|[AMRadio] boat anchors heathkit etc|
doxemf at aol.com
doxemf at aol.com
Fri Jun 23 13:55:33 EDT 2006
From: Merz Donald S
<A small quibble...the light bulb thing is great. But if you're in this
you need a variac and you need to always use it to bring up anything
I can't agree more ! I built my first V/A metered Variac box around
1976 while working on HIFI gear after blowing up a couple of "repaired"
stereo amps. Had used the light bulb trick before that.
A lot of trouble shooting , especially of tube gear can be done while
only operating at 100 vac. The first step with an unknown
unit is to plug in a solid state rectifier module in place of the 5U4,
etc with all other tubes pulled and ramp up the voltage while
monitoring I/R drop around the supply dividers and secondary caps along
with watching the ac supply current.
If this looks near normal after several minutes at 100vac then I just
leave it on while checking for heating of the cans or a current rise.
This can be done while working on something else and just watching
meters. A Heath metered 1-3 amp bench variac works fine for this and a
digital metered 20 amp variac used for the big iron.
<I am often to lazy to do this, but I think the right way to do it is
to use a
Sprague TO-series cap checker (or regulated DC supply, etc.) to apply a
DC voltage while watching the current flow, keeping it at 1ma or less.>
Used the TO-5 and TO-6 for years but you are right , it is a pain if
there are a lot of caps to go over.
1-2 ma appears to be about the minimum current needed for reforming in
a reasonable time period with around 5 ma causing heat buildup if
seriously leaky. The most sucessful reforming has been done with a
CC/CV supply set safely and just let run.
If you have a good 10X scope probe or high voltage version the
breakdown and reforming can be observed by watching the voltage in a
unit under variac control.
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