Need help identifying a diode

[bigbrew25]

Hey Guys. I have about 10 NEC PA-16LCBJ cards that are dead. The source of the problem is a resistor and a diode on the power daughterboard. I have found the resistor no problem but am having a hard time finding the diode. The following readings is the same diode off of a good card:

It is a Glass diode (hole through) with the markings "301." along the anode side.
It has a forward biased voltage drop of .677.
The body dimensions is 1.85mm W x 2.15 mm L (Anything close could work of course). Here is a pic

[bigbrew25]

Re: Need help identifying a diode

Any help would be greatly appreciated. The daughterboard description is :

PZ-PW36-G.

I have no schematic. The symbol on the board is a standard diode symbol.

[budm]

Re: Need help identifying a diode

"It has a forward biased voltage drop of .677." That is about the correct Vf for the Silicon diode. So why do you think the Diode is bad?

[bigbrew25]

Re: Need help identifying a diode

budm, that was the reading on a "known good" one. The defective ones are shorted (out of circuit).

[SteveNielsen]

Re: Need help identifying a diode

Perhaps it is a 1n301.

[budm]

Re: Need help identifying a diode

Can we see the pictures of the board and the closed up pictures of where the shorted diode is mounted? We want to make sure it is not Zener Diode.

[SteveNielsen]

Re: Need help identifying a diode

How would a non-zener test on a zener tester?

[budm]

Re: Need help identifying a diode

Zener diode will show normal forward Voltage jsu like regular diode, to do the test for the Zener Voltage you will need resistor and variable power supply and bias the diode in reverse bias mode until the Zener knee Voltage is reached.
Regular diode you will do Forward Voltage and the break down Voltage test

[bigbrew25]

Re: Need help identifying a diode

Here you go. The transistor that the Cathode connects to is a A988.

[budm]

Re: Need help identifying a diode

A988 = 2SA988 PNP Transistor.
Can you please focus those two pictures and also the pictures of the whole board both side?
If that diode is shorted out I would expect the Base-Emitter junction of that Transistor T2 to be damaged too.
That board looks to be the sub-power supply board.
Darn, this PBX card is expensive to replace.

[bigbrew25]

Re: Need help identifying a diode

Here are some better pic budm. R20 usually goes bad with this problem, the resistor closest to L4. I will check the transistor on one of our bad ones.

[budm]

Re: Need help identifying a diode

I made the virtual PCBA and I need those
Capacitor designators which I cannot see in the pictures.

[budm]

Re: Need help identifying a diode

A1 IC spec for M51977P.
P-CH MOSFET T1, INDUCTOR L3, Diode D2 form -28V Buck regulator switching power supply that take the -48VDC input on J2 to produce the -28VDC at J7.
That D3 is hit with the reverse bias of at least 28V, so it is possible that this diode is Zener diode, that transistor (2sa988) Collector drives the pin 7 (Voltage DETECTION pin) for controlling the output Voltage.

[bigbrew25]

Re: Need help identifying a diode

Do you need the values of these caps as well?

[bigbrew25]

Re: Need help identifying a diode

C2: 22uf 25v
c15: 10uf 63v
c14: 100uf 35v
c6: 47uf 63v
c5: 47uf 63v
c3: 10uf 63v

[budm]

Re: Need help identifying a diode

So BTW, when that D3 shorted out the resistor R20 (220 Ohms 1/8W) will burn out due to 28V being applied across the resistor, but I would expect the Transistor T2 E-B junction to be damaged as well as the R19 (100 Ohms).
The board is used for taking in -48VDC and then produces the -28VDC and -5VDC outputs.

[bigbrew25]

Re: Need help identifying a diode

Is there a certain zener diode part # that you suggest, budm?

[budm]

Re: Need help identifying a diode

At this point I do not know, but you do have a working board that you can pull that diode to verify if it is ZENER diode or not?
Did you have a chance to check the transistor and the resistors?
BTW, thanks for the good pictures, I am able to make the virtual PCB and trace out the circuit.

[bigbrew25]

Re: Need help identifying a diode

budm, I did check the transistor on one of the bad cards and it read fine. On most of these cards, R19 does get dark, but usually reads fine. I told you wrong about the diode. On most of these bad cards, the diode is OPEN, not shorted. I just had one of them that was shorted. Don't know if this changes your theory or not. I have not done a zener test on it, but i did ohm the good one. It read OPEN one way and 4.5 M the other. Do you have any plans on how to make a tester?

[SteveNielsen]

Re: Need help identifying a diode

I am impressed with your virtual PCB Budm.