TRS-80 Fireworks

[Guy Dunphy]

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.


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