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For the benefit of those of
you who also own the Tokyo
Hy-Power (THP) HL-2.5KFX
auto-tune HF amplifier and
HC-1.5KAT ATU, here are a few
notes about using them.
Warning - lethal high voltages inside
With a potentially lethal 240
volt mains supply, the
amplifier finals run at over
100 volts, under some
conditions pumping several
kilovolts of RF through the
ATU to the antenna. RF burns
are nasty enough even at 100
watts so I imagine at the
kilowatt - or killer-watt -
level they are more than
merely painful. RF penetrates
the outer layers of the skin
and cooks the underlying soft
tissues. If you
don’t know what
you’re doing, leave the
lids on and don’t
meddle. Even if you DO know
what you’re doing, and
are prepared to take the
risk, pay great attention to
safety as you probably
won’t get a second
chance.
THP HL-2.5KFX auto-tune solid-state HF amplifier
 The HL-2.5KFX amp is rated
at 2½ kW input, producing
up to 1½ kW PEP output on
all except 10m where it
reaches about 1 kW flat-out.
Our license limit in ZL is 1 kW
so it’s ideal for us.
The design was described in
some detail by its proud
creator, Toshiaki Ohsawa
(JE1BLI), and the THP
president Nobuki
Wakabayashi (JA1DJW), in
an excellent technical article
in QEX. It uses a push-pull pair of ARF1500 MOSFETs in the output: these are about the same price
as QRO HF valves.
The owner’s manual is online here and there are a few notes about QSK timing etc. here.
THP HC-1.5KAT auto-ATU
The HC-1.5KAT is an auto-tuning HF Antenna Tuning Unit rated for 1.5 kW at up to 2:1 SWR
covering 160m to 10m, with a direct pass-through facility for 6m. 
The ATU circuit is a straightforward T-match using a
pair of air variable capacitors (with additional fixed
capacitors switched in parallel for the low bands) and a
string of air core and toroidal core coils selected by
relays. The T-match is inline for both transmit and
receive, so long as the ATU is turned on. When it is
switched off, the transmitter is directly connected to
antenna 1 by a relay.
The clever part is the control circuitry that measures
the SWR on the antenna feedline and rapidly adjusts
the T match to present a low-SWR load to the
transmitter. It monitors the SWR continuously so long
as sufficient RF is present, retunes itself if necessary
and drops the PTT displaying an error message if it
can’t find a match.
To match more rapidly, the ATU pre-tunes itself as each band is selected (either manually from the
front panel or through the data cable from a connected THP amplifier). It evidently uses factory
default settings, presumably pre-defined in the firmware for its 16-bit microprocessor. When it
senses sufficient input RF to get a SWR reading (about 10-20 watts), it rapidly fine-tunes the
capacitors using stepper motors and if necessary selects more/less inductance with the relays,
achieving a good match within a second, usually much less if the antenna naturally presents a good
match. Only then does it complete the PTT connection to key the amplifier.
For some reason, the ATU doesn’t store and recall the actual settings used previously on a band
instead of those factory defaults. If it did that, I wouldn’t need to switch the amp to standby, tune
off a spotted DX station’s frequency to find a clear channel, send some RF to get the ATU to retune
itself, then put the amp back online - all to avoid QRMing others in the pileup. The ATU would
already be tuned and could quickly sense the SWR and short the PTT with hardly any delay. I know
I can just start transmitting normally but for the second or so until the ATU retunes itself, the amp
is offline, truncating the start of my transmission. [Please, THP designers, rewrite the firmware to
store and recall previous band settings, returning to the factory defaults only if necessary].
The ATU has 3 antenna connectors, any one being automatically pre-selected for each band. It is
possible to change to another antenna, or to tune the ATU manually, using the front panel function
button and a turn-and-press selector knob. A 2-line LCD display gives enough of a hint to know
what you are configuring although the cunning interface takes a bit of getting used to.
Noise-reducing fan mod
As supplied, the ATU is surprisingly noisy due to a little 4 cm computer-type fan running flat out
inside. This draws air in from a rear panel side grill, past the control boards and stepper motors,
then out past the T-match coils to a rear panel top grill. As installed, the fan/chassis combination
seems to resonate creating a pronounced and annoying whine around 1.5 or 2kHz, audible above
the whooshing noise of the mass air flow.
A US-based THP dealer has advised owners who complain about the noise to disconnect the fan.
Although I have near-resonant antennas and seldom run QRO, I was concerned about the
possibility of overheating the circuit boards or stepper motors in contests or when using RTTY.
Somehow I doubt the designers would have put the fan in there unless it's needed under some
foreseeable circumstances. So, I would rather just slow it down than stop it altogether.
The fan in mine is an EUDC12B8 from Japan Servo Co, rated to supply up to 6 CFM of air flow with
no back-pressure at 13.8V and 75mA or about a watt - pretty standard for a little fan. It is
supposedly fairly quiet at just 30 dBA but evidently not when installed in this ATU.
 After cutting the fan’s red supply wire to insert a small variable resistor, I found that about 100
ohms in series with the fan substantially reduces the noise, but also of course reduces the fan
speed and cooling effect. I initially tried a parallel pair of 150 ohm ¼ watt resistors which improved
things, but then swapped them for a parallel pair of 220 ohm ¼ watt resistors instead, giving 110
ohms in series with the fan. This alone substantially reduces both the air noise and the whine.
I used a small length of heat-shrink tubing to insulate the resistors. Just remember to slip it over
the wire before soldering the resistors in place!
 The resistors drop about 5 volts when the fan is running normally, dissipating about ¼ watt. A
single ½ watt resistor of 100 or 120 ohms would probably do nicely, whereas a single ¼ watt
resistor might burn out in this application unless it was very carefully positioned in the air flow, but
why take the risk? Don’t use too large a resistance value, however, as the fan may not start
reliably.
 I also put little triangles of thin plastic padding under
each corner of the fan, between the fan and the
chassis panel, to help deaden the vibrations at
resonance. I cut them from a bag used to protect
something in the mail, and punctured holes with a
probe so that they would fit over the fan’s mounting
screws.
By the way, although the upper fan screws are easily
accessible, the lower ones took some ingenuity to
loosen and tighten with a 6 mm hex screwdriver bit
held in a pair of pliers. Much easier than removing the
front panel or dismantling the entire chassis to get at
them with a normal screwdriver!
 To test the fan mod with the ATU case off, you’ll have
to defeat the safety interlock temporarily. It’s called a
safety interlock
for good reason: check that all your
tools and wire clippings are well clear of the innards,
double-check that you cannot transmit any RF through
the ATU (turn off your transceivers), reconnect the 12
volt power cable, gently depress the safety interlock
microswitch with something suitable such as the handle
of a screwdriver (see left) and switch the ATU on at
the front panel. The fan should start and run
immediately. Provided you haven’t yet shrunk the heat
-shrink tubing into place, it’s easy to tell the difference
you’ve made by shorting out the resistors with a croc
-clip lead or piece of wire.
The fan is much quieter now but I'm tempted to fit a
switch to short out the resistors and run the fan at full
speed when I'm contesting, just so long as I’m wearing
noise-cancelling headphones anyway! There's plenty
of space on the front panel for a switch but it would be
nice to find a source for a rectangular plastic switch to
match those in the ATU and amp ... or maybe I’ll fit a
miniature switch out of sight on the bottom of the
case ... or maybe I’ll buy another fan to see if that
helps. Who knows, maybe the original fan will become
whisper-quiet once it has been fully run-in.
Now I need to rearrange my desk a bit so the ATU doesn't sit right on top of the warm amp, then
tackle the noisy fan in the THP amp (why does it idle so fast I wonder?)!
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