Help with blowing transistor NAD T752 receiver

[rddube]

Re: Help with blowing transistor NAD T752 receiver

Also R574 is open on the side that connects to Q533. The other side is ok....

[rddube]

Re: Help with blowing transistor NAD T752 receiver

Sorry guys, got confused and didn't realize that it's not the left channel that went berzerk, it's the center channel Q533 and Q534 that are short.

R520 wasn't heating up, and the left channel was working, the right channel was not (no sound) and it's while adjusting the bias on the right channel and monitoring heat and voltage, that the center channel popped. This was the good channel up until now.

So I am really confused. No heating, but I think I saw a glow somewhere in the middle of the card when it happened, but can't find any trace of heat or burnt mark????

[eccerr0r]

Re: Help with blowing transistor NAD T752 receiver

Something still doesn't seem right, if you were monitoring the voltage across the resistor and shut down immediately if you saw anything but a few millivolts, should have been able to save it. Having R520 burn out also will kill the finals so that should have been your clue... did you ever fix that problem? Should have been able to run the amplifier without the finals and R520 should not have been burning hot.

[petehall347]

Re: Help with blowing transistor NAD T752 receiver

the new transistors could be fakes ..

[rddube]

Re: Help with blowing transistor NAD T752 receiver

After a few more tests, trying to adjust bias while monitoring temperatures which were ok, the finals on the left channel popped and now they are shorted again - both. So I give up, will use the surround amplifier instead to listen to music.

Thank you very much to Eccerr0r for all your time and to Pethall347. It has been a long journey which doesn't end like a fairy tale, but what the heck you can't win'em all! Thanks again!

[eccerr0r]

Re: Help with blowing transistor NAD T752 receiver

Nope, have to do it in situ, powered up...

[rddube]

Re: Help with blowing transistor NAD T752 receiver

Hi,

I do have an osciloscope and a sine wave signal generator. Can i do this with the board out as there is not much room to go measure on the board itself when it's installed deeply into the receiver?

[eccerr0r]

Re: Help with blowing transistor NAD T752 receiver

Now for debugging stuff like this you need all those jumpers again so you can measure voltages...

And the right tool for this is probably an oscilloscope (and a sine wave signal generator) so you can see where the signal distorts or disappears...

But posting voltages at different points of each transistor in the path is what's needed to see where the voltages seem out of whack?

[rddube]

Re: Help with blowing transistor NAD T752 receiver

Ok, so did everything by the book, kept the connectors in to test voltages and so on comparing to the good center channel, and all seemed fine. Proceeded to adjust bias, and was able to adjust bias just like in the manual. Checked temperatures, everything looked cool.

Removed the board to take out the connectors, reassembled everything nice and neat, rechecked bias before connecting speakers, everything seemed perfect. Connected speakers, powered it up and only the left channel was giving sound and not perfect sound.

Redid the bias adjustment being careful to follow the instructions exactly, same result.

So I am getting discouraged, not too sure what's wrong this time. If it wasn't for you Eccerr0r, I would probably send this damn thing to recycling. Is there any hope? Tks again!

[eccerr0r]

Re: Help with blowing transistor NAD T752 receiver

Probably okay to remove them but best to keep them in... in case things haven't been totally solved yet. If you're confident the L + R channels match in behavior now which is close to C channel, then you probably can remove the test wires.

One thing that you might want to do is put a meter on each of the bias resistors, i.e. P501/P502 test pins, and ensure it doesn't go up to a few volts when you power up for the first time after installing the finals...if it does you need to shutdown right away and look for the problem... Do not use autoranging.

[rddube]

Re: Help with blowing transistor NAD T752 receiver

Question for you Eccerr0r - when I put the finals in place (should get here tomorrow), do I leave the test wires I had put on Q506, Q518 and Q530? Also, should I replace all boards or test it with minimal boards?

I guess I should, but that means I will have to dissassemble it again if everything is ok, but I'd like to have your advice on this. Thanks!

[rddube]

Re: Help with blowing transistor NAD T752 receiver

very well, if both right and left match, reduce both to the lowest voltage and then put in the new transistors, and carefully monitor their temperatures as you power up. If they still get hot fast, shut off right away. If they don't get hot, go through the bias tuning in the service manual.

ok, got it. Will report back in a few days when the finals get here. Thanks again for all you help Eccerr0r!!

[eccerr0r]

Re: Help with blowing transistor NAD T752 receiver

very well, if both right and left match, reduce both to the lowest voltage and then put in the new transistors, and carefully monitor their temperatures as you power up. If they still get hot fast, shut off right away. If they don't get hot, go through the bias tuning in the service manual.

[rddube]

Re: Help with blowing transistor NAD T752 receiver

Sorry again Eccerr0r, I just realised you asked for Q507 and Q508, and I wrongly assumed you meant the same as Q506 on the other channels, so I measured C-E of Q518 and C-E of Q530 and reported them as Q507 and Q508. I can't measure Q507 and Q508 because they are deep down under.

So what I just did is compare what I get on Q506 C-E when I change R522 to Q518 when I change R545 and get exactly the same readings i.e. 2.1V when turned completely clockwise which gives a resistance on R522 and R545 of 1.3K, and 2.75V when R522 and R545 are set like the good channel R571. When I measure C to E on the good channel, I get +2.9V.

[eccerr0r]

Re: Help with blowing transistor NAD T752 receiver

I think we're back to square 1, and still have more unknown issues unless you can properly adjust the spread via R522. Q506 C-E spread should be around 4.2V and adjustable by R522. The voltage drop across R5143 should be around 2.8V and finally the voltage drop across R520 should be around 1.4V, both of these should be likewise adjusted with R522. Just ensure that you can adjust these voltages, and then turn it down as low as possible before putting the new finals in. You should be able to hear that the audio sounds awful with harmonics and then as you increase the voltage it should start sounding less harsh and raspy, making it sound good once more. Of course you should ultimately follow the service manual's R522 adjustment by using P501.

[rddube]

Re: Help with blowing transistor NAD T752 receiver

Ok, so Q507 gives me +2.78V and Q508 (good channel - center) +2.972V. Each side of R520, I get -0.244V and -0.156V.

[eccerr0r]

Re: Help with blowing transistor NAD T752 receiver

Well, worst case is you fry R520 but you should be able to go the full gamut like the other channels, but it shouldn't be able to get so high to do that if it were designed properly and no bad components. Well, if it matches the other channels now approximately it should be ok, just check the voltages at Q507/Q508 bases and the two ends of R520 to ensure the spread is less than what you see at Q506, then you're probably good to try reinserting finals.

[rddube]

Re: Help with blowing transistor NAD T752 receiver

Eureka, with everything soldered right I get between E and C of Q506 +2.1V when R522 is at the 1.2K level (fully clockwise), +2.5V when at +- 0.800K level (about mid turn) and +2.75V when set like the other 2 channels...I didn't go over that just in case.

What do you think Eccerr0r ?

[eccerr0r]

Re: Help with blowing transistor NAD T752 receiver

Yeah I typoed that, good to always doublecheck any lies spewed on the internet

[rddube]

Re: Help with blowing transistor NAD T752 receiver

C518 is parallel with C-E of Q506 and is a filter capacitor for that voltage.

Presume here you mean C508 as C518 is in the right channel? Tks.