Pure Electronics Repair 3 Learn Methodical Fault Finding Techniques / Methods To Fix Almost Anything

LER #304 This time I was asked to repair the control PCB from a large clock tower on a commercial building in the UK! Obviously I know nothing about clock towers and how they work but I do know some stuff about electronics... so of course I accepted the job. I mean, how hard can this be? It's all electronics at the end of the day. Yeah?

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TEST METERS
Aneng AN8009
KM601
VC480C+
MESR-100 ESR METER
XC6013L CAPACITOR METER
TM-902C TEMPERATURE METER
FNB58 USB ANALYZER
PCI POST ANALYZER
TL460S Plus PCI_E ANALYZER
MULTIMETER PROBE KIT KET05
OSCILLOSCOPES
FNIRSI 1014D
FNIRSI DSO-TC3
FNIRSI DPOX180H
BENCH PSU
NPS3010W
SOLDERING
T12 Station with M8 9501 Handle
SUGON T26D
QUICK 861DW
PROS'KIT SS-331H
THERMAL CAMERA
Infiray P2 Pro
MICROSCOPES
Amscope Optical Microscope (copy)
ANDONSTAR AD407

EEPROM Programming
TL866 II+
CH341A

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Thank you
Richard

[LearnElectronicsRepair]

*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

[frieddo]

love this methodical fault finding series.

[JosepsGSX]

Another extremely interesting video. I really enjoyed both the procedure and learning about how varistors and triacs work and are used. Thanks!

[shagreobe]

Being a beginner, I always thought they were caps. I learn something from each of your videos. Thanks!

[carlos]

Great work, I would change the second one too because MOVs generally will have degradation caused by many times seeing voltage spikes on main

[romancharak]

So, the Varistor did what is designed to do - protect the circuit board from voltage spikes. RIP Varistor.

[orionRS]

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.

[amrelsharkawy]

It's very kind of you to explain in such a rich details.. thanks so much for the info.. I really appreciate your efforts.

[ziggyironic]

Brilliant, thanks once again for sharing your knowledge and taking the time to make this video. 👍

[Colin]

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

[newmonengineering]

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.

[anthonydenn]

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.

[darkdepth]

One of the best technical teachers on the internet !

[jernejkurincic]

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.

[wiedapp]

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...

[skanderbenmrad]

hi richard, from tunisia i make the same profession and i agree in all, thank you for your videos they inspire me very well !

[scotshabalam]

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.

[gregreynolds]

Great video and analysis! Would love to see more of this kind of repair.

[ralphj]

Well discovered. No fuses inside the transformer, just confusing symbols meant to represent the maximum secondary loading.

[surgingcircuits]

Thx so much for your vids. I was getting stuck working on boards, and you have helped me move forward. Cheers!