Help with blowing transistor NAD T752 receiver

[rddube]

Hello everyone,

I guess I need help from the masters to try and find what is frying a 2SC5669 transistor on the left channel of this front center Amp for a NAD T752.

Was working perfectly and all of a sudden I heard a pop, and then a glow from the amp. Not only did the 2SC5669 burn a leg, but R520 which is a 220 ohm resistor partially burned - it now measures 110 ohms.

I bought this amp second hand fairly cheap as it was not working, it went into protection mode. I finally traced it down to the same transistor Q510, the one that just popped. It was shorted and so was the paired Q509 2SA2031. I replaced them both along with Q507 (2SC4159E) and Q508 (2SA1606E). I also changed Q511 which is a 2SC2362 which was shorted. Checked every other component on the board for the left channel, removed every transistor and checked them with my transistor checker, checked all caps and changed a few out of range, all resistors, etc. Everything looked fine so reinstalled and it was back to life!

It played for approximately 4 hours with a beautiful sound. I checked and set the idle current on each channed to 1mV and all was good. After listening to a track fairly loud, is when it popped as shown in the pictures.

Any ideas what might be drawing so much current on R520 to scorch it and what might be making Q510 pop again? Many thanks for any input, as I'd love to get this amp back to life, it sounds so beautiful!

[rddube]

Re: Help with blowing transistor NAD T752 receiver

Here is a photo of the schematic in the pdf file.

Help and tks!

[R_J]

Re: Help with blowing transistor NAD T752 receiver

What type of insulators did you have on the output transistor that blew up? it looks like the collector arced to the heatsink.
Did you use new insulators and fresh heat sink compound?

[rddube]

Re: Help with blowing transistor NAD T752 receiver

What type of insulators did you have on the output transistor that blew up? it looks like the collector arced to the heatsink.
Did you use new insulators and fresh heat sink compound?

R_J how are you doing!! Hey, you hit it right on the nail, look at the picture. I feel a little dumb not having thought of that! The insulator moved while reinstalling and that's exactly what happened.

I made some insulators from a deck of plastified game cards...ok, this time around I'll be careful and check if I have a ground or something. I'll also add new heat sink compound on every transistor to make sure. Thanks a million R_J!.

[R_J]

Re: Help with blowing transistor NAD T752 receiver

Get some proper insulators, playing cards don't transfer heat very well.

[eccerr0r]

Re: Help with blowing transistor NAD T752 receiver

Get some proper insulators, you were lucky the playing cards didn't light on fire...

[rddube]

Re: Help with blowing transistor NAD T752 receiver

Ok, so I got some insulators from Digikey for TO-247, installed them with fresh thermal greas and changed Q510 and Q509 + Q508 and Q507 + Q562 and Q563 at the same time. Change R520 and checked all other resistirs and diodes in the left channel, and all was fine.

Reinstalled, plugged it in and it worked like a charm. Adjusted the bias voltage on all 3 channels to 1mv as stated in the service manual.

Worked well for about 2 hours, and then a pop and went into protection mode. Opened it up, and again Q510 and Q509 are shorted.

I'm out of ideas...any help would be appreciated!

[rddube]

Re: Help with blowing transistor NAD T752 receiver

So something is causing Q509 and Q510 to fry because both of them are short. I checked every other component in the left channel, compared it to the right channel and got the same results.

So now I have no more 2SA2031, will have to wait to get some others (on their way) before doing any more tests, but I have to find what is frying those 2 transistors.

[rddube]

Re: Help with blowing transistor NAD T752 receiver

Anyone, please help?

[lotas]

Re: Help with blowing transistor NAD T752 receiver

Maybe the transistors are not original (remarked) and they will not match their parameters, the Chinese love to do this.

[rddube]

Re: Help with blowing transistor NAD T752 receiver

Maybe the transistors are not original (remarked) and they will not match their parameters, the Chinese love to do this.

Well that is a possiblity, however I did get some for 3 different sources and some of them looked like pull outs from other amps. Unfortunately, this is the only source where you can get them from. I did check them all with my transistor tester and they seemed to have the specs of the originals.

What I could do to test that theory is when I get the new ones, I'll take the transistors from the right channel install them in the left and put the new ones in the right channel. If the left still blows, then it's not fake transistors.

You know it's a real pain replacing transistors on this machine and reinstalling everything, because you have to get the main board and all complementary boards out every time...quite time consuming. Wish I could identify the source ....I read in another thread Budm saying that it could be a transistor from another channel that is the problem but I'm not sure how to check that?

[lotas]

Re: Help with blowing transistor NAD T752 receiver

Transistors and diodes can leak at the junction and break at high current (floating fault) and this tester will not show you this.

[petehall347]

Re: Help with blowing transistor NAD T752 receiver

are your speakers ok ?

[rddube]

Re: Help with blowing transistor NAD T752 receiver

My speakers are fine, when I got it running yesterday they sounded great. After the pop, I checked them with a meter and got 4 ohms on each.

[rddube]

Re: Help with blowing transistor NAD T752 receiver

Transistors and diodes can leak at the junction and break at high current (floating fault) and this tester will not show you this.

Hi, what do you mean by floating fault? And do so I guess you are saying that I'm losing my time trying to repair this?

[lotas]

Re: Help with blowing transistor NAD T752 receiver

Floating (sometimes it works, sometimes it doesn’t work), this is when you check the transistor with a tester or multimeter and they show that it is working, but when it is in the circuit under voltage and high current, it starts to heat up and lose its properties ...

[rddube]

Re: Help with blowing transistor NAD T752 receiver

Any suggestions on where I should look, what component should I test? Please, anything?

One thing I noticed, I am attaching a copy of the board view of the Front Center amp - notice the circle in yellow, it shows a connector J504 that has V+ V- and Agnd on it. This comes from the surround amplifier and I measured +- Plus 63V on the + side and minus 63V on the - side.

It doesn't appear at all on the schematics that I already uploaded and this time I am uploading the complete service manual with all boards and schematics. 63V sounds a bit high but then, when it was working the sound was just perfect - but didn't last more that a few hours before Q510 blew bringing with it Q515.

Any help, any thoughts I would definetily investigate. Many thanks!

[petehall347]

Re: Help with blowing transistor NAD T752 receiver

ok when transistors short it can take other things with it . check everything .
also if replacing complementary transistors replace as pairs . like drivers and output pairs .
aso be sure check emitter resistors and current limiter resistors .
you could have a bad bias pot or bad solder connection in the biasing circuit .
also there may be high frequency oscillation happening that can only be seen on your scope if you have one .

[rddube]

Re: Help with blowing transistor NAD T752 receiver

ok when transistors short it can take other things with it . check everything .
also if replacing complementary transistors replace as pairs . like drivers and output pairs .
aso be sure check emitter resistors and current limiter resistors .
you could have a bad bias pot or bad solder connection in the biasing circuit .
also there may be high frequency oscillation happening that can only be seen on your scope if you have one .

Oh Petehall347, that's tremendous information - can you help me a bit as I am not an expert yet - I get it for replacing transistors..as pairs.

I checked the bias pot it is a 1K variable resistor (there are 2 others for the right and center channel) and the left channel pot acts pretty much like the 2 others ...1.2k when turned one side completely and approx. 3-4 ohms when turned the other side while diminishing is I turn it. But it's funny you say that, although it passes the test, when it was playing ok I adjusted the bias voltage to 1mv as stated in the manual, but turned completely clockwise as the manual states to start, I was getting already 1mv, so no need to turn it. On the right channel, I had to turn it approx half way to get it to rise from 0 to 1mv.........hmmmmm.

Ok, I do have a scope but not sure how to test that...i.e. can I test it with the board out of the amp? Like send a 1khz sine wave along the signal and check it at the output?

Thanks again for your input!!

[R_J]

Re: Help with blowing transistor NAD T752 receiver

Any suggestions on where I should look, what component should I test? Please, anything?

One thing I noticed, I am attaching a copy of the board view of the Front Center amp - notice the circle in yellow, it shows a connector J504 that has V+ V- and Agnd on it. This comes from the surround amplifier and I measured +- Plus 63V on the + side and minus 63V on the - side.

It doesn't appear at all on the schematics that I already uploaded and this time I am uploading the complete service manual with all boards and schematics. 63V sounds a bit high but then, when it was working the sound was just perfect - but didn't last more that a few hours before Q510 blew bringing with it Q515.

Any help, any thoughts I would definetily investigate. Many thanks!

J504B connects to V+ and V- of J781B, via JMP58~61, it will have the full +_ supplies on it. (see page 17) The jack could be either just a test point or it is for a different model.