Variac

[Greg Gore; WA1KBQ]

It depends on the particular piece of equipment in question. No harm would  
be done at all if a Hallicrafters SX-100-101A or any Collins receiver was  
recapped and it would result in a better working more serviceable unit. On the  
other hand, what about a Hallicrafters SX-10 (200 made) or an RME-9D  
(individually hand built), or National AGSX, Breting 14, Sargent 21MA, or a  McMurdo 
Silver 3A? I used to believe every receiver should be recapped until I  realized 
important history was being discarded and lost forever when my goal was  just 
to put it back on the shelf with a "ready" tag and never to be plugged in  
again. Antiques have more value if left entirely in their original state or with  
a simple replacement filter network that can be easily reversed. The problem 
as  I see it with component replacement or re-stuffing work is very seldom do 
you  find workmanship that is up to the standards of workmanship of the 
original. Now  I realize most of these things were soldered together en-mass by 
women  assemblers but they did it every day perhaps thousands of times and the 
quality  and appearance of their workmanship is a part of the history; they did a 
better  looking job than most of us can do. I now prefer to check an antique 
for signs  of life regardless of quality of working order and I consider it a 
plus if any  is found.

For example: I have a completely original Hallicrafters S-1  Special here. 
This was Bill Halligan's first "hallicrafters" communications  receiver and 
there is currently only two Specials known to exist. I have  established that the 
receiver exhibits a loud hum. This is good enough for me. I  could easily make 
it work again but why in the world would anyone want to? The  S-1 could 
probably be outperformed by an Ocean Hopper. The historical value of  this receiver 
far exceeds any questionable noble intent on my part to "make it  work again."

When checking any unknown antique for the first time I  recommend cleaning 
the set first followed by a good visual inspection. Look  carefully at all point 
to point wiring and physical appearance of all  components. If you see a 
damaged component or a charred resistor you don't  proceed with power for example. 
Power the set in steps through a 100 watt light  bulb followed by a variac; 
maybe first without the rectifier tube to get an  indication of the transformer 
first. The 100 watt bulb will protect the  transformer from damage if a fault 
exists. Next I usually plug a  substitute solid state rectifier in the 
rectifier socket for additional safety  because it allows me to start testing at 
lower input voltage but this is not  entirely necessary if you carefully monitor 
the behavior of B+ while advancing  the variac. Rectifier tubes begin to 
conduct B+ at around 50 volts on the variac  setting and B+ will escalate very 
quickly with any small advancement past that  so be careful. I start with some 
measured B+ and monitor how responsive it is to  small advances of variac. If the 
filters are in trouble the B+ will be lazy and  slow to respond and the cap 
may start to get warm. Good filters will remain cool  and allow B+ to build 
quickly to normal levels. Be sure to check component temps  periodically too. Of 
course the plate and screen bypass caps will affect this  too but we are 
primarily answering the variac question here. There are other  means of determining 
if reasonably safe to run with old bypass caps. Usually it  is risky to run 
with old caps but I do it here all the time. Just like driving a  1928 
Chevrolet down the highway would be less risky with a modern replacement  engine and 
hydraulic brakes but I don't know of any antique car guys who would  want to 
experience a 1928 Chevrolet in that manner.

73, Greg
 
PS- A better method would be to disconnect the filters from the circuit and  
test leakage current first before proceeding with power up.
 



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