[AMRadio] Cabinet for the T-368 exciter?

Mon Apr 6 20:04:08 EDT 2009

Looks great!

Brett

----- Original Message ----- 
From: <sbjohnston at aol.com>
To: <amradio at mailman.qth.net>
Sent: Monday, April 06, 2009 7:20 PM
Subject: Re: [AMRadio] Cabinet for the T-368 exciter?

> 
> 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.
> ---------------------------------------------------------
> 
> 
> -----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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