Wine fridge PSU problem

[Mad Capper]

The story so far ...

I am posting this here because I believe/hope the my problem lies in the power supply side of the board.

This is a PCB from a Franklin Chef FCW16T wine refrigerator. Four years ago it blew the fuse and I replaced it and it blew again before I noticed it had a blown cap, the one on the right. I replaced the cap with as close to the value I had at the time. It worked since then just fine.


Last week the power shut off and came back on around four times with in about ten minutes, after which I could hear what I call a zitsing noise the after another cycle of the power, maybe a slight high pitch noise. Both did not sound good and so I unplugged it. When I took things apart I only saw a blown fuse so I replaced that and plugged it in. With out making any of the previous sounds or any at all that fuse blew as well. The two large caps look ok, but, as I have read hear before a capacitor does not need to show a bloated top to be bad.


As I do not know very much I am guessing/hoping that if it is blowing fuses there is something wrong on the power side rather than the logic side. So based on my description and the image of the board is there anything that you can suggest to test/try? Unfortunately all I have is a meter for test equipment. Let me know if there is anything that I can better identify on the board.

I know based on its history it might just be a cap but since the look ok I, not knowing about power supply circuits, just want to be sure there is not something else it could potentially/probably be. The other components look ok to as far as I can tell but based on the much superior knowledge you all have I thought I would get an opinion on what else it could be.

The older of the two large caps is a 200v 120uf, I have a 200v 330uf, would this work as a test or is 330uf too far?

Thanks for any assistance, I appreciate it.

[budm]

The two 120uf 200v are not connected in parallel, but they are wired up as voltage doubler set up, so you need to change both of them, since they are in the primary side, the inrush can be too high and may blow the main fuse.

[Mad Capper]

Thanks for the info.

So would it be a good probability that the big caps are shot and the sole problem and that the rest of components, the transformer, diodes, disc capacitors and the yellow metalized film capacitor are still good?

Thanks

[budm]

You should have cap meter to verify the value and the ESR of the caps. Since the main fuse did not blow so it may safe to say that you do nothave short circuits. You should repalce the two caps first with good caps like PANASONIC FM/FR series if you have no way of verifying the condition of the caps.

[vinceroger69]

just read your first post are you saying the mains fused you replaced blew as soon as you plugged it in? if so yes there could be a item on the board which has a short on it.

[budm]

Ah I did not catch that the new fuse is also blown.

[vinceroger69]

do these power supplies have a bridge rectifier on?

[Mad Capper]

Yes, the glass tube fuse blew when it was replaced. Just like it did 4 years ago, but then one of the big caps did show bloating.

The two parts with heat sinks below, in the picture, the caps are output transistors and the other component with a heat sink is a diode and the last one is a voltage regulator.

As far as google shows what a bridge rectifier looks like I do not see any on the board.

Would the possible shorted component be on the power side?

Thanks

[999999999]

^ Yes, in the picture there are 4 individual diodes forming the equivalent of a bridge rectifier to the right of the two large high voltage capacitors.

What has most likely happened is you had a power surge when the power was cutting out and it has damaged the switching transistor(s). Certainly you should check the 4 diodes too, but those two components on the heatsinks right under the large HV capacitors are where I'd focus first, the caps do not appear to be the problem based on their appearance and when the fault occurred.

Once you feel you have found and repaired the fault, power it up without being connected to the rest of the fridge. If it does not stay on due to having no load, put a power resistor on the output to cause a few hundred mA of current flow on the primary power rail which is presumably the one with the blade style connectors soldered to the PCB.

I doubt the inrush current upon power on is too high, there's inrush limiting via an NTC thermistor behind the AC input power pins.

[Mad Capper]

Ok here goes, hopefully I tested correctly.

I lifted one leg of each diode. Black lead of meter to cathode (striped end) red lead on anode, shows between .58 and .6 on all of them. Leads reversed shows nothing. Am I correct that the means the diodes are good?

As for the transistors with my meter on the diode check setting I get the same reading of .002 for every combination of base, emitter, collector both with the red or black lead, i.e. what I read is forward or reverse bias. When using a continuity setting every combination beeps. Am I correct to say that these then are the problem and thus are the short that is causing the fuse to blow?

Would it be advisable to replace the two large caps along with the transistors?

Thanks for all the help, you guys are great.

[budm]

Take out those two transistors out and check them out off the board again to be sure they are shorted.

[Mad Capper]

Sorry I forgot to say that I did take out the transistors before testing.

[budm]

OK, you need to replace both of them and might as well replace the caps with good quality caps.

[Mad Capper]

Alright then I will get my parts order together and get back to this when time is available.
Hope this fixes everything.


Again, Thanks to all

[TimH]

I'm newbie -- I followed the post since I'm experiencing the same issue and have the same wine-refrig. This unit was given to me since it was not working. I opened the cover and at first glance noticed that their was no fuse in the fuse holder. I have no idea if this is common to this board but layman's thought that it should have a fuse. Could anyone advise of this point? I have removed the two caps, one of which was blown and now want to replace them. I noticed there's no fuse in the photo that is attached to this thread. Thank you, Tim in Tucson

[999999999]

^ It MUST have a fuse installed to work, the fuse is right after the AC power input plug so no power gets to any portion without the fuse. Well, you could just jumper across the fuse holder contacts but don't do that for safety reasons.

Odds are the old fuse blew, someone pulled it out and either gave up on the repair or blew another fuse before removing it too.

Fortunately it's marked on the PCB that it's a 6A fuse so the guesswork replacing it is removed.

[TimH]

Greatly appreciate your speedy reply and the info.

I'm ordering the caps and will solder them in.
Thanks,
Tim

[kaboom]

Originally Posted by 999999999

fortunately it's marked on the pcb that it's a 6a fuse so the guesswork replacing it is removed.

1.6a, not 6A! Look at the line rectifier diodes...

Usually slow-blow, too.




If it keeps blowing fuses, check the rectifiers, looks like 2A06s in there, sometimes 4004s/7s. Also check the switching transistors, usually 2sc4242, MJE13005/13007. There are also two fast-recovery commutating diodes, one across each transistor. Check the big schottkey on the secondary side, and the small FR diode, that taps power to the controller, before the output inductor.

Sometimes, the two 2sc945/2sc1815s feeding the drive tx get leaky. The supply cannot "kickstart" in this case, which has to happen for control power to be developed.

If any power transistor is shorted, completely rebuild the base drivers circuits. In any case, replace the two "back bias" caps in the base ckts- they are from 2.2u to 4.7u.

[999999999]

^ OOPS. Thanks for catching that!

[TimH]

Thanks Kaboom, I did catch the fuse confusion.
I measured the resistance of the black diode (labeled RTC) that is adjacent to the two 120μf 200v that I'm replacing and it seems to be okay. As stated, I'm very new to electronics and trying to learn. This forum is extremely helpful but as I read you detailed "what to do" I find myself lost. My cooler did not have a fuse in the fuse holder when I received the unit (free 2 me). I have a smaller unit that has stopped working and I'm attempting to use this as a training ground. I had hoped that all I needed to do was replace the blown caps … I now see it is more involved … Question, since I've replaced the caps and will install a fuse, could I damage any other part of the circuit if I connect power and start tup the unit?
thanks in advance, Tim
p/s I appreciate your quote, "Schools should teach how to think, not what to think" and this forum is a good example.