[AMRadio] Something strange


Larry Szendrei ne1s at securespeed.us
Wed Nov 19 07:26:02 EST 2014


On 11/18/14 8:56 PM, CL in NC via AMRadio wrote:
> By any chance where the the first filter cap  after the rectifier? Could be a peak to peak ripple issue
>
>
Could be.

I had a recurring problem with electrolytics repeatedly failing in a the 
low voltage supply in my Viking Ranger. The power supply was not stock, 
because the original power transformer had failed (I never owned the 
Ranger when the original power xfmr was present, so I've never ieven 
seen it.) Instead the power transformer was from an old tube-typed TV, 
with a silicon diode bridge around the HV secondary and low-voltage B+ 
taken from the center-tap of the winding. The high-voltage B+ went to a 
choke-input filter, but the low-voltage B+ was capacitor-input - simply 
a 450WVDC electrolytic in the 40-80uF range from the HV center-tap to 
ground. (I used several capacitors of different values in this range as 
they kept failing, and I replaced them with whatever I had on hand. The 
original low-voltage filter choke was also missing from the rig.) The 
failure mode was always open, or high series resistance. I finally 
eliminated the problem by installing a replacement choke from the 
junkbox to make the low-voltage B+ supply filter choke-input, like the 
rig originally used, and like the high-voltage B+ supply. This was 
probably over 10 years ago, and I have not had a failure since. I had to 
conclude it was a peak voltage or peak current problem killing the 
capacitors. I had used many different brands of capacitors, both modern 
and NOS, all failed eventually before I added the choke.

Also, I have had experiences of modern off-shore manufactured 
electrolytics overheating and letting their smoke out, sometimes 
spectacularly, when used as replacements on the input side of a 
conventional pi capacitor-input filter in high-voltage supplies. I 
started using Nichicons and have not experienced this since. YMMV.

73,
-Larry/NE1S


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