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Unknown diode marked "1329" in a faulty Tag McLare

 
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flambart



Joined: 24 Jul 2011
Posts: 3
Location: Italy

PostPosted: Sun Jul 24, 2011 9:03 am    Post subject: Unknown diode marked "1329" in a faulty Tag McLare Reply with quote

Hi, I've just bought a beautiful Tag McLaren AV32R.
The front fluorescent display and all the leds on the front panel are totally off.

I've checked the mainboard, and all digital & analog voltages seem to be ok.
There is even a test point called "Powerfail" that has to be normally high (+5 volt), and so it is, so I suppose that the mainboard is correctly powered.

Unfortunately, I have no schematics at all, but it seems that some AC 2,5-0-2,5 voltages are fully dedicated to feed the panel, with no rectifier bridges in between.
More precisely, the 2 pins corresponding to 2.5-2.5 AC volt are connected to the fluorescent display device.
Instead, the central pin is not directly grounded, but there is an SMD diode ("D4" on PCB, 2.65 x 2.60mm, marked "1329" or "B29" ) between it and the ground. In parallel to D4 , there is also a 22uF/35 volt electrolyitic capacitor.

This diode is apparently short-circuited (about 2 ohm in both senses).
I've been not able to find out any info about it, so I don't know if it's a zener or anything else

Someone could help me? Any schematics would be obviously really appreciated, but even knowing more about this lovely diode could probably be really useful

I also wish to understand if the AC voltages described above are correct, as they seem to me so low in value.

Thanks so much in advance for every hint

Ciao, Paolo
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vtech



Joined: 08 May 2006
Posts: 1264
Location: USA

PostPosted: Sun Jul 24, 2011 7:14 pm    Post subject: Reply with quote

Was that a highend unit-- defunct company?
Either way while I am not familiar with the specific unit, I am rather familiar with the circuit. The idea of using a FL display was/is very common on many brands across the board. While there is no one specific circuit, the basics are ALL the same. (NOT VALID FOR LED TYPE DISPLAY)
In basic terms; For a flourescent disply to opertate, you'd need a DATA driver(almost always part of timer/display IC), some sort of an AC potential (~2to8V depending on the display to illuminate the filament portion) and a negative potentiol to illuminate the actual elements. There are variety of ways to accomplish this which is beyond the scope of forum but utilizing a zener is rather common. Based on your description, the diode in question is definitely a zener. I have seen them to be anywhere from 20 to 40 volts however in your case, the electrolytic's voltage rating puts you within the range. (around 27~30V).
PS: make sure to check the diode outside of the boared since I have also seen caps short out. Also, the main cause of the diode failure can be a power surge or defective display driver--on some cases it is simply under rated.
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flambart



Joined: 24 Jul 2011
Posts: 3
Location: Italy

PostPosted: Sun Jul 24, 2011 10:21 pm    Post subject: Reply with quote

Hi vtech, thank you very much for your kind answer and especially for your illuminating explanation.

I've measured the diode outside the board and I've found it faulty by itself, but I think it's the worth to replace the cap anyway.
The strange thing, normally I'm used to find 0 ohm in short circuited diodes, in this case I've measured a pair of.

Any idea about the zener voltage ? The cap value suggested me the same reflections of yours. Can I make some attempt, or it can be too dangerous ?
Otherwise, how can I retrieve the diode specificatons passing through the marked code ? I took some picture about, I can send them via email, if it's not disturbing you

Thanks a lot again, Paolo
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vtech



Joined: 08 May 2006
Posts: 1264
Location: USA

PostPosted: Mon Jul 25, 2011 2:48 am    Post subject: Reply with quote

Paolo,
Chances of finding the value of the SMd zener is next to impossible. There has never been any set standards in marking SMD semiconductor components.
Therefore, it is safe to use any volatge rating below the electrolytic's rating(preferably a few volts lower). Depending on what you have available, I'd choose a zener somewhere around thirty volts.
As a rule of thumb, electrolytic's voltage ratings are a good indication of what DCvalue is present on that line. The voltage value is selected close to (or higher) than the actual DC level present.
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flambart



Joined: 24 Jul 2011
Posts: 3
Location: Italy

PostPosted: Mon Jul 25, 2011 6:17 am    Post subject: Reply with quote

I will try, I have some zeners in my drawer. Just a general question: why to choose an indipendent AC source to feed the FL display?
In case the zener becomes open, instead of short-circuited, isn't it dangerous ?

I'm not a professional, so maybe my questions could seem a little obvious, or even naif...

Nevertheless, thanks a lot again: your answers are so useful as you gave me a complete scenario, not only a solution.

I will keep you informed

Ciao, Paolo
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