XT control board - +5V to ground short [solved]

Hello,

So after getting the power supply working on my 4PM filter board, I plugged it into my XT control board, with no response whatsoever… except a VERY hot +5V regulator. Stupidly I had not previously checked the control board for a short between the +5V and GND pins, but on checking it now, that’s what I find.

I’ve given the board a very thorough visual inspection and even tidied up a few joints, but I can’t see any connections between pins that shouldn’t be there. I’ve removed all ICs but I still get the short. I’ve taken some pics, but not sure they would be very useful to anyone.

There is one connection I’m concerned about; it’s for potentiometer R20, and is shown in the picture 8, the last image of the PCB rear. Right at the start of building, I go some solder between the connections and decided to remove it with a scalpel. I slightly scratched away some of the white surface revealing copper. Now, as far as I can see, this isn’t in connection with the pads for the pot, but I just don’t know. Does this potentially mean I’ve ruined the PCB?

Any help on tracking down where the short might be would be great. Are there any components that could be faulty and causing this?

Thanks,
Pete

Nowhere near enough solder on your pads, you shouldn’t be able to see the holes.

When you test your XT do you place something non-conductive between the two boards? if not then you need to, the metal tab on the regulators is connected to ground and will touch pads underneath causing shorts.

OK, thanks. Could too little solder be causing the short though?

I made sure there was no contact between the control board and the regulators when connecting. Also, I can measure the short between +5v and GND when the control board is not connected.

No, but you should get an inspection light and magnifier and ensure there’s no offcuts or blobs of solder creating shorts.

A real short will be 0 ohms resistance.

Tanks. Yeah I’ve checked it over 10 times now, also gently running a scalpel tip between all pads to remove any traces of solder.

My multimeter settles at about 0.7 ohm, but it’s a cheap multimeter.

OK, I gave it another go over, cleaning up the joints with solvent and I really can’t find any place where there is a connection between two pins that shouldn’t be there.

Any ideas what I should look into next? Any components that, if faulty, might cause this?

Cheers

Well if there’s no shorts visible on the bottom look on top?

Hi. Almost all of your solder needs rework. Have a look on internet how to solder through hole components. Don’t get me wrong but proper soldering matters a lot.

I understand, and will re-work the soldering. But, assuming I’m correct and there are no cases where the solder is making contact between two pins/pads, could this be the source of my problem? I’m trying to solve a short circuit and it seems to me that a bad solder connection would cause an open circuit, not a short circuit.

Basically, I don’t want to improve a lot of soldering, if I’m then just going to have to start un-soldering the same components immediately afterwards

Those scratched pads look suspicious. fortunately, the two pads involved are the wiper and GND. Can you turn the pot all the way to the left (anti-clockwise) and measure again? I the short is gone and it reads something in the kOhm range, then you have a connection between the wiper and the GND plane.

As you can see on picture blue area is GND . notice how close 95%of all pads on this pcb are close to GND and how easy is to make short between them.

@heniek: only if the paint is scratched off. So all the other pads should be fine

You don’t have to un-solder all those parts. Just need touch up with fresh solder or if you have flux to add.

TheSlowGrowth: Thanks. I was really worried about the scratch yesterday, so I measured between the ground and wiper and I get a variable resistance when I turn the pot, so I guess this means it’s not shorted by the scratch. I’ll take another look when I get back from work though, and make the measurement between +5 and gnd rails with the pot turned to both extremes.

@heniek: thanks for the nice soldering info-graphic thing. Very helpful :slight_smile:

Hi , yes , mostly all solder pads are not good . do you measure with or without the IC’s??
Solder all again and clean it after solder with methylalcohol.
I see that some solder points on the IC sockets have maybe a short to ground .
look here - afer solder and cleaning it shoul look like this.
Greets
Andre’
TubeOhm

Thanks for all the input so far…

I’ve given all the soldering a good tidy up and cleaned the board with some alcohol. Looking much nicer now:

I also removed the screen to get to the connections underneath, and then the midi connectors, because I was on a roll.

But I still have a short between 5v and gnd. I really cannot see any source of a short between pins due to soldering. I’ve checked again and again.

I checked the pot R20, where the paint was scratched, but there is no continuity between the wiper and ground when the pot is set to mid way. So that is not my problem.

So, any advice as to the next step? What to check/remove?

s. @tubeohm: I have measured with all ICs removed.

Cheers,
Pete

hmm, does the resistance between +5V and GND change on different areas of the board? It could help you to locate the error. if the short is on the left side and you measure on the right side, it might actually read a tiny bit higher. Worth a try. But its not unlikely to be the same resistance everywhere on the board.
Be aware that the contact point between your multimeter and the board is very important for such low resistance measurements. Use pressure to ensure a good contact.

Also don’t foget to check top side of pcb. It might be something under ic sockets or pots. Some debris or piece of wire.