Re: Cleaning TM500 module pushbutton switches

Chuck Harris <cfharris@...>

Petroleum benzine is a redundant redundancy for white gasoline.

White gasoline is an additive free (typically) mixture of heptane
and hexane and their isomers. It is also called stove gas. And
a more pure form also exists as a rubber cement solvent.

-Chuck Harris

Colin Herbert via wrote:

You mention something you call "petroleum benzine". Though I am a retired research chemist living in Britain, I have no idea of what this is and others may not also. It _could_ be "petroleum ether", which I am familiar with, but that comes in various boiling-point ranges and you wouldn't want to use a high-boiling fraction; 60-40 is the most likely (boiling-range between 60 and 40 degrees Centigrade/Celsius). Be cautious, this solvent is very inflammable and not a little smelly. If you are actually thinking of "benzene" (note the "e", not "i"), then I doubt that anyone in an EU country would be able to buy that, as it is carcinogenic (causes cancer) and is therefore controlled. Perhaps you could clarify?

-----Original Message-----
From: [] On Behalf Of Nenad Filipovic
Sent: 10 July 2020 15:42
Subject: Re: [TekScopes] Cleaning TM500 module pushbutton switches

On Fri, Jul 10, 2020 at 3:05 AM W1PJE <> wrote:

I have multiple TM500 modules that have developed sticky / intermittent
pushbutton switches (the square small ones). Is there a recommended
cleaning procedure? I was going to do the standard use of Caig's DeOxit in
a sparing manner, but perhaps there is a better way.
In some cases hard patina forms on contact surfaces and DeOxit can be
ineffective or just a short term solution. Additionally, DeOxit cannot
restore the original grease which is likely fully decomposed and dry due to
age. In that case I use the following procedure to restore these without
any desoldering:

- Gain access, carefully uncouple and remove the pushbutton shaft.
- Remove the clamp next to the spring, carefully remove the tiny parts
(spring, tiny rocker bolt and its elastic support).
- Carefully pull out the switch shaft at the back of the switch body - care
must be taken to go slow and grip the two metal contacts (use tweezers)
before they fully come out (otherwise they may fly off and get lost). Note
(or take photo of) the orientation of contacts with respect to their slot
in the shaft (the bend in the middle should be oriented away from the
switch body).
- If the switch is radio button type it will contain a "clutch" sliding
plate that needs to be pushed to one side so the shaft could be released
and removed.
- Sometimes a PCB component may get in the way of shaft removal, I almost
always find it easier to remove the offending component rather than the
switch body.
- If switch parts are clogged by ancient dried up grease, clean/wash
everything (both plastic and metal parts) with a paintbrush in petroleum
benzine (pharmaceutical grade, in Europe we get it in regular pharmacies;
however most non-polar solvents which are not aggressive to plastic would
do). Do not let the plastic parts sit in benzine for too long, all this can
and should be done in just a few minutes. Let the clean parts dry.
- Using a soft brass brush (I get these at dentist supply shops) gently
scrub the patina off the active side of the metal contacts. Soft brass
brush should not damage the contact plating that needs to be preserved.
- Use cotton ear buds soaked first in petroleum benzine, then IPA to clean
the internals of the switch body. I spend at least two sticks per switch.
- Grease all sliding surfaces of parts sparingly (innards of the switch
body, shaft, contacts, rocker bolt pathway in the shaft) with plastic safe
light grease. The goal of contact greasing is protection from moisture and
oxygen that may re-deposit the patina again in time. Light grease won't
interfere with low current low voltage electrical contact properties.
- Re-assemble and exercise the switch.

Best regards,

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