TenTec 238B

This article is a step by step “how-to” repair a Ten Tec 238B Antenna Tuner. This tuner came to me as a junk parts unit. I knew what it was really worth in working condition, and had always been curious about how well it functioned. The original owner priced out the replacement one piece board for the B version; it was nearly the price of a new unit. I priced out the switch wafer only, not the board, and with a lot of sweat equity, it might be repairable. I also replaced ALL the fixed capacitors at the same time, and the parts cost was about $85 total. Ten Tec support is legendary for helping out owners who self maintain their Ten Tec gear. They were great about tracking down the stuff I needed. Here is the ugly view when I first opened the case. I separated the burned up capacitor selector board from the good SWR sensor board and the good antenna switch and balun board with tin snips. This shows where to do the cuts. It also shows the old switch and the replacement switch as received from Ten Tec.

This is the separated and salvaged SWR sense board and the antenna selector and balun board.

Clean up the burned up board for capacitor and match selector switch and use it as a gage to locate the new holes on the rear panel required to directly mount the new replacement switch directly to the panel. The large center hole is used to insert the switch shaft at the very end. You can put a plastic goof plug in the large hole to keep foreign material out.

Add a ground buss bar to the rear panel inside to wire the capacitors later. Drill and mount it now to avoid damage to the new switch.

Use new insulated washers and spacers to mount the new switch as shown. Select the length to allow the original shaft to properly engage the switch wafer.

Assemble the salvaged coil to the new switch. Enlarge the holes in the switch connections to allow the wire from the coil to pass through. Do not just tack solder the coil to the connections on the outside. Also add wires to the back of the switch to complete the circuits from the old burned up pc board. I used red and blue teflon wire. Refer to the schematic in the manual to be sure you get it right. These pictures will be helpful. When you are done wiring the switch, mount it to the back panel with the new hardware.

Wire up the lower and upper capacitors. I replaced all of them with NEW Ten Tec original parts. I strongly recommend that you do not attempt to re-use the ones from the burned out switch. The leads are too short anyway. On the web, there is a posting of someone who used USSR surplus doorknob capacitors. These would definitely be an upgrade, since the Ten Tec capacitors work really hard, especially on 160 Meters.

Install and wire the salvaged balun and antenna switch board.

Install the SWR sense board.

This the completed rear panel ready to assemble to the chassis. 

Wire the SWR sense board to the chassis and the variable capacitor. Be sure also to plug in the connections for the front panel meter and so on.

Wire the variable inductor, connect to the balun and antenna switch board, and the necessary standoff insulators for the balanced line on the rear panel.

Use the new rear panel hole to insert the salvaged shaft thru the back panel. Orient the burned end toward the front panel so that good non conductive material is next to the switch wafer. Be sure the switch wafer function for straight through operation aligns properly with the knob and detent label before tightening the shaft coupling. Use a black plastic goof plug in the rear panel hole for the shaft. This makes it look almost factory original. In fact, it now resembles the original 238 before the B version. The one piece board cut assembly costs. Unfortunately, it made repair harder and more expensive.

I played with this tuner a bit and found its matching range on 160 Meters was not as wide as my Heathkit tuner. Also, I was not impressed with its power handling capacity. If I ever need a balun, I would use an external current balun, not the types employed in either the Heathkit or the Ten Tec. But I satisified my curiosity and performed a resurrection on a nice tuner which somebody got from me at a hamfest.

If someone has a melt down with a 238, I hope this article helps them get it back to operation.

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I recently saw in an older QST a photo of the internals of the 238 C latest version. It seems that Ten Tec realized the expensive repairs on the B version were a turn off. The newer C version fixes all this, except for the wimpy ceramic fixed capacitors. If the center board and switch blows, at least you do not have to go through what I did rebuilding this B version. As a plus, it now includes a nice cross needle meter, replacing the obsolete single meter unit. For half legal limit operation the 238C should do nicely for 80 through 10 meters. The disc capacitors will continue to be a problem, unless you upgrade them. On 160, the tuning will drift with the stock capacitors if you run any power or run AM.


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