Help finding a 15v zener 1 watt to replace .4watt

[Voxman]

Hi Everyone,
Hoping someone has the knowledge to help me find a replacement zener diode with the same characteristics as the one I want to replace but with a higher wattage rating.
The zener diode is a HITACHI manufactured HZ15L-3 as specified in the NAD service manual. I could not find the exact diode but came close I think from Digikey. Their part number is HZ15-3LTD-E and specs to .4 watts. The circuit these diodes and a few resistors are very hot to touch with no audio applied. 2 of the 8.2K 1/4w carbon resistors are also very hot and are discolored. The circuit board in the area is discolored too and was what caught my eye to the potential problem. Since the receiver works fine (at the present moment) I would like to keep it that way and think perhaps replacing these components would lessen the chance of something going south. Of course there is a possibility that something already is wrong but I couldn't find anything out of spec or voltages wrong in this area.
With those thoughts, I had hoped to find a very similar zener diode with similar specs but at a higher wattage. I may be wasting my time and money but this is a really nice NAD receiver and I'd like to keep it working as long as possible.

Thanks for time and any advice on a suitable replacement is much appreciated.
Gary

[fzabkar]

What about 2 x 7.6V, 400mW Zeners in series?

HZ7L-C2

https://suzushoweb.com/pdf_file/56e62e1481d2a.pdf

HZ7C2L

https://rocelec.widen.net/view/pdf/wmk9lvuhon/RNCCS08552-1.pdf
https://www.digikey.lv/en/products/detail/rochester-electronics-llc/HZ7C2L-JTA/12110255

[Voxman]

Well yeah I reckon that would work and I'd get to use the OEM diode. I'll keep it under advisement.

Thank you very much for your response sir.

BTW I've been attempting to upload a photo of the circuit board and hope I can use this reply to send it.

Gary

[coppercone2]

elevate the replacement parts with a P - bend so their not right on the board and have some air flow

what makes a zener hot is the load drawing more current then it should. for instance drifted resistors, degraded transistors.

like if there is some load circuits powered by transistors desolder their resistors and check em out.

[Voxman]

I've been analyzing the circuit these components are part of and determined they perform an impedance switching to 8ohms or 4 ohms. You can see the switch part of the circuit. Since everything checks out okay so far, I'm puzzled why this peripheral circuit would be trying to cook fingers. The amp plays fine. Voltages provided all are spot on or very close. No distortion with sine wave input at rated power.
Someone suggested that the diodes and resistors getting hot would be causing by excessive current flowing through.Yes, true but I can't find anything out of the ordinary. So is this a design mistake or is there really something causing this.
BTW, in case I didn't mention it, the components get hot without speakers connected and no signal applied.
I'm going to replace the 15v zeners along with the 8.2k resistors ( which all tested fine out of circuit).
After that, I'll see if this issue persists and I can jump back in to do more troubleshooting.
I believe the zener diode used is obsolete so I may have to find a substitute however I'm still wanting to upgrade the power rating on diode and resistors.
Gary

[coppercone2]

the board damage won't be helped too much by upping the power because it still needs to dissipate heat. It's not like its gonna have less heat generated. Its spread out over a larger area, but elevating the component is huge compared to putting a slightly fatter cylinder

a 5 watt diode might do better, but the diff between a 0.4 and a 1 watt diode IMO is small for heat dissipation area for a board. And the convection cooling is huge. There is static pressure under the part. If you lift it up there is IMO gonna be a big air flow increase.



the AK-

To repair the board, you really need to get like a mini carbide ball mill on a dremel, or tedious work with sand paper, then refill with a board repair epoxy mixed with fiberglass.

Not to mention, those components will be more squished together. IDK how much of a benefit it is for the PCB. I think its dubious to expect a vast improvement.

[Voxman]

Thanks so much for the advice and ideas.
I ended up cleaning the solder side of the board and it cleaned up nicely. There isn't any damage - what I found was just black coating that I removed with alcohol swabs. So that problem is nigh.
I think you're absolutely right about the value of increasing wattage ratings. Stands to reason one would have to have a much larger physical size in order to dissipate the heat to a safer level. Still the same amount of heat being generated but spread over a larger area. The only reason to go to higher wattage rating is to protect the components from stressing under near maximum wattage levels. I guess I can calculate the amperes and power using the known resistance and voltage drop. That may be useful in deciding there is really something wrong..
The components in the circuit are already way off the surface of PCB. Only the resistors are snug to PCB and I plan to raise them up off the PCB.
Anyways it gives me something to do this winter!
Thanks again,
Gary

[coppercone2]

that board looks like the cheap crap and likely the components are much more durable then the PCB. And again if you lift the components they get more convection cooling and run at a lower temperature.

I replaced a zener diode with a bigger one in my welding machine (igbt driver supply) and the bigger diode did not make much of a difference on a thermal camera test. Ceramic made a bit of a difference compared to plastic IIRC. The board was also damaged there... but I am talking small difference I think like less then 10C, and it was ugly and scrunched together. I don't think there is component magic you can work, it needs either heat sink or elevation and further then that, some kind of forced air if absolutely required. I wanna say the difference between a 900mW? and a 1.5 watt zener diode was 5 degrees C on the thermal camera.

And also, if there is a catastrophic failure, the elevation will probobly prevent grevious board damage with the lead bend.

Also, the diode lasted approx a few seconds longer (something was messed up causing them to bust, I replaced the diode like 10 times trouble shooting the PCB lol. A bigger resistor is impressively more tough, it seems that a bigger diode.. not so much. It seems that when zeners fail, they fail quick, regardless if a slightly higher power is substituted. I figure a 5 watt diode might take the condition a little longer but in the end, that might just be a bigger glowing turd on the board doing more damage. I don't renember exactly, but I think I even tried to put a 2 or 3 watt diode in there, and it still got smoked mad fast. I just wanted more time to see if I could find something that was heating up. With a resistor you can sometimes put a beast in place temporarily to let the circuit run for a bit to see what is over heating without just shorting it out or whatever, at the risk of blowing up a source.... but in some cases you have spare parts for a simple easy to access source when the trouble is hidden in a mess of complicated bullshit . So long you don't damage a transformer, replacing some power transistor after you are done.. who gives a shit? (especially if you are worried about sequencing problems or whatever to prevent you from easily running a power supply into that node)

Maybe I am wrong, but empirically I noticed that, the 'overload' resistance of a diode is meager in relation to size compared with a resistor, in typical circuit conditions. IDK if its the heat being generated mostly in the junction (spot heating) or what, but it seems to just not have much more strength, while a power resistor gets more durability scaling.

[CaptDon]

Part of the answer to scaling in power and size and why a resistor is more durable. Let's say a 1 watt resistor runs very hot so you put in a two watt replacement and raise it up off the board a bit. It will still emit that one watt of heat but it is possibly spread across a bigger mass and may run slightly cooler. The 2 watt resistor could also take a circuit failure that placed 1.4 times as much voltage across it until it ran as hot as the 1 watt resistor it replaced. What happens in a circuit failure that tries to put 1.4 times more voltage across the knee voltage of a zener......Smoke!!! Also most zeners tend to be in parallel with the load and act as a regulator after a resistive voltage drop supplying the zener and load current so it is when the load current goes down that the zener must absorb the current that the load once consumed and then the zener will fail, and not usually from 'excessive load'. 99% of the zener failures I have ever encountered were from 'money saving designs' where they had .5 watt zeners running at .6 watts and more. I have repaired some of the subwoofer 'plate' amplifiers and it was criminal what they were getting away with!!! 15v .5w zeners running closer to .8 watts and the dropping resistors which were 2 watt carbon running at 2.5 watts or more. Cheap chinese garbage!!! Works about a year and dies and when you start troubleshooting you realize Everything in it is underrated for the job. Total garbage!!

[coppercone2]

if its incorrectly specified then thats another story