DECmate III and Rainbow 100+

[jpero at sympatico.ca]

> From: "Geoff Reed" <geoffr at zipcon.net>
> Sent: Thursday, February 24, 2005 11:38 AM
> > At 07:39 AM 2/24/2005, you wrote:
> > >Geoff Reed wrote:

> > >>> > And yes, the whole thing is powered, rather poorly, from 12 volts.
> > >>> > (Poorly: if you flip significant quantities of white pixels on and

> > >>>I've never had this problem. Are you sure you've not got some dried-up
> > >>>capacitors somewhere?

> > >>> > off, the scan area expands and shrinks very noticeably.  So clearly
> > >>> > the supply regulation is seriously inadequate.)

> > >>the VR 201 monitors have a couple of points of failure, the most common
> > >>is a coil (IIRC) in the vertical that is potted in wax, they potted this
> > >>-before- soldering, and wax tended to get down into the solder joint
> > >>making a bad joint that fails after a while, there is a resistor right

> Basically you are referring to cold solder joints, the bane of classic
> hardware.
> 
> I tell people that it is usually caused by vibration, invariably they say
> "what vibration it just sits there".
> 
> Randy

I have no problem with wax *but* one material I'd stomp on their 
countless feet if they are caught using is hot glue to secure 
components!  Total reliablity nightmare and costs maker in warranty 
claims.

One particluar maker was notorious for this till late 2002 for 
decades.

What happens at first often certain components and crimped on wire 
stakes (these looks like a crimped terminal with connector missing, 
just a tiny barb on the end to lock into thru hole).   Then 
they're secured with hot glue (usually!) in certain spots.  Solder 
wave is then performed successfully.  Over time.  subsonic 
vibrations, thermal and expansion cycles worked many ways to make 
solder bond fail caused by hot glue flowing into "break" and slowly 
working through the solder & copper or brass "junction" bond.  
Eventually fails, ranging from minor fix for little cost all the way 
to $300 bill and multi parts replacement.   Multi parts blown very 
common in some TV designs, just one part lost the solder bond and one 
part ahead of it gets dirty signal and fails, taking out other parts 
along with it.   Zzzzzzt, finally usually fuse goes if the owner 
didn't heed the power supply screaming as sign to yank that power 
cord out of that socket!

Usually this happens 
ranging from 3 months to 10 years.  Always failed at the component 
leads or stakes where solder wets that leads.  Never the trace lands.

Every time, granteed.  The ONLY solution was desolder them and pull 
all the parts that was "slopped on" with glue and scrape off glue and 
clear the thru holes of those horrible glue.

Simply resolder those parts without hot glue clean up will earn you 
call backs or fails again just after the repair warranty.  Not nice 
and learned that hard way.

Right now we're busy making good money with this!   Just this!  Along 
with occasional items that has true failures not from glue, other 
normal causes like bust flyback transformer, badly installed 
transistor on heatsinks, maybe a burst capacitor or dried up.

What I think was that your monitor might have another type of equally 
nasty glue, that starts out very light tan, very strong bond and very 
stiff stretchy like gum but much stronger bond.  This does very 
poorly with heat, as it ages with heat, glue hardens and turn brown, 
in extreme cases went black.  Some has specific formulations that is 
is equally nasty it turned corrosive after a time as glue breaks 
down.  Some went conductive causing wildly interesting cases.  Some 
turns so brittle that crumbled when scraped off like a burnt 
bread.

Make sure that coil coated in this is really wax.  If it is hot glue 
or other kind, get that freaking thing OFF!!   After soldering parts 
in, sometimes needs to be secured with silicone which is good.  Great 
for stopping whistling noise or shrieks, high pitched buzz parts.  
Like loose ferrite beads on component leads, other items, transformer 
cores is ideally with hard or very firm curing glue or varnish for 
those.

By the way, watch for new no lead solder problems to roll in!  This 
late 2004 and later begun to use those, there will be problems I 
promise you!   I just starting to see the potiential problems with 
this.  :-(   No lead solder doesn't solidfy nicely like a normal 
solder did.   Still mushy, dull look and poor stress resistance 
compared to good old lead-based solder.  I know lead is really bad 
and those bastards is too cheap to properly recover all the lead 
before going to dispoal.  Old type of solder does melt rather easily. 
 In fact you can melt solder without burning plastic if thermal 
control is controlled tightly.

Cheers,

Wizard  (TV servicer).