[AMRadio] Cabinet for the T-368 exciter?


sbjohnston at aol.com sbjohnston at aol.com
Mon Apr 6 19:20:04 EDT 2009


At a local thrift store I found a cabinet for my T-368 exciter and 
power supply.  It was a two-drawer CD/DVD cabinet made of plastic and 
masonite, with a lovely black toned wood grain pattern on the outside.  
After a couple modifications to make it a little stronger it has turned 
out pretty decent:

http://www.wd8das.net/T368ex.jpg


Steve WD8DAS

sbjohnston at aol.com
http://www.wd8das.net/
---------------------------------------------------------
Radio is your best entertainment value.
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-----Original Message-----
From: D. Chester <k4kyv at charter.net>
To: amradio at mailman.qth.net
Sent: Wed, 25 Mar 2009 3:35 pm
Subject: Re: [AMRadio] Cabinet for the T-368 exciter?

Probably the best bet is to make one.  I modified one and use it as my
station VFO, capable of driving any of my rigs.

I still found the drift objectionable when the PTO is turned off during
stand-by, particularly when working 40m CW, so I modified mine to let 
the
oscillator run all the time even when the rest of the unit is turned 
off.
The problem with doing that is that the oscillator is audible in the
receiver, which can be a problem with weak signals when  the band is 
quiet.
I built a metal shielded enclosure to encase the  whole thing, PTO,
buffer/multiplier unit and all.  I used about 1/16" sheet aluminium and 
some
1/4" square rods of aluminium stock salvaged from something long 
forgotten.
I cut side and top panels, using the original bottom cover, rear panel 
and
front sub-panel behind the mechanical dial assembly.  I carefully 
drilled
and tapped holes for 4-40 screws, using the square stock as a skeleton 
to
hold the sheet metal panels together at the corners.  The front and 
rear
sub-panels in the unit are thick enough that I was able to drill and 
tap
directly into the edges, without the square stock.  I used standard 
circuits
for TVI filtering and bypassing for all power supply, filament and 
control
leads going in and out of the unit, except that the inductances and
capacitances were optimised for the 1.5-20 mHz range instead of for 
VHF.  I
was able to reduce the rf feed-through to the  receiver to negligible 
with
the receiving antenna connected, although it is faintly audible when 
the
antenna is removed from the receiver.

I  replaced the type 6000 tube with a 6AG7.  I had to re-wire the octal 
tube
socket, but the capacitances of the two tubes are nearly the same.  I 
put
only about 150 volts on the 6AG7, and that gives me about 200-250 
milliwatts
of power out, which is comparable to the output from a ham type VFO 
like the
Johnson 122 or Heathkit VF-1.

The next problem was coupling the output to the transmitters.  In the
original T-368 setup, a short length of coax is used to directly couple 
the
plate of  the output  to the grid of the final amplifier, and its
capacitance was not enough to cause a problem with resonance at the 
output
coil. Some of my rigs are as far as 10' away from the operating table, 
and
that much coax loads down the output circuit with too much capacitance, 
and
the alignment parameters changed with different lengths of coax to the
different  rigs.  So I decided to make mine  link  coupled.  I salvaged 
a
spare bandswitch wafer from a T-195 multiplier/buffer unit I had on 
hand,
which is very similar to the one in the T-368.  I disassemble the
bandswitch, cut two of the hollow tubes used as spacers between wafers 
to
accomodate the additional wafer, and re-assembled the bandswitch.  I 
then
took out the output coils and removed the shields, then wound a 
coupling
coil over the cold end of each coil and brought the new  lead out the 
bottom
of the  coil.  I don't recall if there was already an extra banana plug 
at
the coil bases, or if I added one or simply brought the wire out 
through a
hole (I made these mods about 20 years ago).  The new wafer section was 
used
to select the coupling coil.  As I recall, I used about 2 turns for the
highest frequency coil, 4 for the next, 8 for the 3-6 mHz coil and 16 
for
the 1.6-3 mHz coil.  I removed the original BNC connector and relay at 
the
top of the buffer assembly, and relocated the BNC to the rear of the 
unit as
the output jack.  With the modified output, I can get about 1/4 watt of 
rf
into a 50-ohm dummy load.

Before the mod, the unit would track perfectly over each one of the 
tuning
ranges.  I was amazed that I could put an RF voltmeter at the output 
and it
would show a constant reading no matter what band or frequency I tuned 
it
to.  Adding the coupling links somewhat screwed up the linearity, and I
could never get the ranges to track as well as they did before the
modification.  I possibly could have experimented with  the exact 
number of
turns of coupling coil on each output coil, and been able to get the 
unit to
track perfectly over each frequency range, but I didn't go to the 
trouble,
since I use each frequency output range of the unit for only one ham 
band:
1.5-3 for 160, 3-6 for 75/80, 6-12 for 40m, and 12-20 for 20m.    I 
found
that I could align the output coils well enough that the unit would 
still
track perfectly across the entire ham band in each frequency range, but 
the
output would fall off at the extreme ends.  Since I don't do pirate
broadcasting on shortwave and have no use for frequencies outside  the 
ham
bands, the tracking error made zero difference to me, so I didn't 
bother. I
haven't tried using it on 30m or 17m, but hopefully I would still be 
able to
get enough output on those bands to drive whatever transmitter I might 
use.

The final modification was to add a Jackson Brothers planetary dial 
drive to
the tuning knob, because the stock tuning rate was too fast on the 
higher
frequencies. The reduction drive magnifies the small amount of backlash 
in
the tuning mechanism, but it still allows for more precise tuning to
zero-beat a signal, particularly on 40m, than with the tuning knob 
running
straight through.

Using a drill press, I drilled a large number of holes in the top panel 
of
the sheild I added, in a pattern that places ventilation holes above 
each
tube, to reduce temperature build-up inside the unit and possible 
warm-up
drift. I used a small drill, something around a size #48, to make the
ventilation holes.

Don k4kyv


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