Fire!

[David Roper]

My 5th grade teacher told me the story of her TV set catching fire back in the 50s. It was playing fine and she happened to wander away (as you do) for a moment or two. Next thing she knew flames were coming out of the back.

At least twice in years past the chassis of a set undergoing restoration at the Early Television Museum has caught fire. IIRC both were British prewar sets plugged into a 220 line, accidentally left on because the switch that killed 110 to the bench was a separate line. Good reminder not to get too casual with the old things.

[Chad Hauris]

I have noticed for old equipment that was designed for commercial/industrial use (jukeboxes, PA amps, broadcast equipment) and which was designed for 24 hr. a day use there is always fuse protection in the power line and usually additional protection such as a B+ fuse. The notable exception is Hammond organs.
However most consumer equipment seems to have no protection.

I always retrofit any old device I repair with a fuse, even table top plastic radios. The only ones I don't are series string radios with a tube rectifier as the rectifier cathode is designed to be fusible in case of a short and usually a portion of the rectifier filament acts as a fusible/surge resistor.

Also I don't let there be anything on the power line in the set before the fuse. If there are noise suppresion capacitors across the line I just remove them and have had no trouble.

I replace all electrolytic/paper capacitors and any old silicon rectifier diodes before using a piece of equipment. I have had too many failures of these old things from previous experience to trust them. One was a shorted capacitor or diode in a Harman Kardon A-300 amp which worked perfectly on original components for years but luckily had a fuse which blew when the short occurred to prevent serious damage to the power transformer.

I also have seen too many cases of burned/shorted wiring in places like Hammond organ tone cabinet connectors, and melted 5U4 tubes due to shorts where damage likely would have been minimized had a fuse been in the unit.

The only time I actually experienced a fire was a flyback fire in an RCA CTC-31 when I was a teenager. Everything inside the HV cage was burned and melted. Luckily there was no other damage to anything else but that was very scary.

Another potential problem can be (i am theorizing) when the enamel insulation on magnet wire degrades due to heat. I have had several motors in electric fans burn up and my theory on this is that the motors were overheating for some reason in use which led to the insulation breaking down to the point where a short finally occurred across the windings and burned them up. This sounds like the kind of failure which may have occurred on Dave's set.
If a motor has seized due to lack of lubrication and the windings look discolored, even if I can relubricate it and get it turning again I just replace it due to perhaps the increased possibility of a short.

[compu_85]

Originally Posted by Chad Hauris

I have noticed for old equipment that was designed for commercial/industrial use ... there is always fuse protection in the power line and usually additional protection such as a B+ fuse. The notable exception is Hammond organs.

I've heard this is because Hammond got sick of all the warrenty calls related to blown fuses.

[cbenham]

Originally Posted by Chad Hauris

I have noticed for old equipment that was designed for commercial/industrial use (jukeboxes, PA amps, broadcast equipment) and which was designed for 24 hr. a day use there is always fuse protection in the power line and usually additional protection such as a B+ fuse.

During the 6PM News I ran camera at an ABC TV affiliate in Florida. I saw an orange flicker down to my left which turned out to be the flyback in an RCA B&W Studio Picture Monitor burning brightly.

I dragged the beast by the power cord out of the studio and hit it with the fire extinguisher. The important point I made in my report was that the monitor had a 0.25 amp fuse in the B+ line to the flyback and a 2.5 primary line fuse. Both were the right value and NEITHER FUSE BLEW during the excitement.

So much for fuse safety.

Cliff