[R-390] R-390A R390 Stuck Micro Switch

[David Wise]

The switch contacts like to weld.  If I understand correctly, it's a two-step process.  First, the spark from opening the switch degrades the contacts.  Second, when the now-damaged contacts close, that spark welds them together.  You can slow down this process by adding an RC snubber network across the switch.  Sized correctly, it minimizes the opening spark.  (And maybe the closing spark - I can't predict it and don't remember the case studies.)

An inrush limiter will limit the closing spark energy and help prevent welding.

My radio has a snubber and a limiter.

Dave Wise
SWL in Hillsboro Oregon
________________________________________
From: r-390-bounces at mailman.qth.net <r-390-bounces at mailman.qth.net> on behalf of Barry <n4buq at knology.net>
Sent: Friday, April 26, 2019 7:49 AM
To: R-390 HF Receiver List
Subject: Re: [R-390] R-390A R390 Stuck Micro Switch

I had that problem with one of my R390As.  I removed the switch and merely exercised it several times (maybe applying a little contact/switch cleaning solution to the plunger at the same time).  Pressing it down (with a screwdriver, fingernail, etc.) and then letting it pop up several times allowed the plunger to start working properly.  If I'm not mistaken, the plunger simply gets gunked up over time with, well, gunk and won't allow it to pop back into position.

A little less risky than the "fine alignment tool" method but glad that worked out for you.

Thanks,
Barry - N4BUQ

----- Original Message -----
> From: "Ed Rafferty" <dktedr at gmail.com>
> To: "R-390 HF Receiver List" <R-390 at mailman.qth.net>
> Sent: Thursday, April 25, 2019 11:19:06 PM
> Subject: [R-390] R-390A R390 Stuck Micro Switch
>
> My Motorola R=390A had the dreaded micro switch issue where it would not
> shut off when switched from Stand by  to Off.
> The previous care taker of this marvelous radio had purchased the
> replacement switch but had not installed it when I acquired it. So I've had
> the part in my parts drawer for a few years now, Waiting for the opportune
> time to perform the surgery.
>
> Today, while reading the"Approaches to the Dead Receiver" compilation, I
> read the post from Mark Huss 12/30/2006 describing the (do not attempt this
> at home ,professionals only) procedure of deploying the fine alignment tool
> technique" to the targeted area of the ascribed "FUNCTION" label to release
> the stuck microswitch.
> One "calibrated" whack later, My 390A is now performing beautifully, as
> advertised , the function switch is now back to normal. My spare switch
> remains ,spare.
> I used a wood block and a 8 oz ball peen to administer the corrective
> inertia.
> As a former Navy Avionics Technician, I have been trained in the benefits
> of the technique described, and it worked for me today.
> Thanks to Mr. Huss for his Post,
>
> Ed , N4MDA  , USN, Ret.