Dirty traces and its effects - Page 1
Forum section: electronics / repair

author: Yamin
author_rank: Frequent Contributor
timestamp: October 02, 2020, 07:44:08 pm
content: Hi guys,I am working working on a digital piano which is having some issues with the key scanning (my suspicion). Anyway this is a general question about dirty pcb.I have attached a photo of the PCB (keybed). As you can see some parts of it has got some kind of black spots and tarnishing and there is another spot which has got some greenish/turquoise colour I am assuming its some kind of corrosion.I have measured for continuity from these questionable traces, and to the closest points it showing almost 0 ohms and to the furthest point which this trace goes it shows around 0.7 ohms.My first questionis would this tarnishing cause any problem? I know its not ideal but seems the connections are intact. Would something happen when the circuit is in use?My second questionis how do you recommend cleaning this kind of tarnish, I have seen similar kind of tarnishing on other boards. I have used IPA and a brush, not much improvement.I have a suspicion that key scanning is hiccuping due the dirty pcb and its affecting only some keys. I measured the voltage at the diodes of the 'faulty' keys and I get a reading of 4.3V and I get close to 5V at the diodes of the 'good' keys. I know the scanning process would involve a square wave and I can't directly measure it using a DMM, but 4.3V measured at the faulty keys cannot just be a coincidence. However I measure 4.3V at the microcontroller pin which leads me to disregard voltage drop from the dirty traces.Thanks in advance.

author: wraper
author_rank: Unknown
timestamp: October 02, 2020, 08:05:31 pm
content: It's not dirt but corrosion. The only way to clean it is to scrape it together with solder mask which you probably should do in the most affected places to prevent it growing further. As long as traces don't become open, it should not affect operation.

author: fzabkar
author_rank: Super Contributor
timestamp: October 02, 2020, 09:11:33 pm
content: I see exactly this kind of corrosion in electronic equipment belonging to a friend who lives by the beach. The salt air is what does it. In fact in his case the corrosion is so bad that it eats through wire shorting links.

author: Audiorepair
author_rank: Frequent Contributor
timestamp: October 03, 2020, 08:04:11 pm
content: I would agree that this is corrosion, I have seen this many times.Spilt drinks on the keyboard often cause this, particularly if it is Coca Cola, which is highly corrosive.If there is evidence someone has been in this keyframe before you, it is likely they have cleaned off the contamination and left behind the corrosion, which will only get worse.I see several other tracks in your photo that you haven't highlighted that exhibit the same corrosion.The only option is to physically bridge all the corroded tracks with wire.I would seriously consider whether this repair is a write off, unless you want it to come bouncing back for ever, or your reputation gets shot to pieces for having charged for a repair that failed a short time later.

author: fzabkar
author_rank: Super Contributor
timestamp: October 03, 2020, 08:23:07 pm
content: The OP is from the Maldives. I think that explains the corrosion. :-)

author: Audiorepair
author_rank: Frequent Contributor
timestamp: October 03, 2020, 09:06:58 pm
content: Quote from: fzabkar on October 03, 2020, 08:23:07 pmWell, if that corrosion is througout the whole keyframe then that makes perfect sense!If, however, that corrosion is local to a particular part of the keyframe, that suggests a spilt drink.

author: packetbob
author_rank: Contributor
timestamp: October 03, 2020, 09:08:32 pm
content: I just fixed a Axiom25 keyboard with similar issues...Only 2 of the keys still worked...When I opened it up I found quite a few PCB traces with similar damage..It seemed something was able to eat away at the trace under the solder mask...When I carefully scrapped away the bubbled up bit you could see the trace was completely gone...I cleaned the area with alcohol and soldered in jumpers to go over the damaged bits..Got all the keys back working...

author: Audiorepair
author_rank: Frequent Contributor
timestamp: October 03, 2020, 09:15:47 pm
content: Yes, it is likely someone spilt something on the keyboard and perhaps didn't admit to it.This happens a lot.

author: Yamin
author_rank: Frequent Contributor
timestamp: October 04, 2020, 06:22:54 pm
content: Thanks everyone for the input. Heheheh yes we are surrounded by salt water. The unit is used at a venue near the beach but the place is not completely open, maybe there are some other factors coming into play as well. Its used inside an air-conditioned room. The tarnishing/corrosion is on the whole keybed. Funny thing is I have worked on keyboards and other gears which some travelling musicians use which are in far better state. I am talking about gigs couple of times a week and some gigs are right on the beach.I have send some pictures to the owner and have explained that it doesn't look good.But here is the thing - Everything I check seems to be in order as in, can't find and shorts or open connections and to make things even more harder the problem seems to be intermittent. I have to play several keys fast for the problem to occur eg. if I play g,a,b,g,a,b and all of a sudden the b note comes out with full velocity. It remains at full velocity for a second and goes back to normal and then I am able to play all the keys normally.Only thing I could remark is that on the 'velocity' detecting path (I hope I am making sense) when I measure at the diode to ground (of the 'b' note) the voltage keeps going down from 4.3V to around 3V (does not stay stable). With all the other keys I measure something close to 5V.That's why I asked about whether this corrion could be causing that (something weird like stray capacitance or something), I had to disregard voltage drop because I see the same 4.3V at the microcontroller pin which is used for key scanning. I have done the regular rubber pad and contact cleaning as usual and cleaned the whole pcb as well.I am just trying to figure out why this is happening (even if its not fixable) for future reference.Once again thanks for the help.

author: Audiorepair
author_rank: Frequent Contributor
timestamp: October 04, 2020, 07:36:25 pm
content: It's not really feasible to test a Dynamic system with Static methods.Those key contacts are two tiny pieces of floppy rubber with a conductive coating flying up and down at great speed onto a PCB contact area.The volume of the key press is calculated from the time difference between the first tiny piece of rubber hitting its target, and the second tiny piece of rubber hitting its target.If you lose one of these two vital contacts, you will get full velocity by default.Tiny peices of conductive coated rubber wear out, may distort, may bounce, may just not make good contact any more when hammered onto the PCB by a pianist.You can't detect this with a multimeter, you are better off using your fingers and ears.One tip though, the rubber contacts wear out because they keep hitting the exact same PCB contact.If you swap the contact strips around from octave to octave, you can swap the most worn with the least worn, and in the process, the contacts will now probably be using a slightly different section that is not quite so worn out, so you might get another few years out of the system.This is particularly relevent where new contact strips are no longer available, which is often the case.

author: Yamin
author_rank: Frequent Contributor
timestamp: October 04, 2020, 08:26:57 pm
content: Thanks so much for the swap the contact strips tipThe 4.3V I measured which I mentioned earlier was without any keypress. I measured the same when I had just the bare board connected as well meaning I had the contact strips removed because I was also thinking that somehow there might be some kind of contact happening, I inspected the jagged contacts under a magnifying glass on the board very closely and cleaned them. The voltage remained to be under 4.3V and going down. I measure that only on the signal line which has got the second diode to detect the 'velocity' of the 'b' notes, I measure the same on all the other notes which are grouped in that line but for some reason the 'b' note is what gives the issue. Very occasionally I had issue with some other notes which are grouped with the 'b'.All the other keys which are not grouped to the 'b' showed 5V on both the diodes for that note and works fine.I'd really like to have a look at it on a scope and see if its noise or something else during the key scanning unfortunately haven't got access to one at the moment

author: Bud
author_rank: Super Contributor
timestamp: October 04, 2020, 08:56:13 pm
content: You can use WD40 on the corroded tracks, if you have that stuff at your place. You may need to let it soak for some time and then use a stiff toothbrush. It still may take the solder mask but that would be because it was coming off in corroded spots anyways.

author: Gyro
author_rank: Super Contributor
timestamp: October 04, 2020, 09:11:23 pm
content: On the other hand, it will be hell on the key contacts.

author: Bud
author_rank: Super Contributor
timestamp: October 04, 2020, 09:40:37 pm
content: Yes the key contacts would need to be protected.