@@kiphakes Well I picked up 19 subs during Friday - Sunday so thank you very much for the mention and including my channel link, it's really appreciated. I don't know why these things are so complicated with their negative voltages, the chips can't need much power. The VFD display probably uses the highest voltage. Unfortunately I suspect the chips are faulty, but cannabilising the working one is the only real way to find out. 🙄
Zener diodes have a known reverse polarity breakdown voltage. They are the same as normal diodes which break down at high voltages (ie they stop resisting current and become a conductor), but they break down at lower, known voltages. A 5.1v Zener is a normal diode in the "normal" direction, and an insulator in the reverse direction until the reverse voltage is reached. That means if connected across a higher voltage (eg 6v) in the reverse direction, they will effectively clamp that voltage to 5.1v (but heat up if there's big current). Your >50v reading suggests "regular" diodes (remember they're the same as zener diodes but with higher reverse polarity breakdown). The tester just can't test over 50v - if it could, it would come back with a figure eventually.
A lovely video of your adventures in the wonderful, and sometimes frustrating, world of electronics. Just subbed. 😊 As a professional electronics engineer / electrician for 39 years, and electronics geek / hobbyist for 45 years, here's my ''two penn'orth,'' to hopefully help you on your way. 1: The diodes S05, S06 and S09 are small signal diodes. S05 and S06 gave you, 775mV and 772mV (0.7V) forward bias voltage, and they showed >50 Volts on your Zener Diode Tester, which is what to expect from a forward biased silicon junction diode. You didn't test S09 with your component tester, but if it was faulty, the Zener diode tester would have most likely picked it up anyhow. These 3 diodes are fine, but just check S09 with your multimeter on diode test, you will get about 0.7V in one direction, and open circuit in the other direction. Just de-solder one side of the diode and lift it off the board. 2: In my decades of experience, 99% of the time, discrete semiconductors, i.e. all types of diodes, all types of transistors, all types of SCRs etc. either go short circuit or open circuit when they are faulty, and these faults will show up using a multimeter in diode test. The first quick tests can be done without de-soldering any components, just look for around 0.5V to 0.7V forward bias, and open circuit (OL) (OFL) or whatever your meter says when not connected to anything in diode mode when reverse bias. If you get readings other than this, make a note of them, then de-solder one leg, and test them out of circuit, sometimes components connected to them will give the same reading in both directions or a short circuit reading. 3: Bipolar transistors (NPN and PNP Types) can be tested the same way as diodes, they will give the same readings as 2 diodes connected in series, Anode to Anode (NPN) or Cathode to Cathode (PNP). 4: Datasheets can provide a lot of useful information. If you Google NEC 7528C Datasheet, it's a 4 Bit, Single Chip Microcomputer. It gives you the pinout (pin configuration) of the IC. Pin 21 (VDD) is the + Supply Voltage, and Pin 42 (VSS), is the - Supply Voltage, although, strangely enough, the positive and negative (0 Volts) connections on a digital IC are often the other way around, so beware. 🫨 It also tells you, in the text, that it will work on a supply voltage between 2.7V to 6V. Most early digital electronics, 1970s through to the 1990s used a 5V power supply, they still do, but a lot of modern processors now use lower voltages, because they operate at much faster speeds. If you power the board up, you will most likely get around 5 Volts DC between pins 21and 42, because the game is working, and the processor is running. 5: In older electronics, like this, failures are often due to electrolytic capacitors. They age, and the electrolyte dries up, changing their value and /or ESR (equivalent series resistance). The great thing is that the ESR of capacitors can be checked in circuit. The company that makes your Zener Diode Tester also does an ESR meter, I have several of their instruments, and I highly recommend them. The ESR Meter, Semiconductor Analyser and LCR Meter would be VERY useful additions to your test equipment. 6: An electronics RUclipsr, who I personally hold in the highest of regard, and who I consider to be one of the very best electronics educators on the Interwebs, is Richard over at Learn Electronics Repair. www.youtube.com/@LearnElectronicsRepair He has individual videos on how to test, pretty much EVERY COMPONENT, literally, and they are a wealth of information. Even with my decades of experience, I've learned loads from Richard, and I cannot recommend his channel highly enough, he's like an electronics God.
@kiphakes I'm enjoying the exploration you're going through with this. I believe your glass diode are most likely germanium. They use color bands similar to resistors. You can look up germanium diode color band chart on Google and find some answers. For instance the green yellow diode is likely 1N54A 50V 5MA diode. The red band diode may likely be a 1N60 Schottky Barrier Rectifier (diode) rates for 600V.
Regardless, knowing your voltage drop across the diode (as others have mentioned) is just as important as knowing the voltage at its junctions. Having an idea of the current draw in the circuit the diode is situated in can go a long way too. If you know the current and the applied voltage at the P junction (for instance) then you can determine what diode might meet the application you are desiring. Then you can select a replacement based off those two factors and install and test. When you test see if the voltage drop is the same across the diode. If the voltage drop is lower than expected, then you may be able to drop it down more with a series resistor. If it were me though. I would just replace your diodes with the diodes I mentioned and see what happens.
You need to measure the voltage directly across each diode (one probe at each end). The ones with apparent higher breakdown voltages are 'probably' not zeners, but ordinary signal / rectifier diodes.
If you and the chap that sent you the schematic get your heads together to work all this out, you could come up with a complete kit and sell them on eBay just like the kits you can get for game gear and the snes and other retro consoles. It would be a massive help for the retro gaming/collector community.
a diode only cares what is on its two legs, so measure directly across the two legs of the diode itself. Using the ground as a reference is only valid if you are sure that the diode is connected to it directly, you have a little transformer on the board to create a bunch of voltages and the diodes are connected to its in and output so may not have any connection to ground
Everything @pdrg said, plus the diode is between two different voltage circuits, and it could be any large voltage difference if the diode isn't conducting due to incorrect polarity. If the diode is conducting there will be a tiny voltage difference 0.6v...
my knowledge is about the same as you, and Vince, but pretty sure the black Zenner Diodes are supposed to stop reverse polarity, a black Zenner is classed as dead if it allows current through in reversed polarity, and therefore think the Red Diodes are for increased voltage sort of like an amplifier to boost voltage. I could be wrong like I say as my knowledge is none exsistant in circuit testing, or diagnosing things.
Start with the known Zener diodes and test those same diodes on the non-working board. If they are open, replace them with the same voltage diodes you tested on the working board. That will establish a lot. Without having the boards in front of me I can't figure out much but it's safe to say that a good starting point will be to match your known Zener diodes with what's on the good board. That could be as far as you need to go.
As someone else said, the zener voltage is across the diode, so you should measure from one leg of the diode to the other leg. Pretty sure Retro Tech Repair tested the breakdown voltage of some zeners from these VFD games with 9v batteries in series some time ago and some were upwards of 100 or 150v. Edit: Just watched the first video and looked at the schematic. A number of zeners are in series, end to end arrangement so might be able to subtract the voltages as you were but TX-30 is probably a transformer and hence using AC and the zeners may be clipping the voltage, which you would see on an oscilloscope but not sure what a multimeter might say. Measuring out of circuit with DC would probably be easier. Buy about 20 PP3 type 9v batteries and set up a test circuit as Retro Tech did in his video "Astro Wars Repair (sound but no display or faint display)", about 47 mins in :)
Hey, Zener diodes are special types of diodes! "Normal" rectifier diodes have a polarity and only allow current to flow in one direction. Zener diodes have a breakdown voltage (which is indicated by the voltage number indicated on the diode - and here even on the circuit board -) above which they also become conductive in the reverse direction... But "normal" rectifier diodes also have a breakdown voltage above which they become conductive (that's the 0.7V or 752mV that are shown to you in the multi-tester). But from this 0.7V they only conduct in one direction, not in the opposite! please read the articles about diodes and Zener diodes: en.wikipedia.org/wiki/Diode and en.wikipedia.org/wiki/Zenerdiode. But there are many more different types of diodes ;-) By the way: The cut-off voltage must be measured across the two connecting leads of a diode, not against ground (minus) of the entire circuit!
A Zener diode, often misspelled as "zenea diode," is a type of semiconductor device that allows current to flow not only from its anode to its cathode like a regular diode but also in the reverse direction when the voltage exceeds a certain value known as the Zener breakdown voltage. ### How Zener Diodes Work #### Structure and Characteristics 1. **Semiconductor Material:** - Zener diodes are made of silicon semiconductor material, doped to create a p-n junction. 2. **Zener Breakdown Voltage:** - The Zener breakdown voltage (Vz) is the voltage at which the diode allows significant reverse current to flow. - This breakdown voltage is precisely controlled during the manufacturing process and can range from a few volts to several hundred volts. #### Operating Principle 1. **Forward Bias:** - When a Zener diode is forward biased (positive voltage to the anode), it behaves like a regular diode, allowing current to flow from the anode to the cathode with a typical forward voltage drop of around 0.7V for silicon diodes. 2. **Reverse Bias:** - When a reverse voltage is applied (positive voltage to the cathode), the Zener diode blocks the current flow until the reverse voltage reaches the Zener breakdown voltage. - At this breakdown voltage, the diode allows current to flow in the reverse direction, maintaining a constant voltage across its terminals. #### Mechanism of Zener Breakdown 1. **Zener Effect (Low Voltage):** - For breakdown voltages below about 5V, the Zener effect dominates. - This effect occurs due to quantum mechanical tunneling, where electrons tunnel through the narrow depletion region of the heavily doped p-n junction. 2. **Avalanche Effect (High Voltage):** - For higher breakdown voltages, the avalanche effect becomes more significant. - In this process, the reverse voltage causes electrons to gain enough kinetic energy to collide with atoms, creating electron-hole pairs and leading to a chain reaction that allows current to flow. ### Applications of Zener Diodes 1. **Voltage Regulation:** - Zener diodes are commonly used in voltage regulator circuits to provide a stable reference voltage. - They can maintain a constant output voltage despite variations in input voltage or load conditions. 2. **Surge Protection:** - They protect sensitive electronic components from voltage spikes by clamping the voltage to a safe level. 3. **Waveform Clipping:** - In signal processing, Zener diodes can clip the peaks of waveforms to limit the amplitude of the signal. 4. **Voltage Shifting:** - Zener diodes can shift voltage levels in circuits by providing a reference voltage. ### Example Circuit: Zener Voltage Regulator A simple Zener diode voltage regulator circuit consists of a series resistor (Rs) and a Zener diode connected in reverse bias: 1. **Input Voltage (Vin):** - The unregulated input voltage is applied across the series resistor and the Zener diode. 2. **Series Resistor (Rs):** - The resistor limits the current flowing through the Zener diode and drops the excess voltage. 3. **Zener Diode:** - The Zener diode maintains a constant voltage (Vz) across its terminals once the input voltage exceeds the breakdown voltage. - The regulated output voltage (Vout) is taken across the Zener diode. ### Summary A Zener diode works by allowing current to flow in the reverse direction when the reverse voltage exceeds the Zener breakdown voltage, thereby maintaining a constant voltage. This property makes Zener diodes essential components in voltage regulation, surge protection, and other applications requiring precise voltage control.
My suggestion is to compare the voltages between the two devices, noting all the differences, especially the negative 30 odd volt supply which the chopper transistor creates for the VFD (can't remember if you had sound on the faulty one). Unless someone reverse-engineers one properly, and produces an accurate schematic, you could go down many rabbit holes.
@@kiphakes As a minimum then, note down all the voltages you saw on the working PCB (under the zeners) and fit new zeners (with those voltages) on the faulty one. Then check continuity between the cathodes and anodes of all the diodes on the working one (and the components they connect to) and mark the diode orientation / connections on the schematic you have. You then have a fighting chance of making progress (in my opinion). At a guess, the diodes with no voltages marked under them are not zeners. Of course, I am assuming that you have all the time in the world.
@@kiphakes Good luck. If you are still struggling after that I have knocked up a very rough approximation of how the transformer is connected (which coil is which and connected where), but diode identification and orientation needs to be corrected on the schematic first.
From recollection, zener diodes are reverse polarity, and are designed to break down at a certain voltage. So measuring them in respect to a notional ground isn't doing you any favours... Measure across the diodes, see where they are breaking down, to give you a better idea of what devices they are. Where a diode may be rated for a higher voltage, maybe they are more protection diodes, not rail regulator diodes? Also, never assume polarities... If you connected a normal diode across your little box, then I guess that they would have the >50V if they are in forward conductance mode... Maybe try testing the unknown diodes the other way round? If your test box just applies a voltage across the junction, then, assuming the current is low, then you shouldn't damage them...
Another admirable attempt and thanks for the mention. Feel free to rob the diodes from the boards I sent you to try and get yours fixed. Good luck.
Thanks dude ! Might be able to identify mine now.. we’ll see! Hopefully sent you a few subscribers too!
@@kiphakes Well I picked up 19 subs during Friday - Sunday so thank you very much for the mention and including my channel link, it's really appreciated.
I don't know why these things are so complicated with their negative voltages, the chips can't need much power. The VFD display probably uses the highest voltage. Unfortunately I suspect the chips are faulty, but cannabilising the working one is the only real way to find out. 🙄
Zener diodes have a known reverse polarity breakdown voltage. They are the same as normal diodes which break down at high voltages (ie they stop resisting current and become a conductor), but they break down at lower, known voltages. A 5.1v Zener is a normal diode in the "normal" direction, and an insulator in the reverse direction until the reverse voltage is reached. That means if connected across a higher voltage (eg 6v) in the reverse direction, they will effectively clamp that voltage to 5.1v (but heat up if there's big current).
Your >50v reading suggests "regular" diodes (remember they're the same as zener diodes but with higher reverse polarity breakdown). The tester just can't test over 50v - if it could, it would come back with a figure eventually.
I wish I could bloody help! I am clueless with circuit boards nonetheless very intriguing video Kip ! Start to finish too notch 👍
A lovely video of your adventures in the wonderful, and sometimes frustrating, world of electronics. Just subbed. 😊
As a professional electronics engineer / electrician for 39 years, and electronics geek / hobbyist for 45 years, here's my ''two penn'orth,'' to hopefully help you on your way.
1: The diodes S05, S06 and S09 are small signal diodes. S05 and S06 gave you, 775mV and 772mV (0.7V) forward bias voltage, and they showed >50 Volts on your Zener Diode Tester, which is what to expect from a forward biased silicon junction diode. You didn't test S09 with your component tester, but if it was faulty, the Zener diode tester would have most likely picked it up anyhow. These 3 diodes are fine, but just check S09 with your multimeter on diode test, you will get about 0.7V in one direction, and open circuit in the other direction. Just de-solder one side of the diode and lift it off the board.
2: In my decades of experience, 99% of the time, discrete semiconductors, i.e. all types of diodes, all types of transistors, all types of SCRs etc. either go short circuit or open circuit when they are faulty, and these faults will show up using a multimeter in diode test.
The first quick tests can be done without de-soldering any components, just look for around 0.5V to 0.7V forward bias, and open circuit (OL) (OFL) or whatever your meter says when not connected to anything in diode mode when reverse bias.
If you get readings other than this, make a note of them, then de-solder one leg, and test them out of circuit, sometimes components connected to them will give the same reading in both directions or a short circuit reading.
3: Bipolar transistors (NPN and PNP Types) can be tested the same way as diodes, they will give the same readings as 2 diodes connected in series, Anode to Anode (NPN) or Cathode to Cathode (PNP).
4: Datasheets can provide a lot of useful information. If you Google NEC 7528C Datasheet, it's a 4 Bit, Single Chip Microcomputer. It gives you the pinout (pin configuration) of the IC. Pin 21 (VDD) is the + Supply Voltage, and Pin 42 (VSS), is the - Supply Voltage, although, strangely enough, the positive and negative (0 Volts) connections on a digital IC are often the other way around, so beware. 🫨
It also tells you, in the text, that it will work on a supply voltage between 2.7V to 6V. Most early digital electronics, 1970s through to the 1990s used a 5V power supply, they still do, but a lot of modern processors now use lower voltages, because they operate at much faster speeds. If you power the board up, you will most likely get around 5 Volts DC between pins 21and 42, because the game is working, and the processor is running.
5: In older electronics, like this, failures are often due to electrolytic capacitors. They age, and the electrolyte dries up, changing their value and /or ESR (equivalent series resistance). The great thing is that the ESR of capacitors can be checked in circuit. The company that makes your Zener Diode Tester also does an ESR meter, I have several of their instruments, and I highly recommend them. The ESR Meter, Semiconductor Analyser and LCR Meter would be VERY useful additions to your test equipment.
6: An electronics RUclipsr, who I personally hold in the highest of regard, and who I consider to be one of the very best electronics educators on the Interwebs, is Richard over at Learn Electronics Repair. www.youtube.com/@LearnElectronicsRepair
He has individual videos on how to test, pretty much EVERY COMPONENT, literally, and they are a wealth of information. Even with my decades of experience, I've learned loads from Richard, and I cannot recommend his channel highly enough, he's like an electronics God.
Thank you so much for such a beautifully simple and helpful reply! I’m feeling a lot more confident now! Thank you! 🙏🏼
@kiphakes I'm enjoying the exploration you're going through with this. I believe your glass diode are most likely germanium. They use color bands similar to resistors. You can look up germanium diode color band chart on Google and find some answers. For instance the green yellow diode is likely 1N54A 50V 5MA diode. The red band diode may likely be a 1N60 Schottky Barrier Rectifier (diode) rates for 600V.
Regardless, knowing your voltage drop across the diode (as others have mentioned) is just as important as knowing the voltage at its junctions. Having an idea of the current draw in the circuit the diode is situated in can go a long way too. If you know the current and the applied voltage at the P junction (for instance) then you can determine what diode might meet the application you are desiring. Then you can select a replacement based off those two factors and install and test. When you test see if the voltage drop is the same across the diode. If the voltage drop is lower than expected, then you may be able to drop it down more with a series resistor. If it were me though. I would just replace your diodes with the diodes I mentioned and see what happens.
You need to measure the voltage directly across each diode (one probe at each end). The ones with apparent higher breakdown voltages are 'probably' not zeners, but ordinary signal / rectifier diodes.
If you and the chap that sent you the schematic get your heads together to work all this out, you could come up with a complete kit and sell them on eBay just like the kits you can get for game gear and the snes and other retro consoles. It would be a massive help for the retro gaming/collector community.
Well, hopefully we can all work this out together! 🙌🏻
@@kiphakes yeah for sure my friend
Great video kip watching you go through the process of these components is great to watch thank you for sharing this video.
Thanks for watching! Really appreciate it!
Awesome channel my friend, subbed!
Thanks so much Kyle!
a diode only cares what is on its two legs, so measure directly across the two legs of the diode itself.
Using the ground as a reference is only valid if you are sure that the diode is connected to it directly, you have a little transformer on the board to create a bunch of voltages and the diodes are connected to its in and output so may not have any connection to ground
Perfect. Thank you 🙏🏼
Everything @pdrg said, plus the diode is between two different voltage circuits, and it could be any large voltage difference if the diode isn't conducting due to incorrect polarity. If the diode is conducting there will be a tiny voltage difference 0.6v...
Thank you 🙏🏼
Really intresting
YT ate my comment. Your TC-1 tester has ZD test in it. The KAA pins lower right if using original firmware
Cool! Thank you!
my knowledge is about the same as you, and Vince, but pretty sure the black Zenner Diodes are supposed to stop reverse polarity, a black Zenner is classed as dead if it allows current through in reversed polarity, and therefore think the Red Diodes are for increased voltage sort of like an amplifier to boost voltage. I could be wrong like I say as my knowledge is none exsistant in circuit testing, or diagnosing things.
Start with the known Zener diodes and test those same diodes on the non-working board. If they are open, replace them with the same voltage diodes you tested on the working board. That will establish a lot.
Without having the boards in front of me I can't figure out much but it's safe to say that a good starting point will be to match your known Zener diodes with what's on the good board. That could be as far as you need to go.
As someone else said, the zener voltage is across the diode, so you should measure from one leg of the diode to the other leg. Pretty sure Retro Tech Repair tested the breakdown voltage of some zeners from these VFD games with 9v batteries in series some time ago and some were upwards of 100 or 150v.
Edit: Just watched the first video and looked at the schematic. A number of zeners are in series, end to end arrangement so might be able to subtract the voltages as you were but TX-30 is probably a transformer and hence using AC and the zeners may be clipping the voltage, which you would see on an oscilloscope but not sure what a multimeter might say. Measuring out of circuit with DC would probably be easier. Buy about 20 PP3 type 9v batteries and set up a test circuit as Retro Tech did in his video "Astro Wars Repair (sound but no display or faint display)", about 47 mins in :)
Hey, Zener diodes are special types of diodes! "Normal" rectifier diodes have a polarity and only allow current to flow in one direction. Zener diodes have a breakdown voltage (which is indicated by the voltage number indicated on the diode - and here even on the circuit board -) above which they also become conductive in the reverse direction...
But "normal" rectifier diodes also have a breakdown voltage above which they become conductive (that's the 0.7V or 752mV that are shown to you in the multi-tester). But from this 0.7V they only conduct in one direction, not in the opposite!
please read the articles about diodes and Zener diodes: en.wikipedia.org/wiki/Diode and en.wikipedia.org/wiki/Zenerdiode. But there are many more different types of diodes ;-)
By the way: The cut-off voltage must be measured across the two connecting leads of a diode, not against ground (minus) of the entire circuit!
You could do the same with an adjustable power supply, a 100K series resistor, and a multimeter.
Yep. I say that in the video
A Zener diode, often misspelled as "zenea diode," is a type of semiconductor device that allows current to flow not only from its anode to its cathode like a regular diode but also in the reverse direction when the voltage exceeds a certain value known as the Zener breakdown voltage.
### How Zener Diodes Work
#### Structure and Characteristics
1. **Semiconductor Material:**
- Zener diodes are made of silicon semiconductor material, doped to create a p-n junction.
2. **Zener Breakdown Voltage:**
- The Zener breakdown voltage (Vz) is the voltage at which the diode allows significant reverse current to flow.
- This breakdown voltage is precisely controlled during the manufacturing process and can range from a few volts to several hundred volts.
#### Operating Principle
1. **Forward Bias:**
- When a Zener diode is forward biased (positive voltage to the anode), it behaves like a regular diode, allowing current to flow from the anode to the cathode with a typical forward voltage drop of around 0.7V for silicon diodes.
2. **Reverse Bias:**
- When a reverse voltage is applied (positive voltage to the cathode), the Zener diode blocks the current flow until the reverse voltage reaches the Zener breakdown voltage.
- At this breakdown voltage, the diode allows current to flow in the reverse direction, maintaining a constant voltage across its terminals.
#### Mechanism of Zener Breakdown
1. **Zener Effect (Low Voltage):**
- For breakdown voltages below about 5V, the Zener effect dominates.
- This effect occurs due to quantum mechanical tunneling, where electrons tunnel through the narrow depletion region of the heavily doped p-n junction.
2. **Avalanche Effect (High Voltage):**
- For higher breakdown voltages, the avalanche effect becomes more significant.
- In this process, the reverse voltage causes electrons to gain enough kinetic energy to collide with atoms, creating electron-hole pairs and leading to a chain reaction that allows current to flow.
### Applications of Zener Diodes
1. **Voltage Regulation:**
- Zener diodes are commonly used in voltage regulator circuits to provide a stable reference voltage.
- They can maintain a constant output voltage despite variations in input voltage or load conditions.
2. **Surge Protection:**
- They protect sensitive electronic components from voltage spikes by clamping the voltage to a safe level.
3. **Waveform Clipping:**
- In signal processing, Zener diodes can clip the peaks of waveforms to limit the amplitude of the signal.
4. **Voltage Shifting:**
- Zener diodes can shift voltage levels in circuits by providing a reference voltage.
### Example Circuit: Zener Voltage Regulator
A simple Zener diode voltage regulator circuit consists of a series resistor (Rs) and a Zener diode connected in reverse bias:
1. **Input Voltage (Vin):**
- The unregulated input voltage is applied across the series resistor and the Zener diode.
2. **Series Resistor (Rs):**
- The resistor limits the current flowing through the Zener diode and drops the excess voltage.
3. **Zener Diode:**
- The Zener diode maintains a constant voltage (Vz) across its terminals once the input voltage exceeds the breakdown voltage.
- The regulated output voltage (Vout) is taken across the Zener diode.
### Summary
A Zener diode works by allowing current to flow in the reverse direction when the reverse voltage exceeds the Zener breakdown voltage, thereby maintaining a constant voltage. This property makes Zener diodes essential components in voltage regulation, surge protection, and other applications requiring precise voltage control.
My suggestion is to compare the voltages between the two devices, noting all the differences, especially the negative 30 odd volt supply which the chopper transistor creates for the VFD (can't remember if you had sound on the faulty one). Unless someone reverse-engineers one properly, and produces an accurate schematic, you could go down many rabbit holes.
Unfortunately the original one had pretty much every diode physically broken so it does very little so there’s not much to compare
@@kiphakes As a minimum then, note down all the voltages you saw on the working PCB (under the zeners) and fit new zeners (with those voltages) on the faulty one. Then check continuity between the cathodes and anodes of all the diodes on the working one (and the components they connect to) and mark the diode orientation / connections on the schematic you have. You then have a fighting chance of making progress (in my opinion). At a guess, the diodes with no voltages marked under them are not zeners. Of course, I am assuming that you have all the time in the world.
@ralphj4012 thank you! 🙏🏼
@@kiphakes Good luck. If you are still struggling after that I have knocked up a very rough approximation of how the transformer is connected (which coil is which and connected where), but diode identification and orientation needs to be corrected on the schematic first.
From recollection, zener diodes are reverse polarity, and are designed to break down at a certain voltage. So measuring them in respect to a notional ground isn't doing you any favours... Measure across the diodes, see where they are breaking down, to give you a better idea of what devices they are. Where a diode may be rated for a higher voltage, maybe they are more protection diodes, not rail regulator diodes? Also, never assume polarities... If you connected a normal diode across your little box, then I guess that they would have the >50V if they are in forward conductance mode... Maybe try testing the unknown diodes the other way round? If your test box just applies a voltage across the junction, then, assuming the current is low, then you shouldn't damage them...
Also, there are more typ of diodes than regular ones and zener diodes... Try here for colour codes... www.repaircompanion.com/diode_color_markings
Cool.. thank you!
Not all glass package diodes are zener diodes, and not all zener diodes are in glass packages.