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
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.
- - - - - - - - - - Post Script - - - - - - - - - -
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.