On Tue, 2008-02-19 at 17:30 -0500, John Pelham wrote:
> Several of the HV electrolytics in my Titan 425 failed two weeks ago. At
> first I looked for exact replacements for the Mallory CGS originals. As of
> last week, Digi-Key and Newark didn't have them. Mouser listed them ($23
> each) but did not have them in stock. Allied had them in stock at $21.
>
> I ended up deciding not to go with exact replacements but with an upgrade.
> I replaced them all with 105C-rated 470uF Nichicon caps, much smaller than
> the originals. It required some small mechanical mods to make them work,
> but my Titan is now humming along nicely again. I also replaced all the 2W
> carbon composition bleeder resistors (they had all drifted higher in value
> by varying amounts) with 3W flameproof metal film resistors.
I am reluctant to use the physically smaller Nichicon capacitors because
I believe them to be rated for a much smaller ripple current and I fear
they will overheat in the transmitter power supply. A power supply
running an amp load requires lots of ripple current in the filter
capacitors, especially if the power supply is a voltage doubler as many
ham PAs are. Look up the data sheets on the computer grade vs the
Nichicon capacitors. The large physical size for the capacitance is why
I like the TVA atoms for axial lead capacitors for better heat
dissipation over the super small off shore capacitors. When I've checked
data sheets for ripple current ratings, the super small capacitors have
been lacking.
The flameproof metal film resistors are a superb product. I've tested
them to a red glow (hard on the surroundings and the coating) at ten
times rated power and found when cooled, it took a 5 digit digital
bridge to detect the resistance change after days of glowing. Really
tough parts.
The dummy load used at Collins for the 250 KW AM 821A-1 used maybe 5,000
10 watt metal film resistors in series parallel with high velocity oil
cooling to handle the 1 MW peak/ 1/2 MW average power output when
modulated with a severely clipped sine wave. Dummy load failures were
all too common, mostly from carbon tracking in the insulation trying to
confine the oil flow to the resistor columns (300 ohm balanced load).
That carbon in the oil flow cleaned the coatings off the resistors
rapidly but I think the metal film remained.
>
> 73,
> John W1JA
73, Jerry, K0CQ
_______________________________________________
TenTec mailing list
TenTec@contesting.com
http://lists.contesting.com/mailman/listinfo/tentec
|