*Additional information* Somewhere in the recording I lost the fact that the resistance across the transformer primary was about 1K ohms. I checked the faulty Varistor (MOV) this morning and it actually reads 5K ohms - so this was not a short you would detect with your multimeter - the Varistor was breaking down at mains voltage which is why the fuse was blowing. Regards the circuit showing the SCR (Thyristor), the circuit is correct but the operation is opposite to my description. The SCR turns on when the LED in the optoisolator turns off, and turns off when the LED turns on. So a 'logic 0' or low voltage applied to the optoisolator turns the load ON, and a positive pulse or voltage to the optoisolator turns the load OFF
21:56 - I propose to write/draw with a pen/marker directly on PCB, symbol of component on other side, when explaining like that, because each time board is flipped forward backward we (viewers) have to re-think all previous signals. Then we can follow more precisely. Its something I do, then when I clean with alcohol , it erases all drawings anyway. 45:30 - That metal cap of fuse that went underneath, can make unwanted connection. 54:01 - To viewers, both MOV Metal Oxide Varistor and Thermistor (Different component from Thyristor) explode like fireworks under some conditions, so be careful when you turn on devices that have those components. And this is why good visual inspection of board is important before any measurements. Use magnifier with LED light and watch for cracks on MOV for example, among other things.
21:56 that is a very good idea I will do this on future videos. Thanks Orion 🙂 54:01 yeah agreed as I also mentioned in the video, these blue things that look like capacitors (and are most definitely not) quite often explode when they fail, and the type of fault that I found on this PCB is quite common so it's well worth remembering
Very interesting! I have learned a great deal from the repair. Thanks so much for sharing. I now find that some of the devices I received in a box of misc. parts from a dealer are in fact varistors and not capacitors! Nice.
Great video Richard. I was thinking if you left it in circuit and used the variac to turn down the voltage, maybe it might of worked at a lower voltage. Probably not the safest thing to try. But it might of pointed you in the right direction from the start.
I always say if you do something do it right, a lot of people would of left that component out and said its working, that component is to protect the rest of the circuit, well done for replacing it, I really enjoy your video's
At work I recently had the exact same fault with two control boards of air handler wall units to cool technical rooms. On both boards the MOVs were blown and shorted L to N (both were cracked like yours!), on both of them the primary windings of the transformers were blown as well. I injected power behind the rectifier with a benchtop power supply and so far the boards seem to be working. We suspect someone to have measured the wiring to both units incorrectly with an insulation tester, as they are supplied from the same electrical cabinet. But I need to sacrifice one board, where at least transformer and MOV do work to test that. Unfortunately I don't have time for the next couple of weeks, as big festivities are in order and everything is a mess right now because of that...
I had a massage chair that had that in it. It didn't work as soon as I plugged it it. They send me a new motherboard for it. Before the motherboard got here I removed that MOV and replaced it with one from a power strip I had. It was working before I got the replacement motherboard in the mail. They didn't want the old one back so I still have an extra motherboard for the chair lol. It was super easy to fix.
Measuring / testing fuses protecting low impedance circuits (here on the transformer secondary) is prone to give you false positives - the resistance of the secondary is so low the current runs the other way around the blown fuse (known issue when testing the fuses on tube amp heater circuits) - so for such testing you have to remove the fuse.
About the desoldering gun technique. I watch Adrian Digital Basement use a cheaper one than yours and what he does is move the opening in circles around the pin with a gentle speed and he seems to get all the solder out in one go.
I have a collection of salvaged varistors. I just remove from stuff when I see them .Do you think you would use them at a push or wait a month and replace with new stock ?
I think this depends on the circumstances. In this case, the PCB is used in a commercial environment (health centre) so I feel it is important to complete the repair using only original value components. Also it was sent to me from the UK and will be going back there so it would be more hassle if I have to change the Varistor again.
Can I connect my external power supply with alligator clips to a 500 series module edge card to power it up to test it . Simply connection the ground , neutral and hot directly to the card? Sorry if it’s a dumb question. Really enjoy the channel!
Are you going to replace both MOVs? The other one may have been weakened a bit in the past and the cost of the component it is best to ensure that both of them are new.
Is it possible that the VDE on the varistor is the logo of the german "Verband der Elektrotechnik Elektronik Informationstechnik e.V." and ist therfore some kind of label for quality control?
Hi Rich, I believe you got the logic backwards in your diagram with regards to the Opto and SCR, when the Opto-isolator switches ON, would it not remove the gate voltage/current effectively preventing the SCR from turning ON?
Yeah you are correct, that circuit would work that way round though the principle is the same. A logic low turning the LED off would turn the SCR on, and vice versa
Really enjoying these videos..being a software engineer i also wanted to know more about hardware..and these videos really help..thanks for these explanations
A thyristor switches on when the gate is on a higher potential than the cathode and it stays on as long as there is current flowing. You seem to say it switches on when the gate is on zero potential. I am confused.
@@kriswillems5661 The diode is the right way round but in this circuit the SCR actually turns off when the LED in the opto-isolator is on, and turns on when the LED is off. My mistake and you are correct. Well spotted and hopefully you are no longer confused.😉
@@LearnElectronicsRepair Ah, right, the diode is right. But will it work if the load is connected to the cathode? Shouldn't it be connected to the anode? I guess it doesn't matter because there's almost no voltage on the load when there is no current. But this is only true for a resistive load, if the load be not linear or slightly capacitive there might be a problem. Generally the load is put on the anode, because this way it's easier to control the gate-cathode voltage.
@@kriswillems5661 I'll accept what you say and learn a little from it 🙂 Fortunately I'm not a circuit designer, I'm a circuit fixer, so I just repair what is in front of me. I have a good idea how it works, enough to diagnose it, but freely admit I don't always know the ins and outs of it exactly.
Mythological fault finding... like my ex! 🎉 😂 Just kidding. Nit a thing.... We don't even talk. But it's a funny joke! 😁 Thanks for the unintentional laugh, beyond the educational value of the video too!
*Additional information*
Somewhere in the recording I lost the fact that the resistance across the transformer primary was about 1K ohms. I checked the faulty Varistor (MOV) this morning and it actually reads 5K ohms - so this was not a short you would detect with your multimeter - the Varistor was breaking down at mains voltage which is why the fuse was blowing. Regards the circuit showing the SCR (Thyristor), the circuit is correct but the operation is opposite to my description. The SCR turns on when the LED in the optoisolator turns off, and turns off when the LED turns on. So a 'logic 0' or low voltage applied to the optoisolator turns the load ON, and a positive pulse or voltage to the optoisolator turns the load OFF
One of the best technical teachers on the internet !
Thank You 🙂
love this methodical fault finding series.
Another extremely interesting video. I really enjoyed both the procedure and learning about how varistors and triacs work and are used. Thanks!
Being a beginner, I always thought they were caps. I learn something from each of your videos. Thanks!
Brilliant, thanks once again for sharing your knowledge and taking the time to make this video. 👍
It's very kind of you to explain in such a rich details.. thanks so much for the info.. I really appreciate your efforts.
hi richard, from tunisia i make the same profession and i agree in all , thank you for your videos they inspire me very well !
Great work, I would change the second one too because MOVs generally will have degradation caused by many times seeing voltage spikes on main
Bravo, good detective work and a very clear win. Keep up the great work 😀👍
Great video and analysis! Would love to see more of this kind of repair.
Thx so much for your vids. I was getting stuck working on boards, and you have helped me move forward. Cheers!
So Glad I found your channel. Appreciate you sharing your knowledge.
21:56 - I propose to write/draw with a pen/marker directly on PCB, symbol of component on other side, when explaining like that, because each time board is flipped forward backward we (viewers) have to re-think all previous signals. Then we can follow more precisely. Its something I do, then when I clean with alcohol , it erases all drawings anyway.
45:30 - That metal cap of fuse that went underneath, can make unwanted connection.
54:01 - To viewers, both MOV Metal Oxide Varistor and Thermistor (Different component from Thyristor) explode like fireworks under some conditions, so be careful when you turn on devices that have those components. And this is why good visual inspection of board is important before any measurements. Use magnifier with LED light and watch for cracks on MOV for example, among other things.
21:56 that is a very good idea I will do this on future videos. Thanks Orion 🙂
54:01 yeah agreed as I also mentioned in the video, these blue things that look like capacitors (and are most definitely not) quite often explode when they fail, and the type of fault that I found on this PCB is quite common so it's well worth remembering
Very interesting! I have learned a great deal from the repair. Thanks so much for sharing. I now find that some of the devices I received in a box of misc. parts from a dealer are in fact varistors and not capacitors! Nice.
Nice - First Connect :-)
I always enjoy your very professional Videos. Keep on going
Thank you. I will 😉
Great video Richard. I was thinking if you left it in circuit and used the variac to turn down the voltage, maybe it might of worked at a lower voltage. Probably not the safest thing to try. But it might of pointed you in the right direction from the start.
I always say if you do something do it right, a lot of people would of left that component out and said its working, that component is to protect the rest of the circuit, well done for replacing it, I really enjoy your video's
So, the Varistor did what is designed to do - protect the circuit board from voltage spikes. RIP Varistor.
A noble sacrifice 🫡
Awesome, thank you for explaining the components on the go
At work I recently had the exact same fault with two control boards of air handler wall units to cool technical rooms. On both boards the MOVs were blown and shorted L to N (both were cracked like yours!), on both of them the primary windings of the transformers were blown as well.
I injected power behind the rectifier with a benchtop power supply and so far the boards seem to be working.
We suspect someone to have measured the wiring to both units incorrectly with an insulation tester, as they are supplied from the same electrical cabinet. But I need to sacrifice one board, where at least transformer and MOV do work to test that. Unfortunately I don't have time for the next couple of weeks, as big festivities are in order and everything is a mess right now because of that...
I had a massage chair that had that in it. It didn't work as soon as I plugged it it. They send me a new motherboard for it. Before the motherboard got here I removed that MOV and replaced it with one from a power strip I had. It was working before I got the replacement motherboard in the mail. They didn't want the old one back so I still have an extra motherboard for the chair lol. It was super easy to fix.
Another good one!
Measuring / testing fuses protecting low impedance circuits (here on the transformer secondary) is prone to give you false positives - the resistance of the secondary is so low the current runs the other way around the blown fuse (known issue when testing the fuses on tube amp heater circuits) - so for such testing you have to remove the fuse.
Fun one Richard. Ta.
Thank you 👍 great video, God bless you🙂
Well discovered. No fuses inside the transformer, just confusing symbols meant to represent the maximum secondary loading.
About the desoldering gun technique. I watch Adrian Digital Basement use a cheaper one than yours and what he does is move the opening in circles around the pin with a gentle speed and he seems to get all the solder out in one go.
Heya, nice vlog learned again more thanks
Another great repair, you could have spotted the varistor heating with your current microscope thermal camera, where is the promised video ?
Which promised video? What have I forgotten to do now
@@LearnElectronicsRepair You promised to show the thermal camera from infiray showing current flow for trouble shooting shorts.
Do they usually have heat shrink around them ?
Repairing Master
I have a collection of salvaged varistors. I just remove from stuff when I see them .Do you think you would use them at a push or wait a month and replace with new stock ?
I think this depends on the circumstances. In this case, the PCB is used in a commercial environment (health centre) so I feel it is important to complete the repair using only original value components. Also it was sent to me from the UK and will be going back there so it would be more hassle if I have to change the Varistor again.
informative as usual
Can I connect my external power supply with alligator clips to a 500 series module edge card to power it up to test it . Simply connection the ground , neutral and hot directly to the card? Sorry if it’s a dumb question. Really enjoy the channel!
EFFECTIVELY!!!!!!!!
Are you going to replace both MOVs? The other one may have been weakened a bit in the past and the cost of the component it is best to ensure that both of them are new.
Yeah why not - I'm ordering a pack of 20 anyway.
very good remark
Thanks a lot
Is it possible that the VDE on the varistor is the logo of the german "Verband der Elektrotechnik Elektronik Informationstechnik e.V." and ist therfore some kind of label for quality control?
love it tks-
Hi Rich, I believe you got the logic backwards in your diagram with regards to the Opto and SCR, when the Opto-isolator switches ON, would it not remove the gate voltage/current effectively preventing the SCR from turning ON?
Yeah you are correct, that circuit would work that way round though the principle is the same. A logic low turning the LED off would turn the SCR on, and vice versa
@@LearnElectronicsRepair Yes, I'm not trying to be picky, I just wanted to clarify that for anyone attempting to build something similar.
@@KB1UIF Absolutely -and please continue to do so, I don't always get it exactly right though that does not usually stop me from fixing stuff
Youre a busy man
Super!!
isnt the transformer only rated for 120v only depend on the writings on it ?
There are two 120V AC primary windings, which you can wire them in parallel for 120V or in series for 240V.
Really enjoying these videos..being a software engineer i also wanted to know more about hardware..and these videos really help..thanks for these explanations
Richard I should think changed a few of them in your time , I know I have, I like this type of repair ,just to put my twopence worth in.
A thyristor switches on when the gate is on a higher potential than the cathode and it stays on as long as there is current flowing. You seem to say it switches on when the gate is on zero potential. I am confused.
Everything seems to turned around, even the diode.
@@kriswillems5661 The diode is the right way round but in this circuit the SCR actually turns off when the LED in the opto-isolator is on, and turns on when the LED is off. My mistake and you are correct. Well spotted and hopefully you are no longer confused.😉
@@LearnElectronicsRepair Ah, right, the diode is right. But will it work if the load is connected to the cathode? Shouldn't it be connected to the anode? I guess it doesn't matter because there's almost no voltage on the load when there is no current. But this is only true for a resistive load, if the load be not linear or slightly capacitive there might be a problem. Generally the load is put on the anode, because this way it's easier to control the gate-cathode voltage.
@@kriswillems5661 I'll accept what you say and learn a little from it 🙂
Fortunately I'm not a circuit designer, I'm a circuit fixer, so I just repair what is in front of me. I have a good idea how it works, enough to diagnose it, but freely admit I don't always know the ins and outs of it exactly.
Hi Richard, Can we hire you for a repair?
Sure - email me electronicanaria@outlook.com
Are u based in Las Palmas? 🙂
Mythological fault finding... like my ex! 🎉 😂 Just kidding. Nit a thing.... We don't even talk. But it's a funny joke! 😁 Thanks for the unintentional laugh, beyond the educational value of the video too!
Power surge “cleaned its clock”
Thanks for the cool video. Nus6 check out the one's under this headline 👍🇮🇪💚
👍👍