[time-nuts] HP105B HP 105B 1 amp fuse blowing

[ed breya]

I'd take Taka's suggestions regarding the actual battery and charger 
characteristics. A gross over-current means something's wrong, but a 
random fuse blowout after many years shouldn't be too surprising either, 
presuming everything seems normal otherwise.

If it's not a battery/charger issue, one thing I can think of is one or 
more shorted turns in the oven heater winding. This would still allow 
for heating, but maybe takes too much current while getting to the 
setpoint. A little bigger fuse may get it past such a marginal 
condition, and once the setpoint is attained, the average current should 
drop to a safe zone.

This is one of the common failure modes I've seen in device heating 
ovens  - another being burned open, which is worse. Another deal killer 
is if the heater winding shorts through its insulation to the oven case, 
whether all the time, or only when it gets hot - that pretty much ruins 
it. I have an HP740A reference diode oven like that.

If the fuse clearing always happens after a certain time range, then a 
simple check is to keep upping the fuse to maybe no more than twice the 
proper rating, and try to monitor what's going on, hopefully catching it 
at the right moment. If it's not battery/charger/heater element 
resistance-related, then you may have a dreaded heater to case short, 
which changes everything. In this case, as with an open, you can get try 
to put some other sort of heating system around it all, but it won't be 
quite the same.

Ed