[AMRadio] boat anchors heathkit etc


doxemf at aol.com doxemf at aol.com
Fri Jun 23 13:55:33 EDT 2006



-----Original Message-----
From: Merz Donald S

<A small quibble...the light bulb thing is great. But if you're in this 
hobby,
you need a variac and you need to always use it to bring up anything 
that is
unknown. >

Don,
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 
varying
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.

Bill KB3DKS

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