|[AMRadio] Variac'n the caps|
CL in NC
mjcal77 at yahoo.com
Thu Jun 3 12:36:57 EDT 2010
Like most of you, after initial ohm checks, I always put something old but new to me on the variac and bring it up slowly. I started to heat up an old Swan 117C PS the other day and really saw what the variac treatment was doing to the caps. The Swan 800 volt B+, has 2 dual section caps, with both sections connected together, and the two caps in series, with a divider string consisting of 2 150K resistors across them. As I was bringing up the variac, I noticed the voltage across one cap was higher than the other. I stopped and checked both 150K resistors, they were within 10K of each other. I then put the Simpson in line with the suspect cap and monitored the current. Sure enough, it was leaking about 10ma, making the voltage drop lopsided across the equalizer resistors. I just let it cook and watched the leakage, slowly it went down to about .2ma. Raised the variac and watched it again, did that for quite a while. When I was done, the only
difference in voltage across the equalizers was due to their 10K difference. There was no load on the HV section while I was doing this. I rechecked it the next day to make sure all was healed, and it was.
So if you are patient, and lucky, you can bring those old caps back to life before just whole sale changing them out. Of course I wasn't so lucky with the 4 section cap in the Swan 350. 2 sections are completely shot. But, you may be aware of Hayseed electronics. He is making multisection caps, and will have this one I need for the Swan soon. He is coming out with Drake caps with the copper twist locks so you can solder it down just like Drake did. He is already providing cap kits for several older rigs and receivers.
Charlie W4MEC in NC
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