Is a zener diode (SOT-23) considered broke if it reads 5 Vdc on one direction, but 0 Vdc on the other direction? Or could this be normal?

Edit: by a direction I mean black lead on DMM to earth on the backbox, and red lead to one end of the diode. Then for the other direction move the red lead to the opposite pin on the diode.

It is on a Babcock DMD, GD-032D128-02, which has 2 zener diods z1 and z2. Measuring these while the dmd is connected and game powered on:

Z1:
5 Vdc at one direction, but 0 Vdc at the other direction!

Z2:
104,5 Vdc at one direction, 117,5 Vdc at the other direction.

Shouldn't z1 measure any voltage at the "other direction"? It shows 0... can I assume that z1 broke??
Maybe someone with the same DMD model could measure z1?

The issue I am trying to solve is too high voltage-difference between pin 1 and 2 on the connector. There should be a 12V difference but I get 23V.
I made sure that it is the DMD itself that is malfunctioning. I replaced zener diode z2 (not z1) among with some caps. It didnt help.

The schematics can be found in the wpc-95 manual, although not for this exact model but they are pretty close.

Here is from the schematics, it seems that maybe z1 has one end tied to the ground, while z2 don't... or?

20200903_081602 (resized).jpg

Thank u for this tips! Just measured over the diodes, z2 is showing 12.2V but z1 is showing 5.0V, not 12V.
Since the schematic is not for this identical model, i am unsure if z1 indeed should be 12V or maybe it is a 5V diode on my model. Is there a way to know which voltage a diode is supposed to have?
If magnifying the text om z1, it says something like "UF L".

The DMD works perfectly the first minute or so, then the display starts scrambeling everything and later it dies.

It seems that pin 2 on the connector is measuring too low voltage, or pin 1 is measuring too high voltage. According to pinwiki, the difference should be 12V.

DMD connected and machine on, measurements on the DMD connector pins:

Pin 1: 116.5V
Pin 2: 93.5V
-> Difference: 23V (should be 12V)

I verified the same on 2 machines, it is the DMD itself that is the issue.
Changed some caps and z2 (but not z1), thought it would help but it didn't.

My last hope is that z1 could be the erroring component.

I think this display is not uncommon, I have a little hope that someone with the same display could check the z1 voltage on their DMD

Here is a picture really appreciate it if someone could look up the z1.

20200906_104913 (resized).jpg

I checked the q1/q2 but i will take a look at those again. Should I do a regular diode test while the dmd is disconnected, or should these be tested while dmd is connected, like checking voltage accross them or somethibg similar?

Both q1 and q2 is showing 0,74 so I suppose it is correct :/

Holding red on the side with 1 pin and black on the opposite side shows 0.74 in diode test. Flipping the meters shows 0, on both q1 and q2. Could it be that both q1 and q2 are defect? Should they show a value when meters are flipped??
20200906_175854 (resized).jpg

I realized a while ago that the OL is not working on my DMM, it simply always shows 0 regardless measuribg out of range or not measuribg at all. So I just borrowed another DMM, and this one always shows OL even when not measuring anything. So naturally it showed OL but i dont know if it is out of range or just 0.

Anyway, took new measures with the borrowed DMM.

Pin 1 = pin with red lead in pic above
Pin 2 = pin with black lead in pic above
Pin 3 = pin with no lead in pic above

Red on 1, black on 2 = 0.65
Red on 1, black on 3 = 1.87
Red on 2, black on 1 = OL
Red on 2, black on 3 = 1.71
Red on 3, black on 1 = OL
Red on 3, black on 2 = 0.67

Same for q1 and q2, i suspect that this is OK?

Just googled some more info about testing pnp transistors and it seems that BOTH q1 and q2 could be faulty?!
I think the results i got above are not as they should.

My results translated to Base, Emitter, Collector:

Collector > Base = 0.65
Collector > Emitter = 1.87
Base > Collector = OL
Base > Emitter = 1.71
Emitter > Collector = OL
Emitter > Base = 0.67

So i did replace Z1 today, with the old Z2 that sat on this DMD before I replaced it, so I dont know if this old z2 is working correctly.

Anyway, Same results measured, although z1 now shows 8.4 Vdc and not 5 Vdc.

Really annoying that I cant find the failed component, but guess I have to give up now

Wanted to try to reseat the roms but they seem not removable.

I meant chips, or whatever they are called... These black things.

This is killing me, it annoys me to not find the cause. It is about more than saving money

20200907_181834 (resized).jpg

barakandl please please please can u measure the voltage across z1, with game turned on? Need to know if it should be 12v.

The thing is that i dont know if z1 really should be 12v, cause the schematics is not for this exact model. Thats why i am looking for somene to measure it
I do get about 65V at pin 8.

I replaced z2 with another zener that a pinsider was really kind to send me. That didnt solve the issue. I then noticed that z1 measures only 5v, but i dont have another zener so I took a chance and put the old z2 in place of z1, now z1 measures about 8.1v but still not 12v. Diode test with Dmm seems ok.

No, the original z1 and z2 did not have the same markings.
Z1: "UF L"
Z2: "8 S"

barakandl sorry to hear do u remember if z1 was 12V or 5V?

As I really don't want to give up on this, I measured voltages at some places on the DMD hoping that someone brighter than me can find a hint

Q1, Q2, D1, D2, Z1, Z2 all have 3 legs, measurements below are between the one-leg side and the leg on opposite upper side, with the one-leg side faced to the right.

Q1: 106
Q2: 120.4
D1: 12.4
D2: 0.14
Z1: 8.2
Z2: 12.3

C1, C15: 12.5
C11: 66.6
C3: 4.8
C8, C10, C12, C13: 8.2 (I happened to see that C13 jumps up to 11.5 when I turn off the machine).

R179, R186: 108.9
R194: 59.2

The below measurements are take by having black lead on cabinet ground and red lead on the pin. Pin 1 is the one-leg side, pin 2 is the leg on opposite upper side and pin 3 is the leg on the lower side.

Q1 pin 1 -107.5
Q1 pin 2 0
Q1 pin 3 0.5

Q2 pin 1 -120.4 (when this pin is touched with the dmm, the humming noise from the DMD stops!)
Q2 pin 2 0
Q2 pin 3 0

D1 pin 1 -108.9
D1 pin 2 121.4
D1 pin 3 0

D2 pin 1 -107.5
D2 pin 2 -107.5
D2 pin 3 0

Some of the voltages are increasing slightly by time. A strange thing is that after the DMD has been powered on for about 5 min, some voltages will suddenly increase really high! They will drop back to "normal" after a while, sometimes... at that moment I measured pin 1 to be 146Vdc and pin 2 125Vdc, R179 was 133.2Vdc.

Also, I cant find U8. It is in the schematics but not on the DMD. I will look again tomorrow.

I think my problem is the components getting 8.2 Vdc when they should be getting 12 Vdc, or maybe D2 with only 0.14 Vdc is the problem?

Shit this was long... sorry!

The schematics I am following can be found at page 12 in the wpc95 schematics found online.

Here is a screenshot... shouldnt D1 and D2 have the same voltages?

Screenshot_20200909-015346_Drive (resized).jpg

I've searched a lot but cant find a schematic for this exact model. The one in the wpc95 manual is really close, if not perfect, or at least it seems so. I can follow the schematic and get continuety everywhere it should be, including the area for q1/q2 and d1/d2.

Let's assume that the schematics are correct, are my Q and D values correct?

Barakandl, thank u!
Just measured the 8x 27k resistors,3.34 kohm seems fine.
Also measrured the 11x 100k resistors, got 8.31 kohm, should get 9.09 kohm. I suppose that is acceptable?

Re-measered the zener resistances. Z1 is 0 ohm (pretty sure) one way, while z2 is 7 Mohms same way. But z1 is connected to ground so probably fine.