[Hallicrafters] SX-42 question

[GARDGORE at aol.com]

Without knowing more about your particular receiver, its previous history and 
the test equipment you have available I would recommend replacing the power 
supply electrolytics before attempting to power up the receiver for the first 
time. However, if it was a project I was doing myself here I might decide to 
go ahead and bring it up on the variac first with the original capacitors 
still in place in order to establish a baseline of its working order before 
proceeding with restoration. There are many variables involved with these 
things such as physical condition, evidence of previous repair work or 
modifications and time since last operated if known that would affect my 
decision that it is impossible to say for sure as each case usually involves 
different circumstances. What you are really trying to accomplish in using a 
variac to slowly power up a long dormant piece of equipment is to get the B+ 
to come up to normal operating level with acceptable leakage current across 
the electrolytics. This involves whether or not the capacitor will accept a 
"reforming" as you gradually increase the variac voltage and without 
monitoring leakage current as you go there is no way of knowing when it is 
safe to advance the variac. In the case of power supply electrolytics I 
monitor B+ voltage and leakage current and I usually start the variac out 
with whatever voltage will result in 10 mills of leakage current. I watch for 
how quickly the B+ starts to rise and if the leakage current will drop. It is 
safe to keep advancing the variac as long as leakage current is kept at 10 
mills or below. If the capacitor has accepted being reformed the leakage 
current will be less than a mill or two in most cases when the B+ has reached 
its normal level. For precise leakage current limits consult the 
manufacturers original specifications from a vintage capacitor or test 
equipment spec sheet. Practical experience has shown here capacitors from the 
Thirties if they will reform will usually leak about 3 or 4 mills when the B+ 
is at the WVDC limit of the capacitor. This is very risky however, and even 
though I do it here I would never recommend to anyone to try to save a 
Thirties electrolytic capacitor. I only do it in certain instances here to 
preserve the history on certain pieces of rare and valuable equipment because 
the parts all have numbers and date codes on them and I want to keep the 
original history intact. I have reached the conclusion that some completely 
original somewhat rare items should be preserved in their present state for 
future generations and not every piece in my collection needs to be restored 
electrically with a bunch of orange and yellow Chinese capacitors but this 
opens up a different can of worms as everyone has a different definition of 
what "restored" means. Later good capacitors (if they are still good) will 
leak less than earlier "good" ones because of improvements and advances in 
manufacturing technology and modern Oriental imports will leak practically 
nil. There is another thing to be concerned about here and that is the point 
where the rectifier tube will begin to conduct. My experience has shown they 
usually begin to apply B+ at around 50 on the variac. This is critical and a 
little tricky around the beginning point of 50 as the B+ can try to rise 
quickly (too quick) with just a small advancement of the variac. You want to 
watch the B+ and not apply more than will cause a leakage greater than 10 
mills until you can get the capacitor to settle down take more load. I have 
some tube bases with diodes installed in them to sub for 80's, 5U4's and 
5Y3's that I use to make this less critical sometimes. Hewlett-Packard makes 
an instrument with a hall effect pickup that will monitor leakage current 
that makes it unnecessary to disconnect one leg of the capacitor otherwise 
you will have to disconnect them in order to monitor your progress.

Regards, Greg