I have an old 4.5 digit Fluke 8050A DMM that I'm trying to bring back to life. It came to me as "broken". It didn't display anything properly. Voltage readings and resistance were wildly incorrect and jumping around all over the place. So without even doing any further testing I replaced all the electrolytic caps. All the old ones I took out tested perfectly fine, both capacity and ESR.

After that, resistor readings became more accurate, but still a bit erratic. But I suspect the caps weren't the issue. It probably was the result of me playing with the toggle switches on the front panel. Voltage readings were still wildly inaccurate. So I started tracing out the front end, and the first test point, TP6, which should have been directly connected to the input when in DC 2V mode, was not showing the voltage on the input. So all suspicions were now on the switches.

Since they are open front and back, and I can see inside, I put some drops of isopropyl alcohol in there, and dried them out with compressed air. Rinse and repeat a few times, and sure enough, after everything dried out, the 8050A started reading voltages and resistance dead on when compared to a much more modern 3.5 digit handheld Fluke.

So my question is, should I buy some contact cleaner with lubricant in it? Like say DeoxIT. I believe the switches were lubricated with a "heavy" grease from the factory. Some switches, before I cleaned them, were "slow" to pop back out. So I suspect factory grease.

But on the other hand, looking at the schematic of the DMM, the front end is very much analog all the way, and the switches play a huge role in it. Does lubricant have the potential to conduct ever so slightly and upset the front end?

 

I suspect any grease applied by the OEM would have been for the mechanical latching parts only, since degrading grease on the contacts could be a problem. The switch cleaner I'm familiar with is just a solvent, with no lubricant properties.

 

The switch cleaner I'm familiar with is just a solvent, with no lubricant properties.

From my research pure solvent cleaners would be sold as "electronics cleaner", so they leave no contaminants whatsoever, whereas "contact cleaner" usually has some lubricating properties. DeoxIT even lists that as one of the benefits.

Use DeoxIT® D-Series to dissolve contamination, improve the connection as well as lubricate and protect the surface.

DeoxIT seems like the gold standard for bringing back to life old potentiometers. It has rave reviews in the audio equipment restoration circles. But, I'm not sure about it in the front end of a DMM.

DeoxIT®, DeoxIT® GOLD, DeoxIT® SHIELD, DeoxIT® FaderLube liquids are completely non-conductive, and neither the 100% liquids or the 5% spray versions contain alcohol or any other conductive constituents.

They do claim it's completely non-conductive. Not sure if completely means 0.000000001 or 0 in their world.

Any idea what maker/series those switches are? I wonder what they look like inside, and if they need lubrication of the metal contacts by design.

 

Does anyone at least recognize the switches from years past? Who made them?

They must have been (or still are) some top tier company.

 

Dirty switches are #1 problem with benchtop DMM.
Use liberal quantities of IPA injected with a syringe and work the switches vigorously while the IPA is still wet.

 

From my research pure solvent cleaners would be sold as "electronics cleaner", so they leave no contaminants whatsoever, whereas "contact cleaner" usually has some lubricating properties. DeoxIT even lists that as one of the benefits.



DeoxIT seems like the gold standard for bringing back to life old potentiometers. It has rave reviews in the audio equipment restoration circles. But, I'm not sure about it in the front end of a DMM.



They do claim it's completely non-conductive. Not sure if completely means 0.000000001 or 0 in their world.

Any idea what maker/series those switches are? I wonder what they look like inside, and if they need lubrication of the metal contacts by design.

This is what I use on old switches, pots and just about any electrical contact. The trick is to get the fluid on the contacts. The 'Needle Dispenser' works wonders for that.
https://caig.com/product/deoxit-d100l-25c/

 

So my question is, should I buy some contact cleaner with lubricant in it? Like say DeoxIT. I believe the switches were lubricated with a "heavy" grease from the factory. Some switches, before I cleaned them, were "slow" to pop back out. So I suspect factory grease.

But on the other hand, looking at the schematic of the DMM, the front end is very much analog all the way, and the switches play a huge role in it. Does lubricant have the potential to conduct ever so slightly and upset the front end?

The effects of DeoxIT will not last very long in my experience and you are trying to suppress re-oxidation of the contact surfaces by keeping the air away from them.

Have a look at these people:
https://www.nyelubricants.com/switches
Download the various technical papers to get an overview of what characteristics you should be looking for. Different plating will have a bearing on the lubricant required.

I have used their products and they are well engineered.

Ideally you would dismantle the switches so that you can properly remove all the oxidation, especially from the entire surface of the moving contact, which never gets completely cleaned by leaving it in situ.

 

DeoxIT works if the switch is still viable but it's not a miracle worker, nothing is.

The long term solution for long term reliable operations is to replace the switches like I did on this set of old Gaussmeter probe amplifiers.

 

Dirty switches are #1 problem with benchtop DMM.

With analog switches like in this vintage piece?

The trick is to get the fluid on the contacts.

If the DMM board is stood up vertically, IPA drops slowly into the switch assembly, and comes out of the other side. It's just a bunch of pins on either side, and a plastic shaft sliding back and forth, with some springy metal contacts on the shaft I assume. My concern is getting the fluid everywhere inside the switch. If it will change "off" resistance specs of the switch.

Ideally you would dismantle the switches so that you can properly remove all the oxidation, especially from the entire surface of the moving contact, which never gets completely cleaned by leaving it in situ.

That's why I'm trying to figure out who made them, and if there are instructions of maintenance. They do look like they can be dissembled without undersoldering anything. It looks like the interlock mechanism pops off and the sliding elements might come out and might be replaceable parts even.

The long term solution for long term reliable operations is to replace the switches like I did on this set of old Gaussmeter probe amplifiers.

I doubt the switches can come without damaging the board. It's a pretty poorly made PCB by today's standards. It would not be worth the trouble for sure. And I think the switches are fine, just oxidized. I don't know the history of this DMM. It probably sat in a closet for a decade if not more.

 

It looks like the interlock mechanism pops off

Be certain to make notes / take photos before attempting disassembly. There is another thread somewhere posted by a guy trying to work out how on earth the latching bits fit back together! .

 

One bizarre thing I noticed about the circuit board on this DMM is that the silk screen was applied after all the components were soldered in.