Probably as long ago as 15 years I was at a local auction and a Feedback EW604 watt-meter came up for sale. It was a general sale and I shouldn't think anybody there had any idea what it was. Unsurprisingly nobody bid for it and i got it for a couple of quid.
Apart from checking that it worked I have never used it before and it has been sitting on a shelf those 15 years.
Today I had a use for it. I wanted to compare the output from a push-pull amplifier using 2 good output valves compared to 2 valves that were down to about 50% emission.
I started using the speaker as a load and listened to some music just to see if the meter was still alive. I was just about to turn off to lash up a dummy load and connect an audio generator when I heard a "POP" over the music. I looked down at the meter and the needle was still dancing to the music. "Couldn't have been the watt-meter" I thought. Then smoke started to weep out of a joint in the cabinet of the watt-meter. "It is the watt-meter" I thought. (I catch on quickly to these things you know
).
After quickly disconnecting the meter from the mains supply I removed the top half of the case. A lot of dark grey smoke came out and I opened the workshop door to clear the air.
The RF filter capacitor across the mains input had failed (see pictures). The capacitor is after the switch but before the mains transformer primary fuse and so the fuse didn't blow. The 3 Amp plug fuse didn't blow and so it was as well it was not unattended from the point of view of fire risk.
I shall replace the capacitor with a 0.1uF class X type.
I commonly leave vintage equipment running while I am out of the workshop for short periods. Perhaps I need to review that habit.
_