DM 501A Mode Select Switches Issue


sweetbeats
 

Greetings all…

I’m having an issue with the bank of mode select switches on my DM 501A. Only the resistance mode switch will latch, unless I keep some light pressure on the resistance mode switch will pressing any of the other mode switches. If I do that I can get any and all to latch, but if I don’t keep that pressure on the resistance mode switch while pressing other switches they won’t latch.

I captured a short video to YouTube to demonstrate this but apparently I cannot include a link here?

Cory Oace


 

Cory,

sounds like classic gummy lubrication on the latches for those pushbuttons. Some spray lubricant usually does the trick, but it might only be temporary. The real solution is to get some more permanent lubrication in place, e.g. light machine oil. There's a flat spring over the front of each latching switch and I try to get some oil on each of those springs, then exercise the switch a bunch of times to get things loosened up and work the oil in to the mechanism.

For my instruments I have been spraying DeoxIT into the back end of the push switch and exercising the switches. If the switch is still sticky I hit the front with some WD-40, on the basis that it will penetrate the gummed up works better. That's been working so far, but I suspect that the WD-40 will evaporate and what remains may get gummy again. So long as that takes more than a few years, though, I'm happy. If they gum up more quickly than that I will try using light machine oil instead.

-- Jeff Dutky


sweetbeats
 

Thanks, Jeff.

Actually I don’t think this is that issue. The buttons are not “sticky”. This is a different mechanical issue. The resistance mode button, if not held when pressing other buttons, can hyper extend past its normal stop a little bit. That’s not normal. And like I said if it is held in a little the other buttons will latch, and their action is crisp when released. It’s not that feeling like they got costed in honey I’m dealing with on some of my other modules.


Jared Cabot
 

Here's an article on repairing and servicing these push button switches I just uploaded.
I have serviced a tone of these in my TM500 modules and with a little care, it's relatively easy to do, most often you don't even need to remove them from the PCB.

https://groups.io/g/TekScopes/files/Tekscope%201975%20V7%20N4%20Repairing%20Push%20Switches.pdf

To clean them, I disassemble the switch, clean all the parts with isopropyl alcohol and a small paint brush/Kimwipes (Using a q-tip to scrub inside the switch body), then I use a new q-tip to apply Deoxit red inside the switch body along with some Deoxit on the spring contacts that sit in the central plunger, then finally a reaallllyyy thin smear of plastic safe lithium grease on the plunger and reassemble. I find holding the spring contacts in place in their recesses on the plunger and inserting the whole assembly into the switch body from the rear is the easiest way.
With a little practice, you'll get the feel for it.


Jared.


Mlynch001
 

Jared,

Thanks, very helpful.

--
Michael Lynch
Dardanelle, AR