[Hallicrafters] how to power up old radio with variac

[WA1KBQ at aol.com]

You have already received excellent advice to replace the electrolytics and  
the wax paper caps so I'll just try to answer the question you asked. The  
following reply assumes you want to try to power the set with limited  
availability of test equipment and further assumes a willingness to accept a  calculated 
risk that the procedure may or may not produce the desired result. I  like 
the earlier recommendation of starting out with a 60 watt light bulb in  series 
with the AC line cord because the bulb will protect the power transformer  in 
case of a serious fault.

Answer: It depends upon a number of  variables so a simple single 
recommendation might not be correct. Rather than  determining a time and steps for 
advancing a variac what you really want to know  is that you are not overloading the 
power transformer secondary windings at any  given primary input voltage due 
to a fault. You would like to know if there is  sufficient combined leakage 
current in all the old capacitors that would cause  the receiver to load the 
transformer more than it was designed to handle. Of  primary concern will be the 
leakage current across the old power supply filter  capacitor(s). The 
rectifier tube will begin to conduct somewhere around 50 volts  on the variac but this 
is so unpredictable and unstable that you cannot use any  certain value 
concerning where to start. A much better method would be to  monitor B+ voltage and 
start out carefully advancing until you see about 100V  and advance in 50V 
steps from there allowing several minutes at each step.  Trying to start out at 
100V can be a little tricky because advancing the variac  a small amount 
beyond the point where the rectifier starts to conduct will send  the B+ up fast. I 
would recommend substituting an inexpensive plug-in solid  state rectifier 
built into a tube socket which is widely available for different  type tubes 
because they are easier to control and more stable. You will be  looking for the 
B+ to quickly jump when you advance the variac. Lazy B+ will  indicate a 
problem. If you advance the variac and do not see a corresponding  quick jump in B+ 
voltage the exercise is over until you determine where the  fault lies. If B+ 
rises normally toward its rated value as you advance the  variac toward the 
rated input voltage for the receiver you may be OK at least to  run the set a 
little for now. Remember you are dealing with old capacitors that  will be on 
borrowed time and anything can happen so be ready. If you like a  little 
excitement this could do it.

73,  Greg



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