Need Help Identifying Zener Diode in Sony NS999ES DVD Player

My Sony NS999ES DVD player won't power on. Checking out the switching regulator board, I found a blown electrolytic capacitor and glass encased Zener diode with scorch marks on the PCB underneath the diode. I replaced the blown cap, but I cannot determine the specs of the Zener diode. On the schematic, the only notation next to the diode in question, D302, is "HZ188P." I've searched everywhere and cannot find any reference to this marking. C402 was the blown cap I replaced.

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Unfortunately, Sony's service manual for this player provides a parts list for everything except this switching regulator board, ZSSR107G. Apparently, if something on the board goes bad, one is expected to buy the entire board from Sony for over $100. I'd rather just spend the few dollars to solder in a replacement Zener diode. Any help is greatly appreciated.
 

Mooly

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2007-09-15 8:14 am
The circuit image is to blurry to see. Can you rescan it or at least put a mark against the two parts. Is it the diode marked here... if so it looks like it's used as overvolts protection. A crowbar, that just fails short circuit if the PSU output increases. If so and the PSU is OK then it will work without it for test purposes. As a safety measure it should be fitted when all is OK.

I used to be repair tech and fixed countless SMPSU's over the years, loved working on them and identifying failure modes and so on. Most issues are down to dried out electrolytics so I would replace ALL on that board that in positions that pass high ripple currents. You need to at least do a quick "diode" check across the power devices and rectifiers to make sure non are short circuit. Any caps you replace should be of the same physical size and lead pitch and also of good quality. e.g. 105C temperature rating and specifically designed for SMPS applications.
 

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Hi Mooly,

Thank you for much for the prompt and thorough response. You did circle the correct Zener diode, D302, at the top of the schematic. C402, the 1500uF/16V capacitor is the cap circled with a question mark. This is the blown electrolytic cap that I replaced with a Panasonic FC 1500uF/25V (same lead spacing as blown Nichicon cap).

Please try this link for a higher resolution scan of the schematic.

http://img231.imageshack.us/img231/9407/sonyzssr107gswitchingre.jpg

Unfortunately, Flickr reduced the size/resolution of the image I uploaded.

I am a novice when it comes to these switching power supplies. Can you elaborate on doing a quick "diode" check across the power devices and rectifiers to make sure none are short circuited?

For reference, the output voltages are:

+3.3V, +5V, +11V, +8V and -8V.
 

Mooly

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2007-09-15 8:14 am
That's a much better image.

If the PSU is non working then damage caused by the caps could literally be anything.

So you need to check IC101 pins D to S are not short. Same for D401, make sure it hasn't failed short circuit. Also check IP402 (circuit protector... a fuse) which should read short of course.

I would try and get the PSU working, then look to replace the other caps that are in stressed positions. I can scan and circle the ones to replace later. The fact that the zener is short means the rails have gone high. So is the failed cap the only issue, or, has something else happened and excess voltage has seen the cap off.

I'll have more time later to copy the image and mark what to check...


Be aware that the primary side caps can be charged to full mains potential even when off under certain conditions.
 

Mooly

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2007-09-15 8:14 am
Forgot... diode check means using your DVM on "Diode" rather than just ohms range. It will show 0.00 or something very low if any device is short. A diode forward biased will read around 0.6 on the meter on that range. We are really checking for obvious shorts though.
 

Mooly

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2007-09-15 8:14 am
Very quickly here is what to change and check initially. The caps are all on the rails concerned with monitoring and feedback control. You should be OK with 47uf caps instead of 33uf. Even the 1500uf should be OK with 2200uf which are more common values.

I can't stress enough that the main failure mode of this type of PSU is caps. For what they cost I would replace all the electros.
 

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Wow! Thanks Mooly for the thorough repair and test information. I will definitely check the rectifiers and power devices in the circuit as you outlined.

I am still wondering about that Zener diode, D302, and what to replace it with. My thought is that it could be a BZX55C- series Zener diode, but I am not certain.
 

Mooly

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2007-09-15 8:14 am
Diodes used as crowbars tend to be unique as they not only have to fail short circuit but also withstand the current without literally shattering. An ordinary zener wouldn't survive. Remember though that to test and repair that the diode can be totally omitted.

Also as the PSU is self contained I would perhaps run it up on it's own (not connected to the DVD player) and tag a 60 watt mains filament bulb in series with the mains to limit any damage in case of some undiscovered fault.
 
Hi Mooly,

As you instructed, I tested the power devices and diodes and everything was fine. I connected the variac to the unit with only the switching regulator board connected, and all the output voltages tested fine. Next, I connected everything up, and the unit's playing fine. I'll order new electrolytic caps. It probably doesn't make a difference, but I usually order Panasonic electrolytic caps from Digi-Key. Thank you very much for the prompt and thorough responses.

Stephensank,

Thanks for the information regarding the Zener diode.

I will obtain a replacement Zener diode per the suggestions.
 
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Mooly

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2007-09-15 8:14 am
That's great it's working... definitely replace the caps. Look for high ripple current ratings and low e.s.r.
Variacs... hmmm. Good though they are, not all smps like a slowly increasing voltage and they can either fail to start or just conduct heavily and blow the switching element. This one obviously is OK, something to bear in mind though.
 
That's great it's working... definitely replace the caps. Look for high ripple current ratings and low e.s.r.
Variacs... hmmm. Good though they are, not all smps like a slowly increasing voltage and they can either fail to start or just conduct heavily and blow the switching element. This one obviously is OK, something to bear in mind though.

Well, that just goes to show how unfamiliar I am with switching power supplies. Thanks for the tip on powering up the PSU. So far, the player is playing very well and sounds great. It was modifed by Dan Wright of ModWright Instruments. He's well known here in the U.S.

For replacing the large 270uF/250V electrolytic cap in the primary stage, can I go with a higher capacitance of 330uF/250V, or should I stick with the exact same value? I generally like the Panasonic ED and EE series Aluminum electrolytic caps and use them in power supplies for my tube amp projects. For the other caps, I'll go with either Panasonic FC or FM caps with low ESR, high ripple current ratings and long life.

I noticed the diode you recommended is not a DO-35 glass bodied Zener diode. Does that matter?
 

Mooly

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2007-09-15 8:14 am
A 330uf is fine.
As to the zener, it wasn't so much as to specify an exact replacement (as that depends on what you can get) but rather to highlight the specification of this type of device. The package doesn't really matter as long as it fits neatly.

It was only after I posted that I saw Stephen had mentioned ordinary zeners, which made my comment on zeners look as if it was directed at Stephen... which of course it was not.

If you look at the diode specs in the link, not only are things such as the peak current high (30amp) and peak power (400watt), but the device is also specified to fail in a short circuit mode if the max current is exceeeded. That's important. Any other device can not be guaranteed to do that. If a zener failed open circuit, or went short but exploded then the PSU would start again with no protection and the overvolts condition would cause damage and more caps to pop.

I wouldn't use it at all until the caps have been replaced as whatever fault caused the overvolts is still present. The blown cap and zener are the symptoms not the initial cause.
 
Hi Mooly,

I had also contacted Dan Wright of ModWright Instruments regarding this same issue, since this is a unit his company modified. He kindly responded, and said he would test the same Zener diode, D302, for its operating voltage on one of his working Sony NS999ES DVD players. Dan pointed out that he did install some FRED rectifier diode in his mods.

In the meantime, I am ordering some new replacement caps that you identified in critical spots. I already have a few of the correct values of some Panasonic FC and FM caps in my spares bins.

Many thanks for the help!
 

Mooly

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2007-09-15 8:14 am
That's good...
The zener won't be critical in the normal sense. It has to be rated above the nominal rail voltage and by a safe margin so that it only conducts when there is a genuine fault. The 25 volt rating of the caps set an absolute limit in a round about way. It will be around the 18 volt mark. The feedback monitors the 5 volt rail anyway via R403 and 405 and this is the only rail that is absolutely constant with varying load.
 
Hi Mooly,

I installed the caps as you had pointed out including a large 330uF/250V Nichicon VR cap in the primary stage of the switching regulator board. All the output voltages checked well. I'm still waiting for Dan Wright to get back to me with his measurements of the same Zener diode, D302, on a working Sony DVP-NS999ES unit that he has in his shop. Again, thanks for all the help in getting this DVD/CD player operational again.
 

Mooly

Administrator
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2007-09-15 8:14 am
Thanks for the update.
I wouldn't worry to much over the zener, it's going to be around 18volts give or take... remember it's only function is to conduct (and to fail short by design) when the PSU voltage goes high which will have been caused almost certainly by the caps failing.
You could easily make a small "crowbar" consisting of a thyristor, a small zener, a cap and resistor to replace it if you really wanted and couldn't get the proper original part.

No friendly Sony dealers that would stick one on an order for you ?