[Amps] Fuse holder surprise

Artek Manuals Manuals at ArtekManuals.com
Tue Jun 2 20:33:31 EDT 2020

This is going to seen trivial to many of you but some times the most 
puzzling problem has the most simple solution, worth sharing.

The " 60W-brick" ( an old Mirage C106) on the 220 "lets chase 
DX"-repeater had given up the ghost. No power on light ...hmmm checked 
and the fuse on the back panel fuse holder  looks absolutely perfect. So 
out of the rack and back to the home shop for a more detailed autopsy. 
After 5 minutes of poking around with the VOM it seems the fuse is 
actually a zombie fuse, meaning it looks OK but in reality it is open 
with no sign life or the cause of death, usually 15 Amp fuses die pretty 
spectactularly. Oh well pop in a new fuse and it is off to the races. 
But the amp is only putting out 40-45 W and it is supposed to be 60W? 
Playing for 10 more minutes and nothing seems out of the ordinary..oh 
well it is 30 years old anyway button it up and move on to the next 
project right? As I put the cover back on I happen to absent mindedly 
tighten the fuse holder cap and it is VERY WARM ...now wait a minute 
fuse holders aren't supposed to be that warm? Further measurement shows  
a little better than .5 volt drop across the fuse holder under load. A 
little loosen-tighten cycles and it is down to .25 volts . Upon closer 
inspection the spring inside the fuse holder looks dark and crusty ( 
like the one in your flash light after the batteries leak). So I have 
some really nice fuse holders in the junk box with bright shiny copper 
insides , replace the fuse holder , no more voltage drop and the brick 
is at 70W out.

The moral is the old fuse holder was corroded internally to the point 
that it's contact resistance had gone up enough under load to heat the 
fuse contact end and melt the solder inside the fuse. The bigger take 
away is that if something seems a bit odd keep looking. This one was 
easy but in 50 years of tinkering I had never seen a bad fuse holder and 
now I wonder how many I over looked over the years 8^)

manuals at artekmanuals.com

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