[TenTec] Extra Capacitance Voltage Rating

[Bob McGraw - K4TAX]

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 at aol.com 
  To: RMcGraw at 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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