understand also won't work. Only the remote can do it.

My question is, since i do not have a remote, i have been bending my mind for several days now about how i should proceed as i have no money to invest, and the best way forward i see is to try and disable the max volume limit.. (IF that is the problem which i am really placing all my bets on at this stage, since the guy got both channels to work, how he did it i have no clue, and he'll probably never tell me)

So after looking at it from several angles, the best way i see would be to get a universal remote, i did a lot of research and the conclusion was that most average remotes will only give you basic functionality,(despite what they say?)so only volume tone controls.. etc.. will work with those, but for the advanced features (which Volume limit is a part of on the amp
) You need a more advanced remote.

Looking at my budget and the options locally, i am leaning towards getting this remote: https://www.hificorp.co.za/superior...rJFmVjAjv7QHfk_rrbyfQhCSurHVxWNhoCk-EQAvD_BwE

My question, does anyone know where or if i actually can get the an advanced IR codes / or hex files as a last resort, to program the remote ?

It is a Nad t755.

 

[kevinsonic]

Ok i have the code for a t753 so hopefully it will work but. Will it be worth it to get the remote or after what i said, does anyone think i am on a delusional path here.
I have soaked so much into this amp and am at the point where i need to know if i should abandon it or purchase a remote and try it.

Given the symptoms i described in my last post. Does it sound like i still have a cap/dsp chip issue, or does it sound like the volume is limited in the setup menu ?