Repair PCB or make a new one?

[Cykar]

A while ago I was given an ultrasonic cleaner by a jeweller as it didn't work in case I could fix it for our jewellery business. There are two boards inside, the main control board and the Interference Filter board which is where it is connected to the mains. Originally, the main board was buggered after there was some carbon tracking on the Interference Filter board, causing a short. When I got it, there was some white powder in the unit, what I believe was barrelbrite - a pretty toxic cleaning powder you mix with water for cleaning jewellery in a tumbler etc... It seemed to me that the barrelbrite had caused corrosion and possibly the remains of it is conductive?

Anyway, I cleaned up the original short and fixed the broken traces with a bit of wire, and got rid of all the barrelbrite I could. The main board is available to buy, which I did, and all was fine for about 6 months. Interestingly, the way the pcb sits in the unit, all the corrosion is on one side of the Interference Filter board - the other side is fine.

Then last week there was a bang! Looking inside there were no blown parts that I could see, but just possibly some marks left by arcing between the live and ground pins on the IEC13 socket. I wrapped them in little boots of electrical tape, changed the fuse in the plug (as that is all that had blown) and plugged it in. Worked fine - for about 30 seconds then BANG! again. So I took a closer look.

Again, nothing seemed to have blown, but I spied underneath the IEC socket, there looked like there might have been more carbon tracking. I've removed the socket now to get a good look and I was right. A clear track between live and ground cut into the circuit board, a straight short between live and ground pins so no fuses or other parts were blown in the unit, just the fuse in the plug.

I should point out that you cannot buy replacement Interference Filter boards like you can the main control board, and the unit itself costs around £700 new, way out of our price range, but we need it and use it regularly so I need to fix it if I can.

So, I have two options. Repair again, maybe drilling some holes through the carbon tracks and covering up all exposed copper caused by corrosion with clear nail varnish (should have done that before i guess), or make a new PCB. I have already put the schematic on easyeda, preparatory to making as good a copy as I can of this board, but I wondered - what would you do? Repair (again) or remake?

[rsjsouza]

The PCB is very simple but it does not look in terrible shape, so if it was me I would repair it. One aspect is to indeed put two or three coats of clear varnish to keep it from accumulating crud in dangerous places again. I would also put the clear varnish on the parts themselves to also prevent shorts on their terminals.
Good luck with your repair!

[Fraser]

If it were my unit I would do the following......


1. Thoroughly clean the PCB with IPA to ensure surface contaminants are removed.

2. Try to source a new IEC socket if the original looks to have suffered any degradation.

3. Any PCB track that shows any discolouration has been effected by a contaminant. I would remove the solder mask from such PCB tracks and install a nice clean copper wire on top of the track. Solder the wire to the track along its length, following the tracks original path for a nice neat solution. I use the solid copper core of “Twin and Earth” domestic utility cable as it has good cross section and is easy to work with. I would tend to extend the copper wire well beyond the area of visible corrosion, preferably between two component solder points on the PCB. I would coat the repair with UV activated solder mask but nail varnish will work as well. I prefer clear to ‘pink’ 

4. At any point on the PCB where arc-over is suspected, I do not mess around trying to remove the carbon deposits, I create an air gap between the two points between which the arc has occurred or relocate on one side of the arc path to another part of the PCB, further away. In your case, I suggest a Dremel tool fitted with a cutting wheel is used to cut a slot in the PCB below the spare connector assembly. The slot needs to penetrate all the way through the PCB material to provide the required air gap insulation. This prevents re-occurrence of the problem and will not effect the safety of the equipment as no layout changes are needed. I would carefully coat cut edges of the new air gap with either UV solder mask or a little nail varnish. Do not fill the gap with either though, just seal the edges to prevent moisture ingress.

5. You may also wish to consider fitting a new Schaffner filter to the PCB as these have a habit of failing without warning ! I have had one burn-up and it was not pretty !

6. After repair, I recommend you marinise the PCB on both top and bottom by the application of a suitable insulating varnish. Do not bridge the air gap you created. I suggest you avoid coating the fuse holders so mask them. I have used solvent based car paint clear coat for such marinisation tasks. Polyurethane varnish also works well.

Fraser

[RayRay]

Doesn't look that bad, I'd say it's repairable. Anyhow, have you checked the IEC connector for continuity (shorts) between the pins? How bout the joints on the circuit board (where it connects to)? And if everything's good in those regards, maybe try putting electrical tape on the damaged portion of the PCB paint? And as already been said, the IEC connector is replaceable. On a different note though, if your breaker didn't trip from a short between the ground & live then you have a serious electrical safety fault (and you should get an electrician to look at it ASAP!)

[Cykar]

Thanks for your replies. I will repair, putting in air gaps where the shorts have occurred. I have access to nail vanish, so I'll use that too.

I have tested the IEC socket, and it's fine. I've also checked pretty much the whole board for continuity issues, but apart from the shorts, the board is fine.

On a different note though, if your breaker didn't trip from a short between the ground & live then you have a serious electrical safety fault (and you should get an electrician to look at it ASAP!)

Completely agree. The fuse box is of the old fashioned type with fuse wire wrapped around contacts, no trip switch. I'm in a rented flat and apparently it recently past the yearly electrical test here in the UK. Nothing gets fixed here unless absolutely essential - the landlord just won't spend any money unless forced to by law -  but that's a discussion for another forum ;-)

Thanks all.

[wizard69]

Did you contact the manufacture for an upgrade path?   It is completely possible that they are using an off the shelf IEC inlet with built in noise suppression now.    That would depend upon finding one with the suitable electronics for the noise to be suppressed.   However that should not be too hard to do as such filters have become more important over the years as regulation have become more stringent.    You could try sourcing one yourself.   I'd start by trying to find info on the Shaffner parts that are already on the board and then sketching out a schematic to try to understand what the circuit does.

As for the board you have there I have to agree with others that from the pictures in looks like it is repairable.    Now that can be deceiving in photos and you may need to route out any carbon from arcing.   Repair might make sense but you could end up going down a road where it just fails again.   It really depends upon your ability to uncover all hidden damage.

The reason I'm wishy washy on repair is that it is very likely that the cleaner made contact in a liquid state.  The cleaner most likely has surfactants and other additives to increase wettability.   That mean you could have corrosive materials, possibly conductive, in places you just can't see.   Now how bad this could be is highly dependent upon the cleaner in use which I have no information on.   It is completely possible you have residue under components and even inside components if the cleaner found entrance.   So yeah I'm not real strong on a long term repair here.