Help identify a Diode -

[telovoya]

Hi guys.
I have a small circuit board that got wet on one side, and i can see a few corroded components. I would like to identify them and replace them.
I have basic soldering skills. I have very limited knowledge of electronics. I do know how to perform basic tests on some components using a tester but thats pretty much how far i am. I would like to get involved in the topic and learn to troubleshoot circuit boards, ICs etc. So i will start reading these forums and tutorials. Besides that, i have no clue where to start. I did study electronic theory, basic components and assembled basic circuits by drawing the rails with sharpie and letting the boards soak in acid, etc... that was like 15 years ago and then put it on hold to do something else. But i am still interested.

Anyway... i would like to get by this one time so i can fix this board. It is my motorcycle odometer.. some water got in and i can see a few Diodes toasted. Tester shows OL on either side.
I tried googling and also searching the SMD codebook that came up in many sites, promising to help identify most components.. but im not sure what to look for.

The diodes i have here read:

S
1G
75A

I can't find them by those numbers... would you please tell me how to find them?
The rest of the components seem to be fine.
Here is a picture... the ones burned are right next to the wires where the power come from, so my uneducated guess tells me that if i replace those two, i might get lucky and fix my problem.
Thanks for any input.

 

[KrisBlueNZ]

Hi there and welcome to Electronic Point

Thanks for the good explanation.

I don't see anything obviously wrong with those diodes; they just look very dirty. I think you should start by cleaning the muck off the board and posting another photo. Use a toothbrush then cotton swabs, with a solvent - isopropyl alcohol (tape head cleaner) is best, but methylated spirit will work. Keep rubbing until there's no more improvement.

It looks like there are two circuit boards sandwiched together. Is that right? Can you separate them and take photos of both of them?

Unplug any cables so we can see as much of the boards as possible.

 

[telovoya]

Hi there and welcome to Electronic Point

Thanks for the good explanation.

I don't see anything obviously wrong with those diodes; they just look very dirty. I think you should start by cleaning the muck off the board and posting another photo. Use a toothbrush then cotton swabs, with a solvent - isopropyl alcohol (tape head cleaner) is best, but methylated spirit will work. Keep rubbing until there's no more improvement.

It looks like there are two circuit boards sandwiched together. Is that right? Can you separate them and take photos of both of them?

Unplug any cables so we can see as much of the boards as possible.

Hi Kris.
Thank you for the tips and suggestions.
Here are a few better pictures.. im using my phone to get the images so its still not the best resolution, but should give a better idea.
The 2 diodes im talking about are the ones underneath to the orange and red wires on the first picture. The solder is pretty much gone, is not dirt, i removed the white and green corrosion stuff that was there because of the water and now it looks like dirt on the picture. I am testing the
components comparing against the healthy ones you can see on the other side. The good ones read .520 on my tester set on the diode position. the bad ones just read OL when tested both ways.
The other board is an array of switches to control the functions in the display.
I will surely clean it good by removing the bad components, remove any old solder, cleaning with alcohol and then soldering new ones in. Just need to know how to find and buy those suckers.
Thanks again

EDIT: the forum resizes the images and make them too small.. here are the links to the full images:
http://www.use.com/images/s_3/2014_10_28_1_5605c703f88d25100275.jpg
http://www.use.com/images/s_3/2014_10_28_1_5605c703f88d25100275_2.jpg

 

[KrisBlueNZ]

Those diodes on the left really don't look like they're damaged. They just look very dirty. There isn't much solder on them, but there isn't much on the others either. That's normal for SMT (surface-mount technology) assembly.

You said that the two diodes on the left (D4 and the other one) read open circuit both ways, but D3 and D5 on the right measure normally. Are they all marked the same?

Measure the left side diodes again. Make sure your meter probes are really sharp and dig them into the remaining solder. Make absolutely sure they read open circuit both ways.

Whether they're faulty or not, you should probably remove them and clean the board very thoroughly in that area. Then you might as well fit new ones, if we can figure out what they are.

Is the connector with the wires permanently attached to the board? If so, it's probably worth removing it so you can clean the board under it. Do you have a solder sucker?

 

[telovoya]

Kris.
you were right. I did dig the sharp probes of my tester really hard and got a reading on both diodes!
Yes they are the same as the ones on the right.
I do have desoldering bride, an iron and also a heat gun. I don't have flux tho.
So i will try to remove them tomorrow, clean everything good and see what happens. The wires are permanently attached and there is also corrosion on them at the solder points and inside the white connector. I will also re do those as well.
The rest of the circuit seems to be very clean and i dont see anymore corrosion points because of the water.
If it works again, i will seal it using silicone and voila.
Thanks again. I will let you know how it goes.

 

[telovoya]

Update.
I finally fixed it.
I solder them back in place and had to repair a few traces that were cut by accident when i cleaned the board with the wick.
The speedo works again and it is sealed with silicone.
Thanks again for the help.

If anyone could point me to some tutorials on how to test the different components in a circuit board for basic troubleshooting and repair, that would be awesome.
Good luck

 

[(*steve*)]

If anyone could point me to some tutorials on how to test the different components in a circuit board for basic troubleshooting and repair, that would be awesome.

This is really tricky because how you test a component depends a lot on what else is in the circuit with it and how it is connected.

Even testing component when removed from a board can be tricky because you first need to ID the part (and that's not always easy) then you need to know what characteristics are important in that circuit and measure them. It's not unreasonable to test a capacitor for capacitance, but in some circuits it's not the capacitance that is most important. It may be ESR, leakage, temperature stability, or other characteristics.

Being able to identify which is important comes with experience, and often you can't be 100% certain. If you can positively identify a part then a new replacement is often a good test, but even then a fault may affect more than one component and may have a cause that is remote from the obviously damaged parts. When you get into a situation where you can't positively identify the part, testing it, and even replacement of it become more uncertain.