Help with blowing transistor NAD T752 receiver

[eccerr0r]

Re: Help with blowing transistor NAD T752 receiver

Well in the business, you do what you can do with what you have. If the 220Ω resistor is burned, leave it in and assume that stage is bad and keep going upstream to see what else is bad.

Note that the way this is designed, protection doesn't seem to protect the semiconductors, it only protects your speakers and your house from going on fire. Also if one of Q509 and Q510 burns and shorts out, the other one will burn shortly (pun not intended) too. Your audio will sound really distorted when one burns, and will go silent then protection shutdown when the other burns.

Keep comparing with the other channel. Voltages across the transistors between channels should be very similar when given the same input, and yes test with inputs grounded/silence to ensure there's nothing getting hot when quiescent.

Don't worry about overtightening the screw. I think if you have one of those thin handle screwdrivers like a tweaker screwdriver, if you tighten it as much as you can, it is tight enough. You can overtighten if you use one of those "construction" screwdrivers like what they use to screw wood together when building houses - don't use that for transistor heatsinks, only for tightening screw terminals.

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damn... i'm so poor. can't believe i don't have a thermal camera yet...debugging would be so much easier with one of these...

[rddube]

Re: Help with blowing transistor NAD T752 receiver

Ok, so am waiting for the transistors to come in and when they do, will reinstall and take temperature and voltage measurements. At the places you asked me - I will solder thin insulated wires to be able to do so.

I changed the 220ohm resistor and been comparing all components of the left channel with the right channel, and haven't found anything to be wrong. Will report back when I get the transistors, but it should be within a few weeks!

BTW, the thermal camera I have is a Seek thermal camera that you plug into your iphone..works pretty well, but is not as precise as a dedicated thermal camera like the one Northbridgefix uses in his videos on Youtube.

[eccerr0r]

Re: Help with blowing transistor NAD T752 receiver

iphone... mmm...rich folks phone... not that I'd want one, signing away soul to use one...

but you should be checking voltages between the two channels, not just component values. Want to rule out broken traces too.

[rddube]

Re: Help with blowing transistor NAD T752 receiver

iphone... mmm...rich folks phone... not that I'd want one, signing away soul to use one...

but you should be checking voltages between the two channels, not just component values. Want to rule out broken traces too.

Ok, so do you think I can plug it back in without all the otherboards (just the main board) and without q509 and q510 to take measurements? That would leave me with a lot of space to take those measurements.

Oh and it's not my own iPhone, it's the Company's whom I work for...

[eccerr0r]

Re: Help with blowing transistor NAD T752 receiver

do whatever it takes to run without the case, but of course the heatsink needs to be attached.

[rddube]

Re: Help with blowing transistor NAD T752 receiver

So I spent a few hours trying to get it to start without the boards, endend up having to replace all boards and even then, it goes into protection mode because I suspect of Q509 and Q510 being absent. So I guess I'll have to wait for the replacements to come to be able to do further tests.

In a previous post, you mention that I might have to ground the ouput, do you mean the speaker outputs to ground? Tks!

[petehall347]

Re: Help with blowing transistor NAD T752 receiver

put 1k resistors from where base and emitters q509 q510 connect to on the board .

[eccerr0r]

Re: Help with blowing transistor NAD T752 receiver

Ground the input (i.e. silent input) not output. The worry about the output is if the speakers are shorted or supplying enough load that it affects measurements, probably better to leave them off for now, or at least use higher impedance speakers.

[rddube]

Re: Help with blowing transistor NAD T752 receiver

Ok, so I'll do as Petehall347 suggests, i.e. 1k resistors from E to B on Q509 and Q510. Have to take it apart, so while I at it, can you confirm where you want me to take voltage measures i.e. voltage between the collector and emitter of Q506 and voltage across R5143? Is that it? I'll have to run some wires because with all boards installed I can't reach down there to take the measurements. Let me know and tks.!

[petehall347]

Re: Help with blowing transistor NAD T752 receiver

those resistors will keep the feedback intact and hopefully zero the offset .
do not have any speaker loads connected and keep vol to minium ..if it comes out of protect you can try headphones at low level if it has a socket for that

[rddube]

Re: Help with blowing transistor NAD T752 receiver

Ok, so I finally got it to start but just a few seconds because now it stops and displays overheat, but I was able to take voltage measurements.

Between the collector and emitter of Q506 the voltage is .637V so I measure between the collector and emitter of Q518 (which is the right channel) and got .639V.

As for voltage at R5143, I read 52V ! So I took a reading at R5146 (right channel) and I don't get an immediate reading, it seems to be switching from but after a half second I get -6.08V?? What a difference!

So what do you think??

[eccerr0r]

Re: Help with blowing transistor NAD T752 receiver

Yeah R5143 at 52V would explain roasting R520 and blowing the finals. They should be much closer together (i.e., voltage across closer to 0v) just like the right channel.

Since the voltage across Q506 is reasonable now I'd end up having to once again blame Q562, Q563, Q508 or Q507 badly leaking or shorted. What is the voltage of the two pins of R5143 with respect to GND? What are the voltages of C and E of Q506 WRT to GND (though doesn't quite make sense why it would cause R5143 to "roast" at 52V, even if it were way off.)

[rddube]

Re: Help with blowing transistor NAD T752 receiver

Yeah R5143 at 52V would explain roasting R520 and blowing the finals. They should be much closer together (i.e., voltage across closer to 0v) just like the right channel.

Since the voltage across Q506 is reasonable now I'd end up having to once again blame Q562, Q563, Q508 or Q507 badly leaking or shorted. What is the voltage of the two pins of R5143 with respect to GND? What are the voltages of C and E of Q506 WRT to GND (though doesn't quite make sense why it would cause R5143 to "roast" at 52V, even if it were way off.)

Ok so the two pins of r5143, one side is 51V and the other side is 55V. As for C and E of Q506 To GND, they both give 51.4V. And I had removed Q562 and Q563 and Q508 and Q507 and installed brand new ones even though they had tested good. I had to redo some traces as the pads lifted on Q508 and q509, maybe I fumbled something there however I had taken all resistance measurements (without Q509 and Q510 however) and everything seemed ok. What could it be?

[petehall347]

Re: Help with blowing transistor NAD T752 receiver

" As for C and E of Q506 To GND, they both give 51.4V."
looks shorted to me .

[rddube]

Re: Help with blowing transistor NAD T752 receiver

" As for C and E of Q506 To GND, they both give 51.4V."
looks shorted to me .

Q506 measures from C to E .825K and Q518 from C to E measures .821K. Please note that I do not have 1K resistors on Q509 and Q510 as I was able to start it without them, except that it goes into overheat.

I don't think Q506 is shorted as I measured it before resoldering a brand new one and it looked fine.

Really confused help!!

[petehall347]

Re: Help with blowing transistor NAD T752 receiver

use diode test to check transistors .

[petehall347]

Re: Help with blowing transistor NAD T752 receiver

might be missing negative supply somewhere sending positive high .. open resistor or bad solder joint .

[rddube]

Re: Help with blowing transistor NAD T752 receiver

Damn, while taking measurements on Q518 my probe slipped and I must have created a short, because I saw R544 get glowing hot, so I stopped everything and dissassembled now to find out that Q518 was fine, R544 had a burn mark but is still measuring 218ohms, however Q521 and Q522 were both shorted. Removed them and measurements seemed fine, so reinstalled without Q521 and Q522.

Lo and behold, now I get 55V on R5146. So the transistors need to be in to get an accurate reading of R5143 (left channel) or R5146 (right channel). Guess I might as well wait for them to get here (fortunately I bought 4 pairs) and retest when everything is in place. What do you think?

[eccerr0r]

Re: Help with blowing transistor NAD T752 receiver

yes that sucks when you try to repair something and then cause more damage...

when you say XXX volts on a 2-pinned device, it's assumed it's XXX volts across the device, but you're giving conflicting numbers and it's hard to tell what is what. When you give voltage numbers, need to be clear whether it's with respect to ground or across device.

Where is the full schematics to this, clicking on the ones near the top of the thread is only left/front channel...

[rddube]

Re: Help with blowing transistor NAD T752 receiver

yes that sucks when you try to repair something and then cause more damage...

when you say XXX volts on a 2-pinned device, it's assumed it's XXX volts across the device, but you're giving conflicting numbers and it's hard to tell what is what. When you give voltage numbers, need to be clear whether it's with respect to ground or across device.

Where is the full schematics to this, clicking on the ones near the top of the thread is only left/front channel...

Voltages on R5143 and R5146 is relative to ground on one end of the resistor (I can't see the point of measuring across a resistor, but maybe I'm mistaking?). Attached is the full service manual with all schematics. Tks.