A 1KV disc ceramic will do just fine for the external cap. I have a couple of
RF Doorknobs that I use externally depending on my needs. Voltage is not
critical.
Here is one thing that has shown to be an improvement with the 238 tuner.
Replace the disc ceramics internally with RF rated doorknobs. These may be a
challenge to get in the tight spaces physically but this settles things down
completely if you are prone to run legal limit power in modes like RTTY .
Otherwise, it is not a problem on SSB or low duty cycle modes or low power.
You'll observe the SWR change on some bands and with certain antennas and
configurations. These capacitors are the group of 220 PF caps. I found some
450 pF caps for those group of 8 x 220 pf units that are parallel off of the
switch deck S1B and a single 220 for the other application off of S1A. See the
manual/schematic for details. I also replaced the two 50 pf units in series
with a 25 pf doorknob. All doorknobs are rated at 2500 volts. I ordered my
caps from Fair Radio Sales.
It all has to do with the impedance of the antenna and the type of reactive
component be it Xc or Xl . I have a 75M coax fed antenna that show a natural
resonance about 3925. SWR is about 1:1 at this frequency too. At 3980 the
tuner will easily tune the load to 1:1 SWR but after about 15 sec of carrier at
1500 watts I see the SWR starting to rise. Let things cool for a couple of
minutes and the SWR is again 1:1. Again, same antenna and frequency of 3600
with legal limit, SWR adjusted to be 1:1 the SWR does not rise. Checking
components I find that the internal disc ceramic caps are getting hot on one
freq and not the other. Thus they are changing value as they heat.
73
Bob, K4TAX
----- Original Message -----
From: KnoxCol@aol.com
To: RMcGraw@Blomand.net
Sent: Wednesday, January 23, 2008 3:54 PM
Subject: Extra Capacitance Voltage Rating
Hi Bob!
Just read your comment about using a 2700pf capacitor in lieu of the 1000pf
capacitor that Ten Tec supplies in order to extend the tuning range of the
229/238 tuner series. Can you tell me what the voltage rating of this
capacitor should be?
Thanks and 73!
de KD4BRL Dick Knox
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