Author Topic: How to clean switches in high-impedance environment?  (Read 2987 times)

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Offline e-doc

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How to clean switches in high-impedance environment?
« on: March 14, 2016, 07:47:55 am »
I want to clean the input devider switches on an old Fluke 8060A DMM because they seem to cause some loose connection.
Would it be OK to sink the whole PCB in IPA (without LCD but including switches and piezo buzzer)?
I can use an ultrasonic bath too.

Should I bring some lubricant (which?) on the contacts after washing and drying them to prevent contact corrosion?
Could the lubricant degrade the high-impedance performance of the switches and environment?
What about the adjustment trimmers on the board?
« Last Edit: March 14, 2016, 07:57:53 am by e-doc »
 

Offline Gyro

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Re: How to clean switches in high-impedance environment?
« Reply #1 on: March 14, 2016, 12:50:29 pm »
The trouble with putting the whole board in IPA is that any flux or other residues on the board will be dissolved and might then contaminate the switch contacts further as it dries. I would concentrate on applying IPA or other contact cleaner to the switches themselves. The plastic in the area of the switch contacts probably isn't lubricated, but I would avoid the mechanical interlock area, firstly to avoid removing any lubricant, and secondly to avoid washing it into the contacts.

If you use contact lubricant then you clearly need to use one that doesn't cause leakage, but hopefully cleaning would be enough.

There is another related post at the moment:

https://www.eevblog.com/forum/repair/gang-switch-maintenance/
Chris

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Offline uncle_bob

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Re: How to clean switches in high-impedance environment?
« Reply #2 on: March 14, 2016, 02:43:18 pm »
I want to clean the input devider switches on an old Fluke 8060A DMM because they seem to cause some loose connection.
Would it be OK to sink the whole PCB in IPA (without LCD but including switches and piezo buzzer)?
I can use an ultrasonic bath too.

Should I bring some lubricant (which?) on the contacts after washing and drying them to prevent contact corrosion?
Could the lubricant degrade the high-impedance performance of the switches and environment?
What about the adjustment trimmers on the board?

Hi

It is quite possible that the switch is simply worn out. If so, cleaning isn't going to help.

If it is a dirt issue, you should see some evidence of it visually. In that case, yes, a small brush and something like a Q-Tip with IPA is a good idea. A full soak is normally un-needed and is likely to cause other problems. If you use IPA, get the really good stuff and not something from the local store. Home use IPA often has things like lanolin in it that you do *not* want inside your meter.

If you have a meter that lived a really dirty life and thus must be given a full clean there are better and worse ways to do it. Tear it down as far as you practically can before cleaning. Stuff like plastic panel parts can better be cleaned with soap and water than more exotic stuff. If you soak the pc boards / switches, it's a multi pass process. You do a soak, then a rinse with fresh IPA. You then soak again in yet another fresh batch of IPA and then rinse again in fresh material. Depending on the level of dirt, you may go to further passes or other cleaners. Once the final pass is done, the board should quickly self dry with no signs of residue anywhere. After that, put it in a warm oven with good airflow at a reasonable temperature. The plastic parts on the board will dictate how high a temperature you can run. The lower the temperature, the more hours to bake things out.

Yes, you use a lot of IPA doing this, order up a big bottle of the stuff ....

Bob

 

Offline ModemHead

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Re: How to clean switches in high-impedance environment?
« Reply #3 on: March 14, 2016, 02:56:06 pm »
My experience with these old Flukes has not yet revealed any problem that I could blame on the switches. If you take one apart, you will find that the design is incredibly simple and very robust.   So I've never set out to clean the switches, just the PCB.  My procedure has always been to dunk the board in about a 1/4" of IPA and brush the board as much as possible with an acid brush and/or old toothbrush.

I do recall however that the 8060A designer (who posts here as "drtaylor") mentioned that you should avoid immersing the switches.  I recommend looking through his posts for other good advice.
 

Offline e-doc

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Re: How to clean switches in high-impedance environment?
« Reply #4 on: March 14, 2016, 03:49:21 pm »
Thanks for your replies.
I think I first try cleaning the PCB only (without switches) in the ultrasonic bath.
I cleaned the PCB partially several times before, using a brush and IPA.
The symtoms (fluctuating display on 2VAC range) disappeared for some time but came back.
If I press the case a little bit together or move the switches a little bit, the display changes, or the problem disappears for some time.
 

Offline Gyro

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Re: How to clean switches in high-impedance environment?
« Reply #5 on: March 14, 2016, 06:42:56 pm »
From what you just described it would probably be a good idea to check the PCB for bad or failed solder joints, particularly around the switch pins.

I'm not at all sure that ultrasonic cleaning would be a good idea either.
Chris

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Offline uncle_bob

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Re: How to clean switches in high-impedance environment?
« Reply #6 on: March 14, 2016, 08:01:04 pm »
Hi

Part of the issue here may be the "high impedance environment" rather than the "clean switches". A hand held DVM is not that crazy in terms of impedances. Most of their issues come from being dropped off a ladder onto a hard floor. That results in cracked joints and loose parts. It takes a *lot* of dirt to start impacting the readings.

Bob
 

Offline e-doc

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Re: How to clean switches in high-impedance environment?
« Reply #7 on: March 14, 2016, 08:25:49 pm »
I'm not at all sure that ultrasonic cleaning would be a good idea either.
Why not?

Regarding the "high impedance" and IPA cleaning:
First I thought it is more "voodoo" than a real issue, but every time I cleaned some parts of this DMM it got better.
In the beginning it had big offset on display, and now it was nearly OK.

I think I will resolder the switch pins and clean the board.
 

Offline Gyro

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Re: How to clean switches in high-impedance environment?
« Reply #8 on: March 14, 2016, 08:38:21 pm »
Why not? Mainly just an uncomfortable feeling if I'm honest! - Possibility of shifting the values of close tolerance value resistors, crystal damage?

I'm not a expert in ultrasonic cleaning, I'd just be tempted to leave it until I'd tried everything else.

Edit: Also the question of what medium you're going to use, given that IPA isn't safe unless in a purpose designed cleaner.
« Last Edit: March 14, 2016, 08:41:07 pm by Gyro »
Chris

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Offline uncle_bob

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Re: How to clean switches in high-impedance environment?
« Reply #9 on: March 14, 2016, 08:48:00 pm »
Why not? Mainly just an uncomfortable feeling if I'm honest! - Possibility of shifting the values of close tolerance value resistors, crystal damage?

I'm not a expert in ultrasonic cleaning, I'd just be tempted to leave it until I'd tried everything else.

Edit: Also the question of what medium you're going to use, given that IPA isn't safe unless in a purpose designed cleaner.

Hi

Very specifically because there are a wide range of components that recommend against ultrasonic cleaning due to potential damage. Since there are an enormous range of power levels, frequencies, delivery methods (lots of water in the tank vs nearly none), it will never be a "this part will break immediately as soon as any cleaner is turned on". It's more a matter of digging through a few thousand pages of reports on each component and then evaluating the data against your situation.

Bob
 

Offline ModemHead

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Re: How to clean switches in high-impedance environment?
« Reply #10 on: March 14, 2016, 09:08:11 pm »
Regarding the "high impedance" and IPA cleaning:
First I thought it is more "voodoo" than a real issue, but every time I cleaned some parts of this DMM it got better.
In the beginning it had big offset on display, and now it was nearly OK.
The actual A/D input impedance may be several gigohms, so the areas downstream from the divider are quite susceptible.  Just a fingerprint can allow enough leakage current to cause a residual reading.  It's a common issue for all DMMs, but the 8060A seems to be really sensitive, especially with aging electrolytic caps spewing their guts out.
 

Offline uncle_bob

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Re: How to clean switches in high-impedance environment?
« Reply #11 on: March 14, 2016, 09:09:46 pm »
Regarding the "high impedance" and IPA cleaning:
First I thought it is more "voodoo" than a real issue, but every time I cleaned some parts of this DMM it got better.
In the beginning it had big offset on display, and now it was nearly OK.
The actual A/D input impedance may be several gigohms, so the areas downstream from the divider are quite susceptible.  Just a fingerprint can allow enough leakage current to cause a residual reading.  It's a common issue for all DMMs, but the 8060A seems to be really sensitive, especially with aging electrolytic caps spewing their guts out.

Hi

Except that is not the symptom being reported in this case.

Bob
 

Offline e-doc

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Re: How to clean switches in high-impedance environment?
« Reply #12 on: March 14, 2016, 10:56:51 pm »
Sorry, I should have mentioned that I renewed a leaked electrolytic cap (C12) and another, that has lost some capacity (C21).
https://www.eevblog.com/forum/testgear/old-fluke-multimeters/250/

Although I have some years experience in using ultrasonic bath for disassembling and analysing returned faulty encapsulated electronics, you convinced me, not to use ultrasonic in the next step.
Maybe I should have a closer look at the other electrolytics under the microscope, wash the PCB using a brush and use ultrasonic as a last resort...
« Last Edit: March 16, 2016, 10:38:07 pm by e-doc »
 


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