Modifying the Ten-Tec 238 B Antenna Tuner...


Photo courtesy of Ten Tec, (www.tentec.com)

I recently acquired an additional tuner for the station, a Ten-Tec 238 B manual antenna tuner. The tuner had been damaged by the prior owner by overheating one of the fixed tuning capacitors, toasting it to a nice charcoal color, therefore a repair and upgrade was in order. First let me say that the failure was due to operator error, and should no way be construed as a reflection of the quality of the tuner. The tuner operates silky smooth and the quality is good, much better than tuners I have owned from MFJ. The lesson here is to remember that when using an antenna tuner to match the load to your transmitter, you should only use low power until the tuner has matched the load.

About the 238 B:

The Ten-Tec 238 B utilizes a modified "L" circuit with a 0-18 microhenry variable inductor, 40-500 microfarad variable capacitor, and as necessary, a switched capacitance network of five different fixed capacitors which allows increasing levels of capacitance to be switched into the circuit and configured as either a Low-Z or High-Z. In February 2003, an article was printed in QST on the subject of testing five high power antenna tuners, including the Ten-Tec 238. The author had this to say..."The power losses on 10 meters were considerably greater than those on other bands. However, the 238 performed very admirable on most other bands, quite often exhibiting the least losses on any given band and load combinations." The 238 B has the ability to match a wide range of impedances and should handle most any antenna system matching chore required at AB4D.

The fixed capacitors as factory supplied in the 238 B are rated for 2 KV and normally they are sufficient for most tuning needs. However, after researching the Ten Tec 229/238 series of tuners on the Internet, it seems that there are several conditions which can cause failure of the fixed capacitors, such as attempting to match an antenna that is too short for the band of operation and using an amplifier, especially on the 160 meter band; using too much power when trying to match the load to the transmitter; or running the tuner into a dead short with high power. These same types of situations can damage most other tuners by either destroying components or causing arcing of the variable capacitor(s).

I recently read an interesting article on a web site about an apparent issue with the Ten Tec 229/238 tuners. It seems that in certain situations, the tuner is unable to maintain constant resonance on the 160 meter band when running high power, due to heating of the fixed capacitors. This problem is explained in greater detail by Ron, N6IE on his web site at: http://n6ie.com/Ten_Tec_238.html

N6IE resolved the issue in his Ten Tec 229 by changing four of the five factory capacitor networks to surplus Russian door knob capacitors. I decided to follow Ron's method of correction and performed a similar modification to my 238 B. However, the method utilized to perform this modification is significantly different between the 229 tuner and the 238 B. The 229's capacitance switch is mounted in free space directly on the rear panel of the tuner. When adding the door knob capacitors to the 229, you can simply mount the ground side of the capacitors directly to the bottom case of the tuner, and run wires from the capacitors input to the individual taps on the Low Z/High Z switch. However, in the 238 B the capacitors are mounted on a PC board, and all switching is accomplished on the PCB. Further, there also seems to be less clearance space in the rear of the 238 B when compared to the 229. So in the 238 B the door knob capacitors must be mounted right onto the PC board.

To accomplish this feat on my tuner, I made copper strips from copper sheet to facilitate mounting the capacitors to the tuner's PC board, and then used a metal hole punch to add appropriate size holes in each strip. To mount the copper strips, I bent them into the required "L" shape, crimped the edge of the copper strip that goes to the PCB around pieces of copper wire, put the wire though the original capacitor holes in the PCB, and then I soldered the strips and wire in place one by one, mounting each individual capacitor as I went along to check for the appropriate clearance between each capacitor and the mounting position as necessary.

Capacitors:

My Ten Tec 238 B was supplied with five separate capacitor networks consisting of four identical networks of three 150 pf 2 KV ceramic disc capacitors in parallel (total capacitance of 450 pf @ 2KV), and one network of a pair of 120 pf 2 KV ceramic disc capacitors in parallel (total capacitance of 240 pf @ 2KV). It was one of the 120pf capacitors that was damaged in the tuner by the previous owner. As a replacement for the 450 pf capacitor networks, I was able to obtain four 470 pf 16KV door knob capacitors from a Russian supplier. For the 240 pf network, I was unable to locate a suitable door knob capacitor, but was able to obtain a 240pf 6KV ceramic disc capacitor from a domestic electronics parts house.

The modified 238 B tuner appears to work without any issues, it will tune my 75 meter dipole which is fed with 450 ohm line for all the bands from 80 through 10. I don't believe it is possible to damage the capacitors with proper use, even mistuning at high power for a short period of time should not affect the life of the capacitors.

During the upgrade, I noticed that the rubber rings on the tuning knobs were a little distorted, so I also obtained new tuning rings from Ten-Tec. The new rubber rings are better quality than the original old style two piece/two color rubber rings that were once supplied by Ten-Tec. The new replacement tuning knob rings are the same silicone rubber rings that Ten-Tec uses on its current production of HF radio gear. They are very attractive and soft to the touch.

I took some photos of the modified tuner with the new tuning rings and the upgraded capacitors, Enjoy...

73

AB4D

Unmodified Tuner Board with charred component...


Tuner board modified with new capacitors...





Another View, notice I had to daisy-chain the ground side of the last capacitor due to the location of the original solder pads. This does not affect the operation of the tuner...





Tuner ready for service...




238 B in the shack...





238 B in Orange (9/2012).