Help with blowing transistor NAD T752 receiver

[rddube]

Re: Help with blowing transistor NAD T752 receiver

Thanks again Eccerr0r for your patience and all the help you are giving me.

Ok, so removed R518 and checked it, measures 3.291K out of circuit. Soldered it back in. Checked R542 while I as at it, and that one is ok too at 3.296K. Replaced Q506 although the former reads good in my transistor tester and installed a new one.

Put it back together, and started it without any probes attached, turns on fine goes green and goes to FM. Put the probes on and figured out what was the problem of it going into protect mode, I was holding one of the probes to those wires I soldered on Q506 with my finger - I think I was serving as sort of a ground...as soon as I clamped the probe with a clamp. did not go into protect mode and started up just fine, however without any reading.

Now, my bad...when you said across Q506 I assumed it was the same connections that I have made before, but your last post mentions C-B while I have my connections on C-E.

So instead, I took from C to ground on each channel and got Q506 @ 0.7V, Q518 C to ground @ 1.52V and Q530 C to ground @1.57V....

Let me know if I have this right, so that means I would have to go back and solder some wires from B for each of Q506, Q518 and Q530 to measure "across" from C-B ?

[eccerr0r]

Re: Help with blowing transistor NAD T752 receiver

Q506 C-E voltage is what prevents transistors from frying and prevents you from hearing odd order harmonics in your music. This is the more important number. C-B voltage I just wanted to make sure there's something trying to turn the transistor 'on' - usually R518, and stay on all of the time, regardless of what R519 and R522 (the pot) is set to. In fact I don't particularly care about the voltage here, I just wanted to see if there is any conductivity between the pins (both directions, as you'd be forward biasing C-B in one direction), as if there is none, then the transistor can't turn on properly and cause a whole bunch of bad downstream effects including frying output finals.

When you say "without any reading" what do you mean? You mentioned this twice and I had to gloss over this and ignored if you did anything with the requested measurement as it doesn't make any sense.

[rddube]

Re: Help with blowing transistor NAD T752 receiver

What I meant without any reading was 0V not voltage at all.

[eccerr0r]

Re: Help with blowing transistor NAD T752 receiver

Can't be 0V unless it wasn't powered on. Must be measurement error.

[rddube]

Re: Help with blowing transistor NAD T752 receiver

Can't be 0V unless it wasn't powered on. Must be measurement error.

Probes connected on E and C of Q506 - see photos. Getting 0V in reading with amp turned on?

Last photo shows resistance reading between E and C of Q506 using the same wires (actually I didn't disconnect anything, just turned the dial to ohms).

Maybe I didn't solder back R518 properly, but I always double check my soldering with measurements. What do you think it could be?

BTW, when I removed the board yesterday I inspected all traces with my analog stereo microscope and couldn't find anything cut. I do have to admit that where the Q506 and finals are, from removing and replacing transistors, some traces to the legs were damaged, but I bridged them back and flowed solder so as to make a good connection.

[eccerr0r]

Re: Help with blowing transistor NAD T752 receiver

I still don't accept 0V responses unless the two probes are connected together, or one or both probes missed their mark.

How about an alternate question, what are the voltages with respect to ground of the two points? That would be more useful information and cannot be 0V.

[rddube]

Re: Help with blowing transistor NAD T752 receiver

I still don't accept 0V responses unless the two probes are connected together, or one or both probes missed their mark.

How about an alternate question, what are the voltages with respect to ground of the two points? That would be more useful information and cannot be 0V.

C to ground on Q506 is +0.136V
E to ground on Q506 is +0.148V

[rddube]

Re: Help with blowing transistor NAD T752 receiver

Here is the same test done on C to E of Q518 (same probes, same multimeter). The resistance test is done with the amp turned off, just like I did on Q506.

[eccerr0r]

Re: Help with blowing transistor NAD T752 receiver

Don't know if you took basic electronics but by Kirchoff's Voltage Law, if C is +0.136V WRT GND and E is +0.148V WRT GND (which I cast doubt on because for Q506, an NPN transistor, E must be lower than C to be biased properly) then if you put the black probe on E and red probe on C, your meter should see exactly -0.012V, which is not zero.

In any case the spread (difference between the two pins) is still too low. Does the spread change when you change R522? Note that C may go up along with E going down, hence "spread" because if you measure just C, you miss the change on E.

[rddube]

Re: Help with blowing transistor NAD T752 receiver

I'm really at a loss to explain why it is behaving this way, but I still get 0V when I put the black probe on E and red probe on C. I even got another (a third) multimeter to test out and same result.

It is as though there is no activity between the E and C. Retook the voltage readings for C to gnd (got .717V) and E to gnd (got .710V) with my new multimeter, but there is no change when I turn R522 either way, still get the same readings on both C and E as if they were frozen there.

So what do you suggest, that I dismantle the whole thing and recheck the secondaries, the 1K resistors - Q506, etc. ? Tks again!

Question: could the fact that I don't have the preamp to amp jumpers connected change anything?

[rddube]

Re: Help with blowing transistor NAD T752 receiver

I see what you mean in terms of change when I do the same test E to C on Q518 and turn the trimpot, the voltage goes up and down from 2.1V to 3.7V (2.1V turned fully clockwise, so as to measure approx 1.2K ohms on the variable resistor of R543). I didn't want to play with R571 as that channel (center) has the finals and didn't want to create another problem.

So there has to be something wrong in the left channel to not behave this way.

[eccerr0r]

Re: Help with blowing transistor NAD T752 receiver

Something is clearly wrong with the left channel but I cannot help you if you report nonsensical data. It doesn't help me understand what's going on here. If you tested all the transistors as good then the only thing left is a broken trace somewhere between Q561 Q505 Q506 Q504 and their respective resistors/diodes.
So if I ask you to measure all the voltages with respect to GND:
E Q561
C Q561 (same as E Q505)
C Q505 (same as C Q506)
C Q504 (same as E Q506)
E Q504

This is like a totem pole of voltages, as you go from the top off this totem pole you start at full +60 voltage it should go progressively lower and lower until you hit -60 volts of your negative rail. Note that broken resistors such as R5141 and R513 will also get you bad results as they are also part of the pole (and one of their ends are the true +/- 60V of your power supply).

Also it is a good exercise to test the voltages of the "Same as" pins of those transistors. Since we probably have a discontinuity problem we can't trust the connection and if you find something that doesn't match despite being "same as" - you've pinpointed where you need to look. Any discontinuities between these will result in none of these working and perhaps will give you your 0 volts.

Perhaps what you need to do is copy and paste the schematic around these 4 transistors and measure/write down every voltage at every node in the schematic including between diodes and resistors. Also another thing that may or may not help is on another copy of the schematic, do the same thing with you taking a 10K ohm resistor and shunting one of the nodes to GND and see how the voltages change, perhaps you can induce voltages to show up when you claim there is nothing, and that is where you should take your magnifying glass to look for broken tracks and wires.

[eccerr0r]

Re: Help with blowing transistor NAD T752 receiver

BTW, if you want more wild guesses, see if C508 is shorted. But this only explains the 0V across C-E of Q506, once again does NOT explain Q510/Q509 burning up.

A broken track or solder joint however can explain Q510/Q509 burning up along with 0V Q506, but only you have the circuit board to look at...

---

Here's another possibility.

Q506-C track that leads to Q505-C is broken but Q505-C is still connected to Q562-B and R518. If you measure Q506-E and Q506-C directly at the transistor, you may not get any volts but it would cause Q510/Q509 to fry.

[rddube]

Re: Help with blowing transistor NAD T752 receiver

Hello Eccerr0r,

I understand your frustration having to try and help an old novice like me (67 years old ), but I am trying. I know you would probably look at this and within a minute or 2 you would have figured it out so I am very grateful for your help and guidance.

Now one thing I need to mention is that many of the test points you mentioned in your 2 previous posts are not accessible while the board is live like Q561 which I can only access by the back of the board which is about 1/2 inch from the heatsink down below with cables (boy this Nad has cables!) in the way. Also may resistors and diodes are not accessible because of their placement and the other components and cables in front of them. But I'll try my best like solder a wire on each point of Q561 to take measures there, also I think I can get measures of every point on Q505 and Q504 directly.

I have the board out now, checked C508 and it is fine not shorted and reporting 149.7 nf. Will check all tracks again and components along with the totem pole voltages once I've installed sufficient wires making sure not to short anything. Will be back, and again thank you for your patience.

[rddube]

Re: Help with blowing transistor NAD T752 receiver

BTW, if you want more wild guesses, see if C508 is shorted. But this only explains the 0V across C-E of Q506, once again does NOT explain Q510/Q509 burning up.

A broken track or solder joint however can explain Q510/Q509 burning up along with 0V Q506, but only you have the circuit board to look at...

---

Here's another possibility.

Q506-C track that leads to Q505-C is broken but Q505-C is still connected to Q562-B and R518. If you measure Q506-E and Q506-C directly at the transistor, you may not get any volts but it would cause Q510/Q509 to fry.

When yous say "Q506-C track that leads to Q505-C", I can't seem to figure it out in the schematics? The track that goes from Q506-C, passes through C508 and goes to Q505-E, no? Tks.

[rddube]

Re: Help with blowing transistor NAD T752 receiver

Even though I can't see it on the schematics, Q506-C track to Q505-C reads 0 ohms when measured, so the connection is good. Checked on the same connections on Q518 and Q517 both collectors and got 0 ohms also.

[eccerr0r]

Re: Help with blowing transistor NAD T752 receiver

C518 is parallel with C-E of Q506 and is a filter capacitor for that voltage.
Q506-C is supposed to be connected to Q505-C along with Q562-B.

Conversely also ensure Q506-E is connected to Q504-C, where likewise if this connection is lost and Q504-C is still yanking down Q563-B since they're all supposed to be connected, you'll not see R522 change C-E voltage of Q506 and end up blowing transistors.
The final connection of ensuring Q506-B unfortunately won't cause C-E of Q506 to go to 0.000V (saturation voltage of a BJT will never get this low) so that has to be ruled out.

All in all I'm just saying if one plucks out Q506 while the amp is on, Q506 will "test good", C-E of Q506 will measure 0V (since it's out of circuit), R522 will have no effect on the C-E voltage of Q506, and the finals will overheat and blow.

I did not previously request Q504-C voltage because it's theoretically connected to Q506-E, but do test how many volts there along with Q504-E.

[rddube]

Re: Help with blowing transistor NAD T752 receiver

Eccerr0r, I owe you an incredible apology, I am so sorry. I could have made up all sorts of theories but here is the bottom line truth.

When I said I had checked all transistors on the left channel, it was true I had checked them all by removing them and resoldering them one by one. I then installed wires on Q506 C and E and put a piece of tape to hold them on the back of the board. That's when I took my readings and was getting 0V.

Your latest posts had me going back into checking traces and components, resistors, etc. etc. Lo and behold, when resoldering my transistors I missed the C leg of Q505, it was not soldered for crying out loud!

I'm so sorry Eccerr0r, really really sorry. This time around I recheched everywhere where I had removed a component and cheched all my soldering. And I promise to be more careful and to triple check next time.

So I will reassemble and take voltage readings again, unless you think I should do or check something else before, while the board is dissassembled.

And THANK YOU for your incredible patience!

[eccerr0r]

Re: Help with blowing transistor NAD T752 receiver

It's allright, but it does lengthen debug time when I don't have access to the board and actually can see the status of the board. Thankfully this does explain the weird behavior you're seeing and at least that mystery is solved.

That being said, not having Q505-C connected would indeed give you no reading on Q506. However it would not explain frying finals and the channel would not make any music.

Do come back with new voltages once this is reconnected and maybe you can see Q506 C-E voltage vary with R522 changing now.

[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.