Help with blowing transistor NAD T752 receiver

[petehall347]

Re: Help with blowing transistor NAD T752 receiver

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.

measure across to calculate the current .
and with transistors base to emitter for vf . i often measure this to see if the transistor is likely ok .

[eccerr0r]

Re: Help with blowing transistor NAD T752 receiver

yes, measuring across is useful to calculate current or at least know the differential between two points. In this case actually this is VERY important as this tells the spread between the two bases of the driver transistors. The farther they are apart, the more likely that they will both be on ... and kill each other.

Measuring the voltage WRT to GND of one pin on a resistor without telling which one it is...now this unfortunately is not very useful to me...

Thanks for the schematic, now know what resistor you're talking about.

[rddube]

Re: Help with blowing transistor NAD T752 receiver

yes, measuring across is useful to calculate current or at least know the differential between two points. In this case actually this is VERY important as this tells the spread between the two bases of the driver transistors. The farther they are apart, the more likely that they will both be on ... and kill each other.

Measuring the voltage WRT to GND of one pin on a resistor without telling which one it is...now this unfortunately is not very useful to me...

Thanks for the schematic, now know what resistor you're talking about.

Ok, now I get it..sorry, I'm not that experienced but you're right and thank you for the explanation..I will retake measurements on both pins of both resistors R5143 and R5146 and report back. I will show on the diagram the voltages for you. Keep in mind that bothe Q509 and Q510 + Q521 and Q522 are not installed. Tks again!

[rddube]

Re: Help with blowing transistor NAD T752 receiver

Here are the results of the measurements in the photo.

[petehall347]

Re: Help with blowing transistor NAD T752 receiver

please take note whether its positive or negative voltages .this is important when using ground as reference .
example .. 56v ....-56v ..

[eccerr0r]

Re: Help with blowing transistor NAD T752 receiver

Incidentally, if you short Q518/Q506's E and B together (or short B to the negative rail), this is one way to cause the downstream resistors and transistors to blow. Trying to figure out what you could have done to short them by a probe slip, as the pinout of the transistor does not make this easy as C is in the center...

Theoretically other permutations (C-E short or C-B short) should not cause failure of the drivers as far as I can tell...

50V on either pin is still way too high IMHO and the spread between base pins, still would suspect Q565/Q562 short assuming +50V at the resistor (versus -50V)...what's the voltage at B and E of these two transistors?

Thanks Pete, yes, the sign of the voltage is also very important too when measuring WRT to GND. This will help pinpoint which side of the two (upper, lower) to look for clues...

[rddube]

Re: Help with blowing transistor NAD T752 receiver

please take note whether its positive or negative voltages .this is important when using ground as reference .
example .. 56v ....-56v ..

Redid the readings and they are all positive + both sides of each resistor.

[rddube]

Re: Help with blowing transistor NAD T752 receiver

Incidentally, if you short Q518/Q506's E and B together (or short B to the negative rail), this is one way to cause the downstream resistors and transistors to blow. Trying to figure out what you could have done to short them by a probe slip, as the pinout of the transistor does not make this easy as C is in the center...

Theoretically other permutations (C-E short or C-B short) should not cause failure of the drivers as far as I can tell...

50V on either pin is still way too high IMHO and the spread between base pins, still would suspect Q565/Q562 short assuming +50V at the resistor (versus -50V)...what's the voltage at B and E of these two transistors?

Thanks Pete, yes, the sign of the voltage is also very important too when measuring WRT to GND. This will help pinpoint which side of the two (upper, lower) to look for clues...

I had installed 2 wires on E and on C (to measure between E to C) on Q506 and Q518, and it's when I was taking readings on the E side of Q518 that I slipped and touched the C side of Q518 and that's where I heard a noise and the resistor become hot because the output transistors had gone short.

"50V on either pin is still way too high IMHO and the spread between base pins," remember when I had good output transistors before I cooked them by accident, I was getting -6.08V at one of the 2 sides of R5146, and now I am getting 58V without the output transistors. Could they be required by any chance?

As for the readings on B and E on Q565/Q562 I will have to dissassemble and solder some wires as it is way too crowded down there with not much room to go and probe. Do you want me to to this? I am attaching a photo of what it looks like down there, it is under that little ceramic filter that you see in the photo.

[eccerr0r]

Re: Help with blowing transistor NAD T752 receiver

The resistor getting hot is the reason (or a causal symptom) why the output transistors shorted. When the resistor gets hot, there is a huge differential across the resistor, which means the bases of the output transistors are wider apart -- and because of that, they both can be on at the same time, causing a rail to rail short, heating the transistors and burning them. So no, the output transistors shorting would not cause the resistor to heat up, rather it's collateral failure of an upstream issue.

Well don't know what to say, without data it's hard to make a judgment to pinpoint the rootcause... no access to the bottom of the board?

BTW you do have center channel and surround channels that are still intact? If you're not comfortable probing, don't short and cause more damage. One tech tip is to tape up the DMM probes so that only the tips are exposed, might help against some accidental shorts but won't help bridging adjacent pins.

[rddube]

Re: Help with blowing transistor NAD T752 receiver

Hello Eccerr0r, I can still do some measures, but some of them I have to install wires before, which is what I did for E and C of Q506 and Q518 (in their case the emitter and collector pins are in the back at the bottom of the board - really not easy to access) and with the wires was able to make measurements. Even then my shaky hand touched the E and C wires of Q518 simultaneously, but I'll just be more careful. Let me know what it is you would like me to measure. Just got notice that the new transistors are on their way from China and should be here within2-3 weeks.

[eccerr0r]

Re: Help with blowing transistor NAD T752 receiver

BTW do you have a "Dim Bulb Tester" jig? It may help against shorting mishaps in case you do want to compare voltages between other working channels?

[rddube]

Re: Help with blowing transistor NAD T752 receiver

Yes I do have a Dim Bulb Tester.

[rddube]

Re: Help with blowing transistor NAD T752 receiver

So what do you think I should do now, wait for the new transistors to come in (I ordered 4 pairs, so will be using 2 pairs to put it back into working order), and then test it?

I suspect there will be a difference at each end of R5143 compared R5146, as if you look at my previous posts R5146 was measuring +- 6V when it had the output transistors in there (yes on one of the sides of the resistor, but it was much lower than 55V).

I checked the voltage at J504 that has V+ V- and Agnd on it. This comes from the surround amplifier and feeds the Front/Center amplifier and I measured +- Plus 63V on the + side and minus 63V on the - side.

[eccerr0r]

Re: Help with blowing transistor NAD T752 receiver

Up to you what you want to do. More measurements would be helpful.

The surrounds and center have equivalent resistors as R5143 and R5146 at R893, R896, and R5149 but we know the main l/r channels are messed up.

Still would like to know the voltages on the leads of Q506/Q518 (Center: Q530, surrounds Q811, Q812) so we know where the bias is when there's no input. If there is a bias that turns on all of the 6 transistors at the same time, then we can figure out whether to keep going upstream or not.

For the original channel with R520/R5143 the buck should stop at Q561/Q505/Q504/D501/D502. Beyond (before) this set of transistors it should not cause final transistor frying. Also chances are these three transistors are good else you would not hear anything when it was working for the short period of time. Q504 however could be kind of broken and you can still hear music... Make sure the base of Q504 is ~ -55V WRT to GND (caused by an open diode), if it's higher than that, it could cause problems...

[rddube]

Re: Help with blowing transistor NAD T752 receiver

Ok, I'll take more measurements. I don't think we can use R893 and R896 as reference as they are on the other side of the heatsink on the surround amplifier and right in front of that amplifier is this humungous transformer that I would need to take out to gain access.

So just to make sure I get the right things, I will measure Q506 both E and C to ground, Q518 E and C to ground and Q530 (which is the good Center channel) E and C to ground. I'll also check Q504 when the board is out, maybe replace it and I will solder a wire on the base before I put the board back in to measure base to GND for Q504.

Will also check both sides to ground for R5143, R5146 and R5149 making sure I note the polarity of the voltage. Will report back, and if I forgot anything please shout! Tks!!

[rddube]

Re: Help with blowing transistor NAD T752 receiver

Ok, so here are my measurements:

Q506 C to gnd +61.07V
E to gnd -61.2V

Q518 C to gnd +61.10V
E to gnd +57.73V

Q530 C to gnd +1.76V
E to gnd -1.62V

Looking at board such as the photo I uploaded:
R5143 from left to gnd +58.6V
from right to gnd +58.56V

R5146 from left to gnd +60.1V
from right to gnd +57.9V

R5143 from left to gnd +1.17V
from right to gnd -1.049V

Q504 B to gnd -61.54V

I double checked every measurement.

[eccerr0r]

Re: Help with blowing transistor NAD T752 receiver

Oh yeah duh...forgot. Yeah you're right, you do need to have the finals in, else the diff pair will go bonkers and that's why we're seeing the output railing. The voltage differential on the resistors is more useful information.

The second Q518 channel probably will work just fine with new finals and possibly new R544 put in, the spread appears to match the good working channel Q530.

However the Q506 channel (the first one), it's a bit too close at least when comparing to the other channels. Having them close is good only because it won't let both transistors turn on and kill each other, but it also means something else is wrong. As the voltage drops of the transistors look fairly normal, they probably are working fine. Is the relative position of the R522 potentiometer close to the position of the R545 potentiometer (don't touch either for now, just look at position). Perhaps R522 is turned all the way down for 1KΩ and hence the voltages are so close.

It is good that B-Q504 is very negative as it should be in quiescent state.

I suspect you may well need to insert the finals or sub some others in its place for the time being for further debug. Do ensure that you can see the spread between R5143 terminals and Q506 E and C change as you tweak R522.

[petehall347]

Re: Help with blowing transistor NAD T752 receiver

you can use resistors like i said instead of the outputs for testing purposes .. once you get about +-600mv at the drivers emitters and close to zero volts at the speaker terminals it should be ready for new outputs

[rddube]

Re: Help with blowing transistor NAD T752 receiver

you can use resistors like i said instead of the outputs for testing purposes .. once you get about +-600mv at the drivers emitters and close to zero volts at the speaker terminals it should be ready for new outputs

Ok, so I will try that i.e. 1K resistors. How do I get it though to go to +- 600mv at the drivers? With the R522 variable resistor or by diagnosing further until we find the culprit? Will install 1K resistors and retake readings and will report back. I guess I should focus on Q509 and Q510 for now? So just to make sure, it's 1k resistors from where base and emitters q509 q510 connect to on the board? Tks again!

[petehall347]

Re: Help with blowing transistor NAD T752 receiver

yes base to emitter. don't adjust anything yet .