Identify zener diode ZD801 in video monitor by V7, model D1912-N6

[Rusthugger]

Ya. Mine is burnt out. I need to identify it so I can replace it.

If you have a schematic for this monitor, could you glance at it to tell me the industry standard or manafacturer's part number for this diode? I mean, some ID that will help me obtain a replacement?

 

[c_mitra]

It is usually tough to help this way. Why don't you upload a high quality picture of the offending region of the circuit board?

Perhaps you will understand, at the level of block diagram, almost all monitors are really same. This is something like people: at the level of DNA we are quite similar but we all are quite distinct in our own ways. There are many experts here who can broadly identify distinct blocks in the circuit - even without a circuit diagram.

Once you have identified the block, it has a distinct function and to the experts here the underlying circuit becomes a readable book. Even if the part has been charred beyond recognition, they can usually suggest some meaningful replacement.

It always helps if you provide all the information that you have in the first post; you receive help faster.

 

[Rusthugger]

Great suggestion.

 

[pjmelect]

I don't know what the diode is but the two 1500uF 10V capacitors look like they are faulty and are probably the reason why the diode got burnt. Also there is another capacitor to the right that looks like it could do with changing as well. Are you sure the diode is faulty? what does it measure in diode mode on the meter.

 

[c_mitra]

The three capacitors with buldging tops need immediate replacement. The zener has certainly overheated but possibly alive. You will need to remove it and test it out. You need to be very careful because the copper on the solder side has got loose due to the excess heat and can peel off very easily.

Suggestion: replace the three caps first and test again.

 

[FvM]

Is ZD801 is shorted or open circuit? It must not necessarily be defective.

 

[Rusthugger]

Yes. At a minimum my plan has always been to replace those capacitors. I'm on the lookout for scrap electronics that might have replacements in them. (No more RadioShack - where can someone buy a few measly capacitors without having to pay 4 times the price in terms of shipping? Agh!)

So you folks advise me that the diode might still work. Cool. So I'll work on the capacitors first and see if the whole unit works after that. If not, then I'll remove and test the diode.

 

[Relayer]

Hello Rusthugger,
If you have a multimeter, then just check the diode in circuit to see if its gone short-circuit.
If it has, then you need to trace the tracks backwards to see what other components are attached to it.
From there we may be able to determine its zener voltage rating.
Its likely that the diode is providing a fixed voltage for that rail.
Check the tracks of the diode cathode to see if they go to a point on the board that's marked with a voltage.
DON'T replace the caps until you've checked the diode.
Regards,
Relayer

 

[c_mitra]

So you folks advise me that the diode might still work.

It is possible (very likely) that the zener is there to provide a start up voltage for the main power supply. Perhaps the resistor sitting (lying) next to the zener is connected to a high voltage. The common point (zener and the resistor) may be connected to a capacitor and a diode. This is likely to supply voltage to the circuit during start up and once the circuit starts oscillating, the zener power is shut off (via the diode).

The zener voltage (if my guess is right) will be slightly less that the voltage produced by the power supply voltage that appears at the other end of the diode: but you need to trace out the circuit and also provide a photo of the other side of the PCB. (Best is to have a bright light on the other side so that the copper traces can be made out from the component side too)

 

[Rusthugger]

just check the diode in circuit to see if its gone short-circuit.

Thanks for the tip.

So I tested that diode in circuit.

When I first make contact with both ends, the multimeter goes to zero ohms. But after a few seconds it shows a resistance. In one direction, it settles around 1k. In the other direction, it settles around 10k.

Does indicate that the diode is likely still good?

 

[Rusthugger]

The common point (zener and the resistor) may be connected to a capacitor and a diode.

My recent glance at the tracks shows the resistor is indeed in series with the zener diode. But that common point does not go anywhere else. It's just a private node of intimacy between ZD and R.

 

[c_mitra]

the resistor is indeed in series with the zener diode. But that common point does not go anywhere else.

But then what is that jumper doing? Also the other end of the resistor appear to be connected to 125V (power that must be coming after the power rectifiers).

If my guess is correct, the diode is ok and what you are measuring is a capacitor. (and then other circuit elements)

But you need to replace the sick caps before you worry about the zener.

 

[pjmelect]

Thanks for the tip.

So I tested that diode in circuit.

When I first make contact with both ends, the multimeter goes to zero ohms. But after a few seconds it shows a resistance. In one direction, it settles around 1k. In the other direction, it settles around 10k.

Does indicate that the diode is likely still good?

It looks like the diode is good.

 

[Relayer]

Hello Rusthugger,

But you need to replace the sick caps before you worry about the zener.

I agree with c_mitra. Since the zener has no short, replacing the defective caps is the next objective.
There's a good possibility that due to those caps having their internal resistance go high, has caused
excessive current through the zener diode, hence the reason it overheated.
Those caps would have induced some pretty heavy ripple through the device.
Make sure you replace any other electro caps within the supply that is of the same brand. Its obvious
the manufacturer hasn't got the formula right.
Please let us know how you get on.
Regards,
Relayer

Foot-Note: After replacing the caps and the unit fires up, you can check the zener voltage
to see what it was supposed to be. Just out of curiosity.

 

[Rusthugger]

Yep. Thanks for your help.

I hope to find the replacement capacitors within a year or so. I'm super poor and won't be simply buying capacitors on line. I hope to fix this monitor for zero dollars. But rather, fix it with just my time searching for discarded electronics to scrap for the capacitors.

 

[Rusthugger]

I finally salvaged the required replacement capacitors and swapped them in. Tee monitor is apparently fixed, until further notice!