PCB Repair gone wrong

[Sotiris Bos]

This will be a lengthy post so I will cut straight to the chase. I own a Krups Dolce Gusto coffee maker (https://www.amazon.co.uk/Krups-Nescafe-Melody-Manual-Machine/dp/B00843M0QC) that used to be in our summer house which had a problematic main power cable and thus had serious problems with the electricity. The whole area has a general intermittent voltage problem. The electrician came to replace the damaged cable and cut the power multiple times. It is then when the coffee maker stopped working and wouldn't turn on. I am very inexperienced with electronics (my experience is soldering a molex adapter on some LEDs to install in my computer). I thought I could tackle this.

I took it apart and found a varistor (TVR 14471) with a broken outer sheeth and two traces on the pcb that used to connect to it that were black. I watched a couple of pcb repair videos and soldered two cables in place (must have been aluminum, they are silver in color), after having removed the varistor completely. I tried it and it worked fine. The cables were very thick and the pcb wouldn't fit into its plastic holder, so I removed them and replaced them with two very thin copper cables. THEN, I probably foolishly decided to solder the varistor back on, since I got 1.500 kOhms resistance on it with a multi meter. I soldered it on the way it used to sit (I think). I plugged the pcb correctly inside the coffee maker and then I plugged it in the outlet. That is the moment when the thing exploded and knocked out the power from the power main of the apartment (the box of the electrical company which is in the entrance of the building, not the electrical box in my flat). I removed the pcb and took some photos of it that I am going to post below. Is it salvageble? Did I destroy anything else? (like the thing that looks like a small resistor and is marked as D1) The fuses are inside the coffee maker and I didn't check them.

 

[Sotiris Bos]

 

[davenn]

THEN, I probably foolishly decided to solder the varistor back on, since I got 1.500 kOhms resistance on it with a multi meter. I soldered it on the way it used to sit (I think).

hi there
welcome to EP

what on earth possessed you to think it was a good idea to reinstall a totally blown part ?


firstly ... remove the varistor and THROW IT AWAY .... after noting any part numbers on it

secondly .... use some meths or other cleaner and clean up the circuit board of all the black smoke and crap

thirdly .... remove the wire(s) you put in there. It appears as tho you may have put a short circuit strait across the mains input
hence all the blowing fuses etc in the apartment block

Then finally ... when you have a good clean board, take a new photo of the solder side ... sharp and well lit



Dave

 

[Sotiris Bos]

hi there
welcome to EP

what on earth possessed you to think it was a good idea to reinstall a totally blown part ?
Dave

Hello to the forum!
Well... I thought that it was just the sheath that was damaged because I was getting a high resistance

I cleaned up the soot with 94% ethanol and cotton swabs (I know, I am terrible).

I am attatching photos through imgur because they seem to be less compressed. The one edited in Microsoft paint shows you where the burnt traces where (blue), where the varistor used to be attached (yellow) and where I scraped the pcb to get to the copper so that I can solder wires on it (red, the copper is under the solder, which I was unable to remove as I have no wick or pump). I connected the red and yellow points using the aforementioned wires. The varistor did not look burnt or have that dot before I blew it up.

 

[chopnhack]

Welcome Sotiri! Are you able to borrow a multimeter from someone and check the diodes for correct operation? There should be open resistance in one direction with little resistance in the other direction. That way you can check D1 to make certain that what you are seeing is indeed a crack in the diode and complete component failure. Will you be able to source the TVR at a reasonable price?
Hopefully you can live up to your name and 'save' this board, opa!
-Γιαννης

 

[Sotiris Bos]

Welcome Sotiri! Are you able to borrow a multimeter from someone and check the diodes for correct operation? There should be open resistance in one direction with little resistance in the other direction. That way you can check D1 to make certain that what you are seeing is indeed a crack in the diode and complete component failure. Will you be able to source the TVR at a reasonable price?
Hopefully you can live up to your name and 'save' this board, opa!
-Γιαννης

Hello there! I did check the diode and it seems to be functioning properly (multimeter shows .686V one way and OL the other way). I could only measure from the top of the pcb and not from the pins on the bottom for some reason. I found varistors on ebay so that won't be a problem. I hope I can save this board as well!

 

[davenn]

look forward to hearing how it goes, please report back

 

[73's de Edd]

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Sir Sotiris Bos . . . . . . .

VERY little time left for me today.
Will use snippets and fill in tomorrow.
Referring to your Post 4
Using its first and second photos.
I can now SEEEEEEE the board !
You had a sustained HIGH voltage to your plugged in unit to take that YELLOW Varistor to its present state of destruction.
They are meant to protect from quick line surges but only holding up for a mere fraction of time.
Your unit took out those two fine foil paths that you have BLUE lined in . . .like fusible links.

Here is the way that I am now " reading " your board.
Considering that your AC power got past that area, it then routes to the D3 diode of a VERY MINOR supply derivation of this PCB.
I believe that you said that diode tested OK.
The MAIN path to then examine, is the AC power line branching off to the R3 resistor . . .my best color perception is it being a Sweet Violet-- Green--Black to make it a 75 ohm at 5% tolerance . . . .it LOOKS ? pristine.
That resistors other end then routes over to a YELLOW 0 decimal 47 ufd X2 rated line capacitor which functions as an AC line voltage dropper to get your AC line voltage waaaaaay on down to the possible
24 to 12 volts, or so, that will be created as DC voltage for your PCB's main power supply.
Note that the other / center board direction / connection of that capacitor routes over to another diode . . . . akin to D3's profile . . .it is being given a D2 part identification.
That diode now needs to be tested, to see if it . . . .happily . . . . .is also having a good 600-700 mv Vf junction . Just by virtue of the isolated buffering being provided by the YELLOW cap . . .it just might be.

That diode feeds the nearby KYS electrolytic cap to power your board.
( I can't see it well enough to read its max voltage expected spec )
If that diode tests good . . . . a bench test of the boards just described power supply aspect might be as close as temporary 28 gauge copper 1 inch length jumper fuse wires being used for the opened foil links.

No varistor needed, unless expecting a nearby lightning hit to the power lines, during a quick test interim.

I do see that this varistor is using "intelligent " part marking with the 471 . . .aka . . .470 indicating the peak voltage initiation of its voltage clamping action.
The 14 indicative of the mm diameter of the varistor slug element . . . . . which thereby relates to its power rating capability.

Making a safe test, such that the unit will no make a boom-boom can be assured via utilization of the procedure given at the end of this link:

Go down to post #6 and read its two last paragraphs and note in the ref photo that the lamp is
being inserted in series with one leg of the incoming AC line..

https://www.electronicspoint.com/th...f-an-old-ups-transformer.280632/#post-1707817


If you have that power supply up with its voltage then being across that electrolytic, then, considering no further ancillary damage is in the control circuitry, you are then that much closer to a end of repair.

Thasssit . . . . .


73's de Edd


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[Sotiris Bos]

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Thank you for your reply sir! I hadn't tested the D3 diode, I was refering to the D1 on the other side of the board. I tested both the D2 and D3 and got .528v for the D3 and .537v for the D2 (is that lower than it should be?). The R3 resistor is violet-green-black-golden/yellow color coded and checks out to be 74 Ohms. The KYS capacitor is rated at 16v. I didn't have time to study your link but will do tomorrow. Thank you for your time good sir!