[Boatanchors] Relay Repair - Demagnetization Question

[WA5CAB at cs.com]

That's correct.  As my third sentence said (the post I replied to had 
referred to age-related chemically caused relay failures), I was referring to 
vibrators that don't work after decades long periods of inactivity (as in never 
used).  And Carl, I got this trick from Tom Horsfall 30+ years ago.  He got 
it out of a West Coast antique car collectors monthly journal sometime in 
the late 80's.  

Most antique cars use 6 volt systems.  So that's what the original article 
said to use.  For those (and the 2-volt and 4-volt military units), you need 
a 40 watt lamp to get enough current through the coil once the switching 
contact starts work again.  For 12 volt units, a 25 watt lamp is adequate.  
For 24 volt ones you can usually use a 12 watt one.  For all contacts that 
don't switch the coil, 12 watt ones are fine.  They all allow roughly 170 VAC 
peak across open contacts.  Only the one in series with the coil has to carry 
enough current to operate the coil.  I've also found that for the lamp in 
series with the coil (especially on shunt type vibrators) time to clear the 
contact is reduced if you put a diode in series with the whole mess.

I've always assumed that the most efficient "fixer" would have the AC 
running at the vibrator's natural resonant frequency.  Apparently, it works a 
little better (in a 60 Hz world) if the dirve voltage is running slower rather 
than faster than the vibrator wants to run (typically 100 CPS).  At least 
until the switching contact clears.

In a message dated 2/2/2010 11:10:32 PM Central Standard Time, 
w7qho at aol.com writes: 
> A distinction has to be drawn here between vibrators  that fail in  
> service and vibrators, military or otherwise, that fail to operate  
> following long periods of non-use.  The latter problem as Robert  
> points out is due to a chemical buildup on the contacts.  I am certain  
> this is not limited to military units and is the type of problem most  
> commonly encountered today.  This buildup is very hard and some  
> significant mechanical burnishing effort is required to remove it.  I  
> have never tried the 115vac/series lightbulb treatment that several  
> claim to have used with positive results.  I 'm skeptical (given how  
> hard the gunk is to scrape off mechanically) but plan to give it a  
> shot one day.
> 
> Dennis D. W7QHO
> Glendale, CA
> 
> 
> 
> On Feb 2, 2010, at 5:37 PM, Carl wrote:
> 
> >I wouldnt know about military vibrators as Ive no interest in them.  
> >Nor do I know if they are constructed any different than civilian  
> >versions.
> >
> >Automotive type whether used by hams, public safety, or whatever  
> >often have the contacts stuck together and 6 or 12VDC often wont  
> >budge them loose. Trying it too long is guaranteed to damage the  
> >contacts.
> >
> >It only takes a little AC, no string of lamps or 120VAC involved; a  
> >6 or 12VAC filament transformer works fine. Ive been doing this  
> >since the 60's when I started rebuilding vintage auto radios for  
> >owners. Many hundreds also including the 38 Buick I finished 2 days  
> >ago.
> >
> >
> >Carl
> >KM1H
> >
> >
> >  ----- Original Message -----
> >  From: WA5CAB at cs.com
> >  To: km1h at jeremy.mv.com ; boatanchors at mailman.qth.net
> >  Sent: Tuesday, February 02, 2010 5:04 PM
> >  Subject: Re: [Boatanchors] Relay Repair - Demagnetization Question
> >
> >
> >  No.  At least not the military ones.  The problem with them, after  
> >decades of inactivity, is that the contacts are covered with a non- 
> >conducting layer of tungsten sulfide (or maybe supphate, I never  
> >looked up the relavent valences) caused by sulfur outgassing from  
> >the sponge rubber sleeve inside all the cans.  I've "fixed" hundreds  
> >of them.  You can open up the can and burnish the contacts but it's  
> >a lot less trouble to use an array of 120 volt 12 to 40 watt lamps  
> >and the 120 VAC line to do the job.

Robert & Susan Downs - Houston
wa5cab dot com (Web Store)
MVPA 9480