Hi everyone, I’m new to this community, so please bear with me. I hope this is the right place for this kind of troubleshooting request.

I’m trying to repair an old Daikin outdoor unit controller board. I don’t have the schematic, so I’m reverse‑engineering the circuit on the bench.
So far I have found two failed components:

1. R5 (5 Ω / 5 W)
2. 2SC3751 transistor (it is the switching transistor driving the flyback transformer (trafo) primary)

I replaced both with new parts, but they failed immediately on power‑up. For safety, I also replaced the bridge rectifier and the main electrolytic capacitor (C9). Now I’m trying to identify what is causing the repeated failure.

I would also like to replace C13 and the Zener diode, but the Zener marking is unreadable.
Some components were tested in‑circuit because the board is double‑sided with plated‑through holes, and removing parts risks damaging the PCB.

• D1 seems fine (tested with a multimeter).
• The resistors in the area measure correctly, as shown in my draft schematic.
• I also removed and tested the C1815 transistor, which appears to be OK (I'll replace with a new one)

At this point I’m looking for help analyzing the part of the circuit shown in my draft (attached). Please note that some small components may be missing from the drawing.

My main questions are:

1. What could be causing the repeated failure of the 2SC3751?
2. Is there a typical Zener value used in this kind of base‑drive network?
3. Could C13 (the capacitor in the snubber/clamp network across the primary) cause this failure if it is open or degraded?

The capacitor shows visible cracks on the sides, so I suspect it may be faulty.
Any help understanding this section of the circuit or confirming the likely failing component would be greatly appreciated.

I found a very similar circuit here in this form in an old thread: https://forum.allaboutcircuits.com/threads/is-this-a-class-c-amplifier.132927/ posted by @Jony130 ...maybe you have some ideas?



Thanks in advance for your help!!!

fuzzy logic

 

Here below is a revised drawing, with added components.


Thanks,
fuzzy logic

 

Just a correction, the resistor R5 is 51 ohm 5 w.

 

Hi there, anyone?

In the meantime, I found the zener to be 6.2 volts.
Thanks!

 

1. What could be causing the repeated failure of the 2SC3751?

It's more likely that the diode or filter capacitor on the secondary side of the transformer could be defective.Or the transformer itself is broken, but you can't check it with a multimeter

1. Could C13 (the capacitor in the snubber/clamp network across the primary) cause this failure if it is open or degraded.

I wouldn't think so. The power transistor can usually handle that mode when the snubber is disconnected.
This capacitor can be easily tested by connecting it to a 230V mains.You'll see a spark when connecting the lead

PS.By the way, when doing similar experiments with mains voltage, it's necessary to follow safety rules and practices !!!

 

Thank you @Pyrex for the reply!

Currently I've replaced the following components, for primary side:

1. R5
2. Q1 - C3751
3. Diode Bridge
4. C9 capacitor
5. C1815
6. C13 capacitor (thanks for your observation, though)
   Zener Diode
7. Electrolytic Capacitor between GND and the zener
   

Some of them were faulty, others probably not, but just to be sure...

In the next coming days I'll proceed with a test with a lamp in series, trying to avoid other faults while understanding what's going on. I'll post again with some results.

For the secondary side, I consider to isolate each secondary if the fault is still present.
Not an easy task, probably I'll proceed cutting PCB traces... I know, not so nice... but I can't see any practical way.

Any idea is welcome! Also to what winding I should start.

PS.By the way, when doing similar experiments with mains voltage, it's necessary to follow safety rules and practices !!!

Thanks for the advice! I really appreciate it.
I’m well aware of the risks... I’ve been working with electricity for almost 40 years, I’m quite familiar with safety practices.
I’ve always been passionate about electronics. Back in my school days I used to build a lot of experimental circuits...
I hope I’m not too old to solve this problem as well!

Thanks in advance!
FL

 

Are you certain on the polarity of the zener z, and capacitor c bottom right ?

 

Thanks @drjohsmith , yes, absolutely, it's typical.

You can see a very similar circuit explained in the old post that I linked in my first message in this thread.

Thanks,
FL

 

Thanks @drjohsmith , yes, absolutely, it's typical.

You can see a very similar circuit explained in the old post that I linked in my first message in this thread.

Thanks,
FL

Sorry the +ve side of C is more negative than the -ve side or are you generating a negative voltage ?

Have you considered a simulation ?

 

Unfortunately I'm not an expert on this circuit, I know the capacitor seems to be wrongly placed, but there are good explanations by @Jony130 here: https://forum.allaboutcircuits.com/threads/is-this-a-class-c-amplifier.132927/post-1107738

Thanks,
FL

 

Unfortunately I'm not an expert on this circuit, I know the capacitor seems to be wrongly placed, but there are good explanations by @Jony130 here: https://forum.allaboutcircuits.com/threads/is-this-a-class-c-amplifier.132927/post-1107738

Thanks,
FL

But that circuit you reference , drives an inductor / transformer in which case negative voltages can be generated .
You can't take random bits of circuits without understanding.
I certainly don't understand in detail , it just looks wrong , hence suggestion that you learn to simulate .

 

Hi @drjohsmith , if you look at my second drawing, you can clearly see that this is indeed a circuit that drives a transformer. I didn’t take random parts of a schematic: I am simply reconstructing the circuit directly from the PCB, following traces and components... it is what Daikin realized and has been controlling the outdoor unit since years, so obviously the circuit is correctly realized.

The topology I posted is not something I invented — it is exactly what is on the board, and it happens to be very similar to the circuit discussed in the thread I referenced earlier. That’s why I mentioned it: the structure matches almost one‑to‑one.

I’m not claiming to fully understand every detail of the design, but the reconstruction is accurate, and the similarity with the previous example is real. My goal is only to analyze the circuit correctly and understand the failure mechanism.

Thanks for your feedback anyway.

 

In the next coming days I'll proceed with a test with a lamp in series, trying to avoid other faults while understanding what's going on. I'll post again with some results.

For the secondary side, I consider to isolate each secondary if the fault is still present.
Not an easy task, probably I'll proceed cutting PCB traces... I know, not so nice... but I can't see any practical way.

Any idea is welcome! Also to what winding I should start.

I could bet that this PSU's topology is flyback.So it would be enough to desolder the power diode that's connected to the secondary winding.This way it will be possible to isolate the diode, the filter capacitor, and the rest of the secondary circuit.By the way, the power diode might have an RC snubber, so disconnect one of these components, R or C.