[AMRadio] Something strange


james.liles at comcast.net james.liles at comcast.net
Wed Nov 19 11:11:19 EST 2014


Try a 100 ohm resistor in series from the low voltage diode stack to the 
capacitors.  Should take care of the capacitor problem and reduce the 
ripple.

Kindest regards Jim K9AXN

-----Original Message----- 
From: Larry Szendrei
Sent: Wednesday, November 19, 2014 6:26 AM
To: amradio at mailman.qth.net
Subject: Re: [AMRadio] Something strange

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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