I have to NAD T762 on the table. Both with burned out (shorted) 5,1V zenerdiodes (D718).
I have replaces them with a new 5W type instead of the original 2W.
But the diodes is getting extremely hot and I can't figure out if this is ok (like the way it was designed). They are sitting directly in series with a
7812 from a 22V DC supply. The thought behind the design must bee to divide the power loss between the zenerdiode and the 7812.
The amplifier is working, but I don't like those hot zeners!
Is there anybody out there that have worked on a NAD T762?
On Wed, 29 Aug 2007 17:57:08 +0200, "Peter Andersen" put finger to keyboard and composed:
If there is a series resistor, then I'd measure the voltage across it. That should give you the current, which in turn should give you the actual dissipation in the zener. A 5V1 zener dissipating 1W would be passing 200mA. Any more than that and I'd be looking for a culprit.
- Franc Zabkar
--
Please remove one 'i' from my address when replying by email.
Well I must say, Jamie, I am having some difficulty following those lines of reasoning. If the nominal supply is 22v, and a 5v1 zener is glued in series with that to the input of the 7812, that's going to leave around 17v on the input to the reg. So why is that a problem in as much as you think that the starting voltage should be higher than 22v ? I must confess that I don't really see the point of reducing the input volts by 5 - either 17 or 22 seems like a perfectly valid amount to me, just that at 22v, the reg might need a bit more heatsinking on it. However, on that point, if the original zener was rated at 2 watts, then allowing for some margin and derating to say 1.5 watts, that would allow for a maximum current of 1.5 / 5 or ~330mA, so that would imply that the designer was not expecting more than this to be drawn from his 7812, even though these, in their commonest form, are rated to an amp. Now if that is the case, the 7812 is almost idling anyway with
17v on its input, so I really can't see it caring much, temperature -wise, about having 22v on it rather than 17v.
However, something else just occured to me. The OP was taking about zeners in the plural - two I think. So is that two zeners, in series with each other, and then in series with the input to the reg ? If that is the case, then the maths certainly doesn't stack up, as 10v will be lost across the zeners, leaving only 12v on the input to the regulator, which clearly isn't enough, as Jamie suggests might be the case.
As a matter of interest, as these zeners had apparently failed short circuit, and we know that the 7812 can take up to 36v on its input, how did the OP know that there was a problem, given that he says that the amp is apparently working ok ?
I thought he was suggesting that the starting voltage should be ~40V, and that's why the zener is needed (to get below 36V). Since the amp is working with 22V as the starting voltage, this doesn't seem likely.
As I read it, there are two amplifiers, and the diode at location D718 is the zener in question. Or is D718 a part number?
That is the partnumber on the schematic (if there should be a NAD technichan out there he would know..)
No, there is only ONE zener in series with the 7812. The supply is 22V. The
7812 can be driven with 22V directly, so I located the _shorted_ zener by the smell :-) In the other amp the zener is leaking with 300ohm in both directions.
I got a mail from someone that confirms the theory about the divided heat dissipation. This 7812 is supplying the tuner and preoutboard (with digital volumecontrol, a lot of opamps etc.)
The question is now: is there drawn to much current from this zener/7812 circuit or is it normal(bad bad design!). Who knows?
Peter. As far as whether there is too much current being drawn, the absolute maximum, which would take the original zener up to its 2 watt limit, is
400mA, which is obviously well within the capability of the 7812. However, if I had been designing the thing, I wouldn't really want more than about
1.5 watts going west in that diode, (not that I would even have put it there in the first place ...) so we're talking around 330mA. That would leave around 4 watts or so in the 7812. Now that is basically nothing, especially if a small heatsink is fitted, so I don't really see the point in having the zener there to split the power dissipation. Maybe the 7812 is running without a heatsink, and putting in a 2 cent zener was cheaper than a 3 cent heatsink, but either way, unless we are missing something here, it seems like a nonsense piece of design work to me.
I still don't really follow Jamies point about the zener being placed in a "high resistive state". Once the zener 'knee' point on its curve has been reached, that curve becomes pretty much vertical within a very small span of reverse current. With the full rail current of 300mA or possibly more passing through the one in this bit of kit, it should be in it's lowest resistance fully on condition.
The point about a poor smoother, and excess ripple resulting in a low average measured starting voltage is ok in principle, but if the cap was really in that virtually open circuit condition, I would have expected to have heard hum on the amp's outputs. The OP said that it was working ok.
I'm interested in the failures of these zeners, and how they were detected by "smell". In the case of the one that had gone short, it can't have been the diode itself that was smelling, as its dissipation must have dropped to zero. That must mean that any power that was being 'lost' in it, was passed on to the 7812 to lose, so was it this that was smelling to give the clue ? If it was, then I am surprised, because if its dissipation had risen to the point where it was smelling, I would have expected its SOA thermal foldback to have kicked in. As for the zener that had gone 300 ohms (both ways) leaky, it's hard to see how that circuit even carried on working, as all voltage input for the 7812 would have disappeared across a 'resistor' of this value placed in series with the regulator input, with just a few 10s of mA being drawn.
It's all very curious. Given that the zeners have failed in normal service in two cases, my inclination would be to short them out and let the regs have the full 22v up 'em, and add a couple of heatsinks. Those circular crinkly ones actually work very well, when the gap in the circle is forced down over the devices's tab. I learnt that from a commercial board that I repair, that has 3 TO220 Darlington motor driver transistors on it, heatsunk in this way.
Unfortunately, some of these diodes do run hot. D716 is also doing the same thing as D718 in the T762 for a different 12V regulator, so I'd see how hot that diode (D716) is getting. Of course if the current load is different on the regulator, the temperature may be different as well. I don't have a T762 here to look at right now.
I would certainly agree with the other poster that the input capacitor coming off the bridge rectifier (C704, 3300uf/35V) could be in trouble and high ripple could cause problems. This can easily be scoped by looking at the cathode end of either D718 or D716.
I have had problems on this series of NAD's with the bridge rectifier diodes D701-704, especially if there is the brownish glue on them. They often have horrible solder joints, and sometimes fail. I replace them (originally they are 1N4004)with 1N5399 diodes which are rated for 1.5 amps. I also stand them off the board a bit for better cooling. If you are running with only 3 of these diodes connected, you'll certainly have higher ripple on the cap.
While you have the board out (lucky you) I'd also check D705-708 in the same area and maybe replace them as well. Also check soldering in general of the regulators. You don't want to have to keep removing the board as it is rather time consuming and annoying.
Many owner's don't realize that surround sound receivers in general run quite hot inside and need good air flow around them. I suggest at least
3 inches (8cm) of open air above the receiver if on an open shelf, and a couple of inches (5cm) on either side and rear. The receiver should also be on a hard surface, not carpet or such.
To other repliers: These supplies that we are discussing are low voltage regulators for the input switching, digital board and front panel, just about everything EXCEPT the power amps.
So how much current do you reckon is being drawn off that rail Tim ? If all the logic and opamps and whatever are being fed from it, you'd have to reckon on at least the 330mA that would take the dissipation in that zener up to 1.6 watts or so. 400mA and we're at its limit. Obviously, you see a lot of these. Is it common for that zener to fail ? I don't remember ever having had one, but then if they fail short, I've maybe never had cause to go looking ?? Seems like a piece of design work of marginal validity to me. Does it reduce the temperature of the regulator by much ? Is it worth it for the dissipation created in the zener, and the apparent reliability issues which ensue ? As one service engineer to another, I would be interested in your opinions.
Don't start down that path, pal. I object to the British being collectively described by you as "trash". No one has made any comments about you as "An American" . And what are you talking about regarding solder and "passing the buck" along to you ? Lets just try and all get along without starting another pissing contest, shall we ?
Graham. Leave Jamie alone, and Jamie, stop your name calling.
I know exactly who Graham is - I have been posting on here for a very long time, so I don't need you to tell me how to sit back and monitor on this group. He has not sucked me into anything, and I am not defending him, but in the past, many people have been drawn into endless pissing contests which he may or may not have started, but has certainly helped to perpetuate, with ever-more offensive and sometimes silly retorts. I just didn't want this particular thread, which up until then had been an interesting one, to dissolve into yet another such contest, which is why I suggested that you should *both* STF up, before all of 'the usual suspects' joined in.
Whilst he might have done his fair share of 'America bashing' both on this group and others, up until this point, he, or no one else had actually done any in this particular thread. As far as I am concerned, we are all just service engineers, and except to point out from time to time that the UK and the US are two countries separated by a common language, which sometimes gives rise to misunderstandings, I really don't care what nationality you, or anyone else that talks on here, are. Which is why I object when people start dragging nationality into it, in an offensive way.
Reading again, I can see that you might have been referring to Graham as a single example of what you consider to be "trash", but it just as easily reads that he is a single example of a collective British trash. Anyway, if you intended it as just him, then I apologise, but you need to watch how you put things, and remember how easily people from different cultures can read things not in the way that you intended.
And that's all that I am going to say. If you two want to keep having a pop at each other, do it by direct e-mail, and let's just keep it friendly on here, with disagreements being 'robust' rather than spiralling down into endless tirades of offensive crap. OK with that ?
I've never measured the current on the rail, and while I've touched up soldering on the zeners, I've never replaced one. On the other hand, I've never checked them either, so while I may have let a shorted one get by, I've certainly not seen an open one. I haven't worked on that many of these, maybe a dozen or so. But, I've certainly seen small regulated supplies run what I would call way to hot on any brand of surround receiver that I've seen.
I can't speak for the design, as I've not analyzed it, though I don't like any products where the regulators run hot and bake the boards. As for the bridge diodes failing, it happens most commonly when the factory manages to spill some of the light brown glue on them, effectively insulating them. The glue often darkens and becomes brittle. The solder joints get baked. I've also seen this glue be corrosive and conductive when it gets baked enough.
That would be "piece" then ... And dashvou. WTF is that ??? Are you struggling for the spelling of deja vue ? (should have accents over the 'e' and 'a')
If you don't know the French, and exactly what it means, don't try to use it. And if you do understand what it means, with exactly what relevance are you applying it here ?
ElectronDepot website is not affiliated with any of the manufacturers or service providers discussed here.
All logos and trade names are the property of their respective owners.