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!

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.

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.

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.

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!

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.

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.

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.

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

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

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

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

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

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

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.