NAD T753 receiver repair, dead after storm, shorted microcontroller ALI M6759

[Chungalin]

This sucks, I know.

This complex and crappy receiver is dead after a storm. There's voltage coming out from the standby tansformer, but it's just 5V instead of 13V. This goes to Front board and then to Main, where voltage regulator U704 produces 5V for microcontroller. But then I see that 5V rail in front board is shorted! Disconnect board completely and apply external 5V to it, with some alcohol on top of every IC that eats 5V. U202 heats very quickly. This is an ALI microcontroller M6759, PLCC 44pin package. No external Flash or ROM, so its program is in internal flash.

The big problem is, of course, how to program that IC internal flash. Local NAD distributor says that the part is obsolete. I don't have the memory dump and I doubt anyone will have it. Since original IC is shorted it's impossible to retrieve its content.

Any idea?

[mmartell]

Re: NAD T753 receiver repair, dead after storm, shorted microcontroller ALI M6759

Have to find one from a donor machine I would say.

[keeney123]

Re: NAD T753 receiver repair, dead after storm, shorted microcontroller ALI M6759

Here is the service manual

[keeney123]

Re: NAD T753 receiver repair, dead after storm, shorted microcontroller ALI M6759

Wonder if this is and equivalent?

[keeney123]

Re: NAD T753 receiver repair, dead after storm, shorted microcontroller ALI M6759

Here is the Data sheet for the micro controller you are looking for

[Chungalin]

Re: NAD T753 receiver repair, dead after storm, shorted microcontroller ALI M6759

Thanks, I already had these documents before posting my message. As I tried to explain, the problem is that I don’t have the data file to program these microcontrollers. Even if I had the firmware file, I think that it wouldn’t be easy to setup the wiring and protocol from scratch.

Luckily I’ve been able to contact NAD main distributor in Spain and he’s offered me the microcontroller pair (U201 and U202) for a price that is not cheap neither too expensive. I’ve agreed and now I’m waiting for the parts to get here. I’ve made clear that the ICs have to come properly programmed with the right firmware.

Will report progress.

[Chungalin]

Re: NAD T753 receiver repair, dead after storm, shorted microcontroller ALI M6759

I received the pair of controllers last friday, I replaced them and now it works fine. They (NAD representative in Spain) sent me the U201 ("AVR") and U202 ("VFD") programmed with firmware version 2.12a.

[Chungalin]

Re: NAD T753 receiver repair, dead after storm, shorted microcontroller ALI M6759

This story hasn’t ended yet. The amp works fine in analog mode, all 7 channels, even video is okay, but when I wanted to use digital inputs I found that it was impossible to switch Surround Mode, it always said HEADPHONE on display and stayed in Stereo mode.

Checked again all connections that I touched on first repair and everything was fine. Then I noticed that optical TOSLINK output socket wasn’t emitting red light. This comes from a 74HC251D multiplexer (U1203) that selects one from 6 digital inputs. It’s partially shorted! I’ve ordered another one.

Remember that this was a storm failure, so it was too good to be true that only microcontroller was hit. Since there was a DVD connected to one coax digital input, there’ll be chances that it has dead digital output too.

[Longbow]

Re: NAD T753 receiver repair, dead after storm, shorted microcontroller ALI M6759

I think this is how we all learn to respect the power of lightning and/or power line surges. The basic rule of thumb here is that lightning doesn't break just one IC. Parts unit.

[mmartell]

Re: NAD T753 receiver repair, dead after storm, shorted microcontroller ALI M6759

Don't count him out yet, sounds like he has a handle on it.

[keeney123]

Re: NAD T753 receiver repair, dead after storm, shorted microcontroller ALI M6759

Thanks, I already had these documents before posting my message. As I tried to explain, the problem is that I don't have the data file to program these microcontrollers. Even if I had the firmware file, I think that it wouldn't be easy to setup the wiring and protocol from scratch.

Luckily I've been able to contact NAD main distributor in Spain and he's offered me the microcontroller pair (U201 and U202) for a price that is not cheap neither too expensive. I've agreed and now I'm waiting for the parts to get here. I've made clear that the ICs have to come properly programmed with the right firmware.

Will report progress.

That is how they make the money to pay the programmers and engineers. Otherwise, I think they could program it over the internet without having a chip programmer.

[keeney123]

Re: NAD T753 receiver repair, dead after storm, shorted microcontroller ALI M6759

This story hasn't ended yet. The amp works fine in analog mode, all 7 channels, even video is okay, but when I wanted to use digital inputs I found that it was impossible to switch Surround Mode, it always said HEADPHONE on display and stayed in Stereo mode.

Checked again all connections that I touched on first repair and everything was fine. Then I noticed that optical TOSLINK output socket wasn't emitting red light. This comes from a 74HC251D multiplexer (U1203) that selects one from 6 digital inputs. It's partially shorted! I've ordered another one.

Remember that this was a storm failure, so it was too good to be true that only microcontroller was hit. Since there was a DVD connected to one coax digital input, there'll be chances that it has dead digital output too.

If it was not a direct strike on the transformer or electrical line and your power section was not affected that is a good thing.
So that only leaves the cable line. I do not know over where you are at, but in the US I know Comcast has a box just for this problem. It is a whole box just to protect the cable line lighting strikes.

[Chungalin]

Re: NAD T753 receiver repair, dead after storm, shorted microcontroller ALI M6759

88€ was the total price for the µcontrollers pair +S&H. The most expensive part I’ve paid this year, for sure. 90% of this price is for programming the firmware.

[keeney123]

Re: NAD T753 receiver repair, dead after storm, shorted microcontroller ALI M6759

Yea, that is a lot of money. Still it is cheaper than a refurbished unit or a new one. I guess it depends on how much you enjoy music from it and if you can afford it. Personnel I like a two channel stereo with an A/B Amp.

[Chungalin]

Re: NAD T753 receiver repair, dead after storm, shorted microcontroller ALI M6759

Personnel I like a two channel stereo with an A/B Amp.

Me too, but this is work, it's not mine. I'd never buy a monster like this. I feel dizzy when I see one of those receivers with countless RCAs, HDMIs and speaker clips on back panel.

[keeney123]

Re: NAD T753 receiver repair, dead after storm, shorted microcontroller ALI M6759

Me too, but this is work, it's not mine. I'd never buy a monster like this. I feel dizzy when I see one of those receivers with countless RCAs, HDMIs and speaker clips on back panel.

Ahh work. These companies need to give you better tech support. It gives them people who are willing to understand and repair their product without the cost associated with an employee. It also would give their customers a quicker turn around making the customer happier. A happier customer brings in more customers and return customers. But they just want everyone to fill up the landfill with their junk so they can make more and more of your money. The problem with that is eventually the well runs dry and you end up with many poor people and very few rich ones. At this point usually a bloody revolution happens.

[battlecat]

Re: NAD T753 receiver repair, dead after storm, shorted microcontroller ALI M6759

Hi Chungalin..


Is there any change you could forward the details of the supplier for this chipset for the NAD T743 ?

Same thing happened , lightning storm and nothing was plugged in... I noted the post was in 2014 , but taking a chance here..

NAD support in South Africa went for runner.

[Chungalin]

Re: NAD T753 receiver repair, dead after storm, shorted microcontroller ALI M6759

I ordered the part from spanish NAD representative, and they ordered it from NAD Canada.

I still have it sitting here, because lightning damaged something else in digital board and I found too overwhelming to face it. I had the hope that only digital input multiplexer was damaged, but now it works OK and still cannot detect any digital input. Definitively my most frustrating repair of 2016.