How do I go on cleaning the pcb?

[Sharif]

It's an old AC to variable DC adapter, that sometimes turns of and comes back on after pressing on the spring loaded potentiometer like thing or changing the voltage to a higher value (I know it's not a potentiometer) 

 

I wanted to ask how can I go on cleaning the pcb?  I am speculating a loose contact

Pictures : 

Spoiler

[Cyracus]

remove from power source, isopropyl alcohol higher percent is better, soft bristle brush... be gentle

[Oshino Shinobu]

A sot bristle tooth brush, isopropyl alcohol and light brushing. I use that method for cleaning PCBs with liquid spilt on them, worked to fix my step sister's Macbook that had coke on it. 

[Sharif]

3 minutes ago, Cyracus said:

remove from power source, isopropyl alcohol higher percent is better, soft bristle brush... be gentle

 

2 minutes ago, Oshino Shinobu said:

A sot bristle tooth brush, isopropyl alcohol and light brushing. I use that method for cleaning PCBs with liquid spilt on them, worked to fix my step sister's Macbook that had coke on it. 

 

 

Perfect, I got 70% lying around that should do the job

Thank you 

[Unimportant]

3 hours ago, Sharif said:

It's an old AC to variable DC adapter, that sometimes turns of and comes back on after pressing on the spring loaded potentiometer like thing or changing the voltage to a higher value (I know it's not a potentiometer) 

 

I wanted to ask how can I go on cleaning the pcb?  I am speculating a loose contact

One should find the source of the corrosion first or any repair attempt will be for naught. My guess is there is a electrolytic capacitor on the other side of the PCB that has gone bad and started leaking. (Unless you spilled liquid on the unit yourself ? ) If so, the capacitor should be replaced because:

• It will continue leaking and the corrosion will simply continue.
• The adapter will have a ever bigger output ripple because the capacitor is not doing it's job, that's bad for the device powered by the adapter.

After removing the capacitor, clean the pcb with a stiff toothbrush and a cleaning compound. Normally one would use a corrosion/PCB cleaner but if you don't have that I guess alcohol, as propsed by @Cyracus will have to suffice.

 

Some bits of soldermask (the green 'paint') might come of during cleaning. Not to worry, that means it was already loose due to the corrosion.

 

After that, in order to restore good electrical connection and stop further corrosion as much as possible one should tin the pcb tracks where the soldermask came off and especially the pads for the bottom control knob by applying a layer of fresh solder over them. If the solder won't take, it means there's still corrosion left and you might have to gently scrape/scratch the tracks/pads, with the tip of a screwdriver for example. Don't skip this step, if the solder does not take the pad is not clean and the control knob will continue to make intermittent connection.

 

Finally, use solder wick to remove the excess solder from the control knob pads so they're nice and flat again, otherwise the thickness of the solder will interfere with smooth rotation of the knob.

 

Don't forget to clean both sides of the PCB, capacitor goo goes everywhere. Also try cleaning the 4 metal prongs on the bottom control knob that make contact with the pads on the PCB, gently lift them up and slide a piece of fine sand paper under them. Don't overdo it, you just need to remove the thin layer of corrosion.

[mariushm]

Isopropyl alcohol will clean the circuit board.

 

However, in this particular case, you may also have a problem with that wiper scratching and cutting those circuit board pads under it, so now you may no longer have a good contact on some selections.

You could probably apply a very thin layer of tin (by placing some solder on those pads and then removing it with solder wick for example) but you'd just prolong the inevitable... either you'll get scratches/grooves on the pads or they'll oxidize. Such pads are normally gold plated, to last longer.

 

It would probably be better (and could be a fun project) to just remake your circuit board, but replacing that rotary switch with something more reliable

[Sharif]

1 hour ago, Unimportant said:

One should find the source of the corrosion first or any repair attempt will be for naught. My guess is there is a electrolytic capacitor on the other side of the PCB that has gone bad and started leaking. (Unless you spilled liquid on the unit yourself ? ) If so, the capacitor should be replaced because:

• It will continue leaking and the corrosion will simply continue.
• The adapter will have a ever bigger output ripple because the capacitor is not doing it's job, that's bad for the device powered by the adapter.

After removing the capacitor, clean the pcb with a stiff toothbrush and a cleaning compound. Normally one would use a corrosion/PCB cleaner but if you don't have that I guess alcohol, as propsed by @Cyracus will have to suffice.

 

Some bits of soldermask (the green 'paint') might come of during cleaning. Not to worry, that means it was already loose due to the corrosion.

 

After that, in order to restore good electrical connection and stop further corrosion as much as possible one should tin the pcb tracks where the soldermask came off and especially the pads for the bottom control knob by applying a layer of fresh solder over them. If the solder won't take, it means there's still corrosion left and you might have to gently scrape/scratch the tracks/pads, with the tip of a screwdriver for example. Don't skip this step, if the solder does not take the pad is not clean and the control knob will continue to make intermittent connection.

 

Finally, use solder wick to remove the excess solder from the control knob pads so they're nice and flat again, otherwise the thickness of the solder will interfere with smooth rotation of the knob.

 

Don't forget to clean both sides of the PCB, capacitor goo goes everywhere. Also try cleaning the 4 metal prongs on the bottom control knob that make contact with the pads on the PCB, gently lift them up and slide a piece of fine sand paper under them. Don't overdo it, you just need to remove the thin layer of corrosion.

I was trying to see if the the printed voltage value on casing matched with what I was getting on the multimeter and it was extremely off. I looked at the capacitor, it didn't have any bumps or leaks (Though, I didn't properly check below as the sides and top looked fine) to make the matter worse I ended up cleaning everything after the inspection.. 

 

Thankfully with the light cleaning with isopropyl alcohol most of it came off and I tried to push metal piece a bit towards the contact on the pcb (Not sure if that helped or the cleaning) solved the adapter from turning off now.

 

Regarding tining the solder mask, my solder skills are somewhere below beginner level and I have doubts my cheap solder iron off ebay is getting hot as it is supposed to get to melt solder (Takes a good while to melt the solder even when places the solder on the tip directly)

 

The voltage is overall higher than what it's supposed to be, would this surely indicate the capacitor? 

13 minutes ago, mariushm said:

Isopropyl alcohol will clean the circuit board.

 

However, in this particular case, you may also have a problem with that wiper scratching and cutting those circuit board pads under it, so now you may no longer have a good contact on some selections.

You could probably apply a very thin layer of tin (by placing some solder on those pads and then removing it with solder wick for example) but you'd just prolong the inevitable... either you'll get scratches/grooves on the pads or they'll oxidize. Such pads are normally gold plated, to last longer.

 

It would probably be better (and could be a fun project) to just remake your circuit board, but replacing that rotary switch with something more reliable

I don't really mind taking this opportunity explore the circuit and learn bunch of new things while at it, it's just hard to source parts where I live (things off ebay-china takes 2 months in shipping too) 

[Unimportant]

30 minutes ago, Sharif said:

I was trying to see if the the printed voltage value on casing matched with what I was getting on the multimeter and it was extremely off. I looked at the capacitor, it didn't have any bumps or leaks (Though, I didn't properly check below as the sides and top looked fine) to make the matter worse I ended up cleaning everything after the inspection.. 

 

Quote

The voltage is overall higher than what it's supposed to be, would this surely indicate the capacitor?

You can't really test the capacitor by measuring voltage. A multimeter will have a low pass filter inside that removes any ripple to smooth out the measurement, effectively obscuring the problem. The voltage being too high is simply caused by there not being any load while measuring, if you add a load it will drop, but it says nothing about the cap.

 

If your multimeter has a capacitor test mode you can measure the cap, after removing it from the circuit, and check if it still has it's rated capacitance value. But even if it does it might still be bad as a common failure mode for electrolytic cap's is the ESR (equivalent series resistance) becoming too high. You'd need a ESR meter for that.

 

The fact remains the corrosion has to come from somewhere. PCB's don't just corrode for no reason. Short of spilling some fluid on the unit yourself the capacitor is the only likely source.

[Sharif]

37 minutes ago, Unimportant said:

 

You can't really test the capacitor by measuring voltage. A multimeter will have a low pass filter inside that removes any ripple to smooth out the measurement, effectively obscuring the problem. The voltage being too high is simply caused by there not being any load while measuring, if you add a load it will drop, but it says nothing about the cap.

 

If your multimeter has a capacitor test mode you can measure the cap, after removing it from the circuit, and check if it still has it's rated capacitance value. But even if it does it might still be bad as a common failure mode for electrolytic cap's is the ESR (equivalent series resistance) becoming too high. You'd need a ESR meter for that.

 

The fact remains the corrosion has to come from somewhere. PCB's don't just corrode for no reason. Short of spilling some fluid on the unit yourself the capacitor is the only likely source.

It does not unfortunately, I added a load and tried again, according to the marking it is set as 6v, without load 10v and with load 8.2v.

 

I should have checked the value on the cap, I hope it's a 1000uF cap, I got few of them lying around after being salvaged from a donor psu to fix another psu. 

Thank you for all your help and info, learned bunch of new things

[mariushm]

There's no voltage regulator in this "thing", it's a basic linear power supply  with no regulation.

 

The transformer is a low power one, and because of this the output voltage will vary significantly with the load. For example, If you only use 50-100mA of current, the output voltage will be much higher compared to when you plug something that uses let's say 300 mA of current.

The value on the switch is usually for when a product actually uses close to the maximum current output advertised.

 

Based on the size of that transformer, it's most likely a 10..15 VA transformer, so unlikely to get more than around 200-400 mA from it at any voltage.

 

The capacitor could be faulty, but even with a new capacitor, the output voltage will still vary a lot depending on how much a device would take from this "power supply".

 

It's too long of an explanation to write at 3 AM (here where I am)

 

[Sharif]

11 minutes ago, mariushm said:

There's no voltage regulator in this "thing", it's a basic linear power supply  with no regulation.

 

The transformer is a low power one, and because of this the output voltage will vary significantly with the load. For example, If you only use 50-100mA of current, the output voltage will be much higher compared to when you plug something that uses let's say 300 mA of current.

The value on the switch is usually for when a product actually uses close to the maximum current output advertised.

 

Based on the size of that transformer, it's most likely a 10..15 VA transformer, so unlikely to get more than around 200-400 mA from it at any voltage.

 

The capacitor could be faulty, but even with a new capacitor, the output voltage will still vary a lot depending on how much a device would take from this "power supply".

 

It's too long of an explanation to write at 3 AM (here where I am)

 

Funnily enough it's 3:20AM here as well. It's rated at 1A max and the light I was using as a load was taking in 0.6A, though that was using a different power adapter rated at 12v. I was double checking before I switched to this one while knowing it had an issue before, I thought I could take this opportunity to try fixing it. Now that I think about it, R=V/I so in order for resistance to be same voltage should increase as current increases.

 

In my head I was thinking the voltage outputted by the adapter is the amount the device receives and end of that. On the brightside one misconception is gone from my head