TRS-80 Fireworks

[Dave Wade]

> -----Original Message-----
> From: cctalk <cctalk-bounces at classiccmp.org> On Behalf Of Guy Dunphy via
> cctalk
> Sent: 28 August 2019 10:07
> To: Marvin Johnston <marvin at west.net>; General Discussion: On-Topic and
> Off-Topic Posts <cctalk at classiccmp.org>
> Subject: Re: TRS-80 Fireworks
> 
> At 01:15 AM 28/08/2019 -0700, you wrote:
> >I'm just curious how many people have powered up their TRS-80
> >computers, and ended up with a bang and a room filled with smoke?
> >
> >So far, I've gotten the fireworks in two out of two TRS-80s (model 3
> >and
> >4) when they were powered up. In both cases, the problem was with the
> >main line filter capacitor mounted on one of the power supplies. The
> >computers continued to work which was my first clue the problem was not
> >serious :).
> >
> >Is this a normal problem with these older computers? I'm used to seeing
> >the electrolytics give problems, but this is the first time I've seen
> >one of the X type line filter caps blow.
> 

In the UK it affects the BBC Model B models. Plenty of kits on E-Bay to fix.
Often cheaper than a single cap.

Dave 



> 
> An extremely common problem with all old electronics from around that era.
> The mains filter caps are commonly 'RIFA' brand metalized polyester film,
> encapsulated in a clear-honey-coloured resin.
> The problem is that the resin embrittles and shrinks with age, resulting
in many
> small cracks. (And sometimes large pieces falling off.) The cracks let in
> moisture, which absorbs into the insulating film.
> When that inevitably fails the resulting arc eats away at the thin
metalization
> film, vaporizing the adjacent plastic into foul-smelling greasy smoke.
Often not
> enough mains current gets drawn to trip the breaker, so the arcing ruin
may go
> on for some seconds - producing lots of smoke and stink. Also commonly
> destroying the component value markings, so you have to guess about a
> replacement.
> 
> HP 62xx series bench power supplies have some RIFA caps and a circuit
board
> inside a closed metal box containing the mains pre-regulator TRIAC. When
> those RIFAs blow the smoke condenses on everything inside the box.
Cleaning
> that mess is really a pain.
> 
> RIFA caps may be the most hated components in electronics. Even worse than
> dipped tantalums, popped electrolytics, and decaying urethane foam.
> 
> Replace on sight. Before trying a power up.
> 
> I was recently given a large pile of NIB (old) switchmode supplies of
various
> sizes, all the same manufacturer. About a third have RIFA mains filter
caps.
> Grrr...
> 
> Guy