testing shorted/burnt out circuit board

I think I may have fried my trunk wire, and now my auxiliary 2 wire. Both accidental. Is there any way to test the circuit board? If I did fry it, what would it have burnt out first on the board? The SMD capacitors (brown) just in front of the pins? What are the two things to the left that look burnt? The connecter there works fine.

Thanks in advance

They look more like diodes to me. That might be consistent with the top-right "D" label partly obscured by the red ring, and that caps rarely burn like that but diodes will if shorted or if they fail in short-circuit mode (which caps can, but not very often unless old electrolytics which those aren't).

If simple "isolation" or spike/polarity protection diodes (that have been shorted) then 1N4004 or 1N4007 equivalents should suffice.
But they might be some other diode (Zenor or Schottky) or component...


PCB tracks can be difficult to check unless you know the circuit. But visual inspection is often enough. If missing (fused; burnt away) or annealed (heated - discoloration or bubbled/burnt green mask) then replace or run over with solder.

Multi-layer boards (more than 2 layers of tracks) are another issue, but it looks like that's merely a 2-sides PCB.

It says D11 under the red there. On the diode (if it is) it says T4 like the ones to the right of the brown capacitors. Is it possible that these are affecting my original pins for the trunk and auxiliary? Like I said the connecter right by those diodes works fine. Also the capacitor by the burnt looking diode has weld from it to the pin (under the left side of the red circle), is it suppose to be like that?

I have this picture of the back side of the board by the burnt diodes.

T4 SMDs are 1N4148W, 1N4148WS or 1N4148WT (or equivalent) high-speed signal diodes.
The 1N4148WT appears to be the highest spec (namely 150mV forward voltage drop vs 200mV & 400mV) not that I think it would matter - get whatever is available.


By weld I assume you mean solder?
Re the 2nd pic under the PCB the two "heated" solder joints (the top 2 solder rounds if the 4 rounds forming a square) look as if they have arced. Clean away the discoloration (arc or carbon residue) to ensue no conduction. (By the green solder mask I can see they were isolated and NOT joined by a track.)
Those solder joints could (or should?) be re-soldered, tho they do not look dry to me - ie, tho pitted etc it still looks like a good conducting joint.   


You should be able to tell if the capacitor (by the burnt looking diode under the left side of the red circle) is supposed to be soldered to the diode when you desolder the diodes...
See if a PCB track joins the two.

No track but a solder link could be an intentional post-production link, but that would (or should) be a rare occurrence.
Suffice to say that every 2-termimal device (like caps and diodes) must have s join (track and solder) to each end or pin for them to operate. [ Unless it's an unused component, but then they would not fit it. Besides, I have only seen that in chips (Integrated Circuits) and never at a PCB level. ]
Keep in mind that whilst an SMD only solders to its own PCB side (any vias thru the PCB would be beside the SMD's copper pad & NOT under it), a leaded component could solder to either or both sides of the PCB (and anywhere in between for >2 layer PCBs).

Good luck.
Those diodes connecting direct to a connector means it's definitely and input else output.
If you are having trouble with your trunk & aux then I'd bet those pins go to them else whatever controls them. Being signal diodes I'd expect they are inputs and the diodes are to ensure isolation (no back-feeding) into other circuits (eg, the trunk switch (GND or +12V) effecting the aux circuit.
Now, what direction are the diodes facing? (Which direction is their line-end supposed to be? But that can be deduced from their connection circuit if not confirmed by the burnt SMDs or the PCB legend.)

Thanks I probably read that 5 times, lots of information. One of the connecters by those 4 working pins (by the burnt out diodes) happens to be a ground, so quite possibly the trunk and aux 1 go to something then to the diodes, which then go to the ground wire coming out of that connecter if that makes sense.

Is it possible to us a 1N4001, 1N4002, 1N4004, or 1N4007 diode in place of the burnt out one(s) or are these completely different? I ask because I have some on hand, not of the other kinds. If so which way does the band go, to the more positive side like usual?

Thanks for your help

I just found a place in town that sells 1N4148 with no lettering at the end, can I use these?

The 1N400x may not work if it does require a "high speed" signal diode. And tho I suspect the 1N400x would be ok, I think they are much larger and would not fit.


The non-suffixed 1N4148s sound like traditional leaded components and not SMDs. You may be able to fit them vertically or with bent-back leads, but it may not be as secure as the proper SMD part. (You may have to add glue or epoxy to prevent vibration.)

Alright thanks, I am going to take it to the electronics shop to see what they think, if they say it would be okay to use those then I'll try it out, thanks.