[TenTec] 229B tuner: Slight upward creep of SWR

[MadScientist]

That’s a common problem when ceramic capacitors become a major part of the tuned circuit because ceramic capacitors (especially doorknobs) have very high TCC (temperature coefficient of capacitance). Ten Tec Titans suffered from this as well (likely other amps too). 

It’s usually only a problem in digital modes or FM, when carrier is key down for extended times.


I have a Titan II, in which the ceramics on 160 and 80 were mostly removed to just the point at which one could get correct loading using the variable cap nearly fully meshed. That way, the air capacitors were the lion’s share of the load capacitance.

One could replace the ceramics with vacuum capacitors, which have near-zero TCC. You’d need to find room (and money) for this upgrade. Also, one can find low TCC ceramic disk capacitors, as mentioned by K4TAX had suggested.

Gary

> On Jul 21, 2018, at 8:25 AM, Bob McGraw K4TAX <rmcgraw at blomand.net> wrote:
> 
> Your description is typical of some component or components heating and changing value.  This is not uncommon with this tuner,  but usually with higher power.   Three areas of concern are; (a)  the 4 parallel pairs of 220pf capacitors which are switched by the HI LO Z switch.  (b) the internal balun, and (c) the series combination of C7 & C8.
> 
> My solution to all of this is replace the capacitors and balun.
> 
> (a)  use 470pf  doorknob capacitors, 4 of them, to replace the 4 pairs of 220pf disk ceramic capacitors.   Tight fit but can be put in on the base plate at the rear and keep the leads short. Requires drilling 4 holes in the bottom plate to mount the capacitors direct to the bottom plate.
> 
> (b)  replace the series pair C7 & C8 with a 25pf doorknob capacitor mounted on the rear panel.  Requires drilling 1 hole. And a short jumper to the HI LO Z switch.
> 
> (c)  forget about using the internal balun.  I use a Balun Designs 1:1 balun model 1171T.    An 18" coax jumper connects the balun to one of the SO-239 outputs.  Balun sits on top of the tuner.  The balanced feedline runs from the feed point of the antenna all the way to the  balun.  No long coax jumper is used.
> 
> Further your description of the extra required capacitors and heating are clear indication you are using the wrong balun.  A 4:1 balun will transform the antenna impedance to a Z value divided by 4.   Thus the tuner has to match a low impedance which is the values where loss is greatest.   Thus if your antenna feed Z is 35 ohms, the 4:1 balun will transform that Z to 8.75 ohms, which is what the tuner must match.   Hi Z load impedance has less loss internally.     Many hams believe, incorrectly so, that a 4:1 balun should be used with a balanced feedline, i.e. 450 ohms. This is absolutely incorrect.   You are not matching the feedline but the antenna Z.
> 
> Problem solved, all bands, and up to legal limit high duty cycle power.
> 
> 73
> 
> Bob, K4TAX
> 
> 
> On 7/21/2018 2:26 AM, Peter Klein wrote:
>> I've just noticed something interesting about my Ten Tec 229B tuner. It happens on 80m-30m, where the center switch is in positions Lo2 - Lo5 and various fixed capacitors are switched into the circuit.
>> 
>> I use non-resonant antennas fed with ladder line.  I normally tune the tuner with a noise bridge or at 10 watts.  Then, when I operate at 100 watts, I notice a slight upward creep of SWR over a few seconds after key-down.  Then the SWR backs down a little and remains stable while I'm transmitting. The creep is very slight, from 1:1 up to maybe 1.2 or 1.3:1 on my IC-7300's SWR meter.  My guess is that this is the same issue some people report with the Ten Tec tuners on 160m.  On low impedance loads that require the extra fixed capacitors to switch in, those capacitors warm up slightly and change value as we transmit.
>> 
>> I never noticed this before.  But I just picked up a used Daiwa CN-101L cross-needle SWR meter, because the tuner's switched meter and the IC-7300's multiple displays make monitoring SWR inconvenient.  The Daiwa's "reflected" readings are a bit overly sensitive. On that meter, I could clearly see the creep happening. Sure enough, when I looked on my IC-7300's SWR meter, it was happening there, too. The Ten Tec tuner's meter barely shows it, if at all.
>> 
>> My guess is that everything is fine.  The SWR creep is too small to endanger my finals, and I can always touch up the tuning.  I just want to be sure that the SWR creep doesn't indicate that the fixed capacitors in the tuner are failing. Can someone give me a reality check?
>> 
>> Thanks and 73,
>> --Peter, KD7MW
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