VERY Expensive Fire Alarm Power Supply | Can I FIX it? 

Amazing to think the amount of expensive equipment that ends up in landfill over a 50c component. Great fix

I have been an electronic technician for about 60+ years. I am always very suspicious of these electrolytics, as well as all electrolytics in general. Motorola used these caps in some 2-way radio products, and they were a constant cause of failures. This was 30 years ago. Some things do not seem to change. Brilliant trouble shooting!

Electrolytics in the startup supply are an extremely common failure - maybe half the PSUs I've ever looked at have been down to this. Like this one, they're often close to a hot heatsink. I usually just replace them before looking at anything else.

14:35 That 224 resistor is the start up resistor. The start up cap is slowly charged up via it until the 3845 chip has enough voltage to drive the FET. Once the PSU is running, the chip is powered from the aux winding.

Capacitor.... Why am I not surprised?! 👍👍Nice fix.

Saved myself £350 on a new washing machine years ago by changing a cap out on the control board. Also saved my company £1500 on a new hot water system return pump due to a blown capacitor that cost me £2 to replace! Always a bloody capacitor, mate!

I've been a fire alarm engineer for over 20 years. Gent is a racket. Once their system is in place, it's impossible to replace the panel with anything different without also replacing all the field equipment. Check the price of the gent psu-72 if the price of the Nano psu shocks you! We used to be able to do in field diagnosis and repair on PCB components, but with the way everything is certified these days it puts you on very dodgy ground to replace any components. Certification costs so much and the equipment has to go through so many tests to pass that it's just not worth your while to open yourself up to the liability of someone saying your repair means the equipment is no longer certified. The best thing anyone can do is insist on open protocol systems. Gent CLAIMS to be open protocol because they can link up with bms systems for remote monitoring but REAL open protocol means having something like Apollo or Hochiki field equipment that can work across a range of panels so you can stick two fingers up at any panel manufacturer that tries to rip you off with replacement parts.

Great fix, I would like to see what was the voltage on this IC after capacitor change.

When you use a DMM on a DC range it is important to remember that most DMMs will show you a time-averaged value. For example, a square wave that swings from 0 V to +5 V will usually result in the meter displaying 2.5 volts. If the frequency is very low or the meter is "fast" the displayed voltage may be almost anything between 0 V and 5 V and "unstable." With the UC3845, the voltage reference output will be 5 volts if the Vcc for the IC is above the lower limit. If Vcc is less than the lower limit Vref is pulled down by a resistor of nominally 5k (from text description of Vref in the datasheet). The fact that Vref was reading as 1 volt very strongly suggests that the IC was repeatedly trying to run. The cap that was replaced was almost certainly charging to the turn-on threshold for Vcc. As soon as that happens the current requirement for the IC itself increases and quite a large amount of current may be required to drive the gate of the FET. Normally as soon as the circuit starts running an auxiliary winding on the "transformer' keeps the cap charged. If for any reason the supply doesn't come up quickly enough, the cap will discharge to the turn-off threshold and switching will stop. That returns the IC to requiring only very low current. The cap will charge from the input supply through the high-value resistor, the chip will turn on and another cycle starts. 1 V at Vref suggests tha 3845 was running about 20% of the time, with unknown period. Some additional information can be had with a meter on an AC range if a DC blocking capacitor is used (I think Fluke meters have one internally in most models - check by measuring a battery on AC; the AC reading would be zero if there is DC blocking). If Vref was "stuck" at 1 V you'd expect close to zero AC. If it was switching between 0 V and 5 V you'd get an AC reading of probably hundreds of millivolts or more, depending on the actual waveform and the way the meter interprets it so the value displayed is mostly just a hint of what is going on (consult meter manual). Similarly the voltage on the cap in question would have a moderate AC component (in neither case does the polarity of the voltage actually change but the meter "sees" it as true AC due to the blocking capacitor). An oscilloscope is very much more informative but you absolutely must use an isolation transformer for the AC input to such a power supply unless you have a scope that is isolated from mains ground and designed to be safe if its ground is "hot." Very few fulfill the latter requirement. With a switchmode power supply you can often get some idea of what is going on by simply holding a probe near the power components where it will pick up radiated noise. Use a clip tip for the probe so you have a bit of an antenna. Just don't drop the probe - exciting things happen if the ground on the probe touches something in the circuit and an isolation transformer isn't being used.

That was an awsome repair mate. The company is making a fortune by selling new boards to their customers.

Before retiering, I repaired many hundreds of power supplies with this family of chips. The most common faults when not starting up were: 1. Chip fault. 2. Capacitor fault. 3. Startup resistor fault. 4. Zenerdiode fault. Sometimes the chip does startup, but to continue working, the 220K resistor is too high and cannot supply the power needed to keep the chip working. So the capacitor is discharged below the threshold, and the chip stops working. The capacitor is then recharged, and the chip restarts to stop again quickly. You only get small "spikes" on the output of the power supply. For that reason, there is a diode connected to one of the windings of the transformer that supplies power to the chip, so it continues working. This phenomenon happens when there is a short on the output side. It can also happen when the small capacitor, the one you replaced, is leaking too much. The voltage gets just high enough to start, but the leakage of this capacitor is too high for the voltage to rise to the normal working voltage of the chip. That usually is somewhere between 12V and 18V. That should be the voltage of the zenerdiode.

Great work as always ! I’m a construction superintendent and work with these types of alarms . What I can tell you is that any time something has to do with fire control or life safety systems the prices go up quite a bit . I think because all of these items need to be certified by a second party in order to be used . Almost like paying for a UL listing . For example a commercial metal door frame vs. a fire rated metal door frame . The difference is literally a UL tag that says it was tested and certified for a certain fire rating ie. one hour , two hour etc . Hope this helps .

Nice fault finding! I was shouting at the screen to change cap - I've seen those exact type of SMD electrolytics fail in large numbers. Very common in Linn amplifiers from the 00's.

Fitted many gent panels. Once didn't realise I connected the batteries in reverse and the plug welded into that white battery socket. Amazingly the panel still worked.

Those UC3845 and PC123 were used extensively in consumer products in the US. I worked on stuff like that for 20 years. I would replace all the low value caps < 100uf in the ones I worked on weather they checked Ok or not. Those ICs rarely failed. Also Zener protection diodes on the low voltage secondary lines would short. Sometimes The switching transistor shorts. Other than that those switching supplies work well

If I recall, your data sheet for the ‘UC’ chip listed the maximum voltage where the operation HAD to start as 8.8 V. The typical (most units started) was your expected 8.5V. My experience is you better always keep in mind, and check the WORST CASE, rather than depend on a single “typical” number. Well, you got it anyway. Fine job!

Excellent job, you've just saved somebody hundreds. Although UK power is supplied to domestic premises at 230Volts, Ofgem allows a tolerance of +10 and -6% on what you receive at your Service Head (Meter) that's 216 V to 253 V, so 251Volts is still within what is allowed.

Quite a few years ago, When i was repairing TVs, VCRs, microwaves, and just about anything electronic, I would always check the electrolytics mounted near a heat sink. The heat dries them out. The smaller values are particularly susceptible to drying out.

Bad capacitors being the culprit is all too common. Just fixed an UPS, also with a failed 50V 47uF, but that was bulging so easy to see from the get go. That was also in the battery charging circuit. Thanks for a great video, subscribed!

Nicely done mate! .... it's always the capacitor! 🙂

The dreaded surface mount electrolytic caps strike again. Nice job figuring it out!

I have 3 or 4 of these under my bench waiting for the day i have time to see why they fail, thanks for spoiling my fun and also thanks for saving my time lol, great video as always !!

Nice job. Its crazy the price of electronics that are used in health and safety!

That was a very satisfying repair. I felt elated when the green LED lit up 😊.

Patience, knowledge, skill, and Will to Try ... *Priceless.* Thanks for it All.

Another great video. It's probably just my DIY level experience but, I didn't see anything special that should bring a $500US price.

Always these capacitors. They seem to be in spec and do what they should do, but in the end it always ends with a faulty C again.

I find so many blown domed caps on computer mother board's. Swop them out for new ones and all is good. Cheep caps that last max two years means company's can keep selling new models every two years. Why are we not surprised 😊 Great work great vid Nice channel

There is no greater wealth than experience. Thanks for the example.🙏🙏🙏

That was a great fix without a circuit thanks for sharing it's surprising that quite a lot of faults are only caused by a capacitor or a resistor going open circuit or short circuit. It goes to show that in general a well designed circuit the semiconductor components are pretty reliable. I guess the cap failed because it gets the heat from the heatsink which isn't going to help it. Good job another board saved.

Nicely done sir! Once again a cheap cap takes down an expensive piece of tech.

Wow Mick well done, you have saved your customer £400!!!!! 😁

Same issue with gate motor power supplies, which use a similar IC. I keep the output capacitors and some 47uF 63V capacitors to replace those, though they work fine with any value from 33uf to 100uF on the primary side, just replacing the capacitor there, and the 2 on the output side, will fix most of them. good as those PSU units are nearing the $100 mark now, and the fix costs under $2 to do.

i use the same brand system, and the expensive bit is the certifications, and of course they have the monopole for the spare parts

And once again it proves out the old adage, "When in doubt replace the electrolytic capacitor". It's amazing how they can screw up so many things, nice catch & as always great video.

I spent over 15 years testing and repairing switch modes , I was screaming aux rail cap at the TV lol.

Perhaps keeping the text for one or two seconds longer when displaying text on screen. Otherwise always interesting following your detective work 🙂

Hi Mick just to let you know that i finally got round to looking at one of the faulty ones i had lying around and it was the exact same fault. Thanks, Mark

Fly-backs often broke like this. You can hook up scope to this cap and in most cases you will see. But your method with adding DC to Vcc of chip is also nice and checking out waveform. Maybe I will do this that way some day :)

"Capacitazz, Resistazz" Love that accent.

While the end roll ads are running , I'll comment .... Awesome diagnostics and fix ..... I have one of those component testers and I found that if you use the bottom left pin section it will give false readings , as you know it's for zener diodes , I forgot to switch the pins to the other sections a few times ..... Mine is about 7 yrs. old and the 1 pin in the sections do not work for ESR readings , 2 and 3 are spot on , I double checked with my BLUE ESR tester ..... 1 pin works fine with 123 sections for transistors though ..... I did find an old Micronta Dynamic transistor tester 22-025 model new in the box for 20 bux on QRZ ham site ...... A sweet find for sure , I had one 30 plus years ago and it's great for matching transistors ...... You saved that some money on that repair ....... Take Care Mon Ami ......

Thank you for this fascinating and as always, perfectly edited video.

I'm in Northeast Georgia usa. And when I was working as apartment maintenance supervisor years ago I had to fix a couple of those. I mean the company didn't want to buy them so they knew I was good at electronics and they asked me to give it a try. We had a lot of iron in the ground around us in the apartments were kind of up on a hill. We were getting hit by lightning all the time up there. It turned out it was a fairly easy fix, the ptc's were burnt out on the input protection

If you want a challenge, try working on a power supply for a vintage 80's-90's digital oscilloscope. Like a TDS714L.

As usual unbelievable fix. Great and thanks!! Have great day!!

It’s been known for a while that it was just a dodgy cap that causes these to fail, it was still interesting to see the thought process behind finding the fault though. I’ve had to replace many of these psus over the years, and it’s amazing how many are still out there, but service engineers aren’t testing the systems correctly now because they know the system will probably die when they do it. The most concerning thing is that gent/honeywell seem know about the issue, but never issued a recall, or even admit fault, as far as I’m aware.

Excellent job as usual

I clicked this video from the thumbnail because I swear I have that exact same board and it's broken, that's wild, go algorithm!

Hello mate. Down the road here in GUISBOROUGH not far from you. Just letting you know another Tech shares your frustration. And happiness when it works.

Electrolytic cap failure is very common. I'm surprised it doesn't appear more often on your channel. Maybe is it because you're generally looking at relatively recent (i.e. not old) devices?

23:17 - Slots that you used for green and red wire are K and A, for diode testing. You need to use 1 or 2 or 3 slots as you did. You can also open device and solder wires directly inside, to prevent bad contact at lever thingy. In that way you also have wires always with device. Great findings. ESR just high enough to drop voltage to disable IC.

Really enjoyed the fault finding and thought process on this one. Brilliant work Mick.

Well done!

I only have a high level understanding of what you do but man do i enjoy watching you figure stuff out! Thanks for sharing

I love the disclaimer at the bottom.... And I can read the fine print on my 24" curved computer monitor.

The main reason for the high price is due to the certifications it must have to be used in a life safety system. It’s not technically legal to service fire alarm parts unless you are a licensed and certified service technician. We here in the USA used to be able to send the boards back and have them repaired by the manufacturer but now they either won’t do it or they make the price of repair higher than the already high price of the new board! It’s just a matter of money now…

It was amazing how something so cheap is a good fix how many people would have replaced the power supply with a new one thanks for another great video

Great video! RU-vid needs more of exactly this type of video. Thank you for your time!

After fixing several of these for subwoofers, lights, etc I'm tempted to just replace those cruddy electrolytics before doing anything else. They have ALWAYS been the problem.

Great job on finding and fixing the fault!

The startup voltage is quite often dropped from the hv dc line through high value resistors, so a little leakage through the cap will shunt it down and stop starting.

Lovely repair 👍🏼

Nice fix, and good teardown of the working. I also see that you have a new and bigger thing to remove nasty things.😉

Love your work. Never saw a troubleshooter wearing kilts. Just joking, watched most of your work. Like your thinking process. Surprised not more upvotes. Keep up your sharing and thanks again.

Brilliant diagnostic process! You have a very real talent for this. Thanks for sharing.

Fantastic video never boring to watch always fascinating to see the end result

nice work! I mind fixing a samsung tv, 65", brothers mate threw it out, bridge rectifier gone, 1.25 to fix it. brothers mate was raging when he found out, as he bought a new one at 1000. ouch!

I upgraded from a first gen Aoyue hotair station to the Atten last year and its such a upgrade, I love that station.

Now that was a gorgeous fix.

What a skill you have. Admiration 👍

Good fix. You the man.

another great job,well done!!

I was given a high end car stereo that wouldn't work. Fix was easy. When I lifted the lid, there was a capacitor rolling around inside. It fell out due to bad soldering during assembly. I found where it went, & soldered it in place. Stereo was fixed.

@0:13 - For those that may not know - the thicker pink/purple electronics bags are supposed to be anti-static bags.

Good job as always,thanks

Good job! I was thinking maybe a bad Zener, resistor, or cap on that start up supply. After it's running VCC for the chip is usually supplied by a small run winding on the transformer. Great troubleshooting in this, it's rarely the ic.😊

Electrolytic capacitor next to a heatsink, who’d have thought it? Wish manufacturers would consider this more when laying out boards.

Awesome as usual 👍

Nicely diagnosed!!

Simple fault. Nothing surprising or indeed very interesting. And yet such a big part of our current world. So many things are just simple fixes people with the correct mindset, some cheap tools and a bit of know-how can fix. This is exactly why I also like to dabble with these things and revive equipment back to life. Really fantastic to see your thought process and workflow here, 10/10 job and an easy subscribe :-)

the small cap near the one you changed looks cracked above the inverter 10:44, nice job as always.

14:05 There will be an auxillary winding on the transformer that powers the chip. That SMD capacitor is the start up cap.

Absolutely brilliant bit of fault finding there Mick 👏 👌 loved the meter, scope and power supply at work, I am sure that Mark will be happy about the result, nice 😊👍

Nice work...cheers!

I really enjoy your videos. 😀

Another job very well done, thank you for you great videos or should I say tutorials . Is there not a single thing you can't fix. Cant wait for the next video.

great fix

Gracias hermano

Nice fix well done Jerry interesting

Good job mate.

Great one!

Are you familiar at all with Vicor power supplies? Looking at one right now that is quite complex. It is composed of like 5 chassis mounted cards, 5 or six plug in cards, and has additional sockets for more cards. An input power board feeds 3phase 120/208 into full wave bridge rectifiers and outputs about 280vdc. From there, the power goes to a toroid on the main board and tot he source pin of a MOSFET. The MOSFET gate appears driven by a separate driver board that plugs vertically into the motherboard on the LH side, and there is a supervisory board full of a boatload of opto couplers, thermistors, dual comparitors and the like, which is plugged into the RH side of the motherboard. At the halfway point (fore-aft) of the motherboard is a backplane where several output cards of different voltages plug in lengthwise with the unit from the front. I've never seen anything quite so involved. It looks like the design avoided using inductors outside of the larger ones driven my MOSFETs.

My goodness I have the exact same power supply on my lap right now, it’s from a honey well fire alarm system, I changed the pwm chip with a ca-888 and it worked fine . My goodness

A fault finding masterclass.

Another great video 👍

Nice fix.

Liking the technique, subbed.

Would have loved if you’d checked the voltage on that pin after the capacitor swap. What is a normal startup voltage when everything is working?

Very cool repair, saved a lot of money :)

A friend of mine had a different back-up generator system with a failed board after 6 years. The replacement board was $800 USD! That's crazy!