[TenTec] TT 238C will not tune

[Rick - DJ0IP / NJ0IP]

The Model 238 is NOT really an "L-Network".

Perhaps we should call it a modified "L-Network".
The tuner is reconfigured with switches such that it has 4 positions:

..<> Low Impedance.........(using a modified L-network* [using a 2nd coil])
..<> Very Low Impedance....(using a standard L-network)
..<> High Impedance........(using a Pi-network)
..<> Very High Impedance...(using a standard L-network)

It is a complex switched matchbox, far superior to the circuit used by the
company operating south of Servierville.
Whoever designed the 238 (and predecessor similar tuners) really had his act
together (except for the balun, which he blew)!

*This position is actually a T-Network, but it is not the classical
Transmatch circuit from W1ICP.  It has 2 L's and 1 C, whereas the classical
T uses 2 C's and 1 L.

There is no rule stating that 0 has to be least or most of anything.
You will find matchboxes where 0 means minimum inductance and others where
it means maximum inductance.
You will find matchboxes where 0 means minimum capacitance and others where
it means most capacitance.

There is no low side or high side of an inductor.
As it rolls, it shorts the turns behind it.  You could swap the inductor 180
degrees and it won't make a difference, as long as we have a floating
inductor, as in the Model 238.  

In the T-Network with one side of the inductor to ground, then the side
being shorted should be the ground side.  Not applicable here.

IMO, "0" should always indicate minimum, but that's just my opinion (which
nobody ever listens to, anyway).

73
Rick, DJ0IP


-----Original Message-----
From: TenTec [mailto:tentec-bounces at contesting.com] On Behalf Of Richards
Sent: Sunday, July 28, 2013 11:29 PM
To: wb5jnc at centurytel.net; Discussion of Ten-Tec Equipment
Subject: Re: [TenTec] TT 238C will not tune

Hey... I had one of those once ... during my SWL days.
I thought it might improve my long (i.e., random) wire receiver antenna, but
I could never see any improvement no matter how I set it.

OK, so if that is a simplified version, I would take it Zero (0) on the 
inductor scale means zero (0) inductance.   Although it could be maximum 
inductance if it was installed the other way around.

Comparing the schematic for each, it would appear the low side of the
inductor faces the radio input, and the high side faces the output - but
that does not indicate, for sure, which way the inductor is physically
oriented in the cabinet, and vis-a-vis the front dial.

Or does it?

And the MFJ 16010 description makes it sound like the whole circuit gets
reversed - which would reverse the direction of the inductor relative to the
current flow - rather than just alter the location of the fixed capacitors
relative to the same circuit/inductor direction.
On point, but I am still corn-fused.

Someone on the list talked me into getting an L/C meter but I have yet to
learn how to use it !  So much to learn, so little time.

---------------------- K8JHR  ----------------------

On 7/28/2013 5:15 PM, Al Gulseth wrote:
> (Hope I'm understanding your question right, HI!!)
>
> There is a "budget priced" lower power "clone" (to use the term very 
> loosely!) of the 238 that might help you understand the concept of how 
> the L network functions. It is made by another manufacturer located a 
> little ways southwest of Sevierville in another "S" town: AKA the MFJ
16010T. Here's the manual:
>
> http://www.mfjenterprises.com/pdffiles/MFJ-16010.pdf
>
> The "executive summary" is that an L network is used to transform some 
> low Z to some higher Z, or vice-versa if the network is reversed. To 
> quote from the
> 16010 manual:
>
> "The connectors are labeled properly to match a transmitter to a 
> higher impedance. This is the normal connection.
>
> To match impedances that are lower than your transmitter impedance 
> (such as a mobile whip), simply interchange the normal transmitter and 
> antenna connections to the MFJ-16010."
>
> Simply put, when the cap is on the output side the transformation is 
> L>H Z, and when the cap is on the input it's H>L Z. The 238 series 
> accomplish this via the BYPASS switch, while the MFJ uses the less 
> expensive and low-tech method of reversing the connections to the unit.
>
> The direction of current flow is immaterial in this case. And yes, 
> determining the actual zero inductance setting position would probably 
> be beneficial in using the unit....
>
> 73, Al
>
> On Sun July 28 2013 2:45:22 pm Richards wrote:
>> Well, this is where I get corn-fused.
>>
>> The user manual states it is a REVERSIBLE L-NETWORK, whereby the 
>> fixed capacitors are either placed at the front of the circuit or at the
end
>> of the circuit using the BYPASS switch.   I presume the current always
>> flows in the same direction through the circuit, including the inductor.
>>    (Heck, I am still learning to read schematic diagrams.)
>>
>>   From some of the previous comments, it sounds as if there is some 
>> uncertainly as to the ZERO (0) inductor setting means the most or the 
>> least inductance.
>>
>> Is this not revealed in the schematic?  Or do we have to open the lid 
>> and look to determine the matter?  And, would it not be the same from
>> unit to unit?   Some comments seem to suggest the Zero (0) setting could
>> be different on some units.
>>
>> Sorry if I missed a point along the way.  It is still a mystery to me.
>>
>> ------------------- K8JHR  -----------------------
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