Looks great! Brett
----- Original Message ----- From: <[email protected]> To: <[email protected]> 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 > > [email protected] > http://www.wd8das.net/ > --------------------------------------------------------- > Radio is your best entertainment value. > --------------------------------------------------------- > > > -----Original Message----- > From: D. Chester <[email protected]> > To: [email protected] > 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 > > > ______________________________________________________________ > Our Main Website: http://www.amfone.net > AMRadio mailing list > Searchable Archives: > http://www.mail-archive.com/[email protected]/ > List Rules (must read!): http://w5ami.net/amradiofaq.html > List Home: http://mailman.qth.net/mailman/listinfo/amradio > Post: [email protected] > To unsubscribe, send an email to [email protected] with > the word unsubscribe in the message body. > > This list hosted by: http://www.qsl.net > Please help support this email list: http://www.qsl.net/donate.html > > > ______________________________________________________________ > Our Main Website: http://www.amfone.net > AMRadio mailing list > Searchable Archives: http://www.mail-archive.com/[email protected]/ > List Rules (must read!): http://w5ami.net/amradiofaq.html > List Home: http://mailman.qth.net/mailman/listinfo/amradio > Post: [email protected] > To unsubscribe, send an email to [email protected] with > the word unsubscribe in the message body. > > This list hosted by: http://www.qsl.net > Please help support this email list: http://www.qsl.net/donate.html ______________________________________________________________ Our Main Website: http://www.amfone.net AMRadio mailing list Searchable Archives: http://www.mail-archive.com/[email protected]/ List Rules (must read!): http://w5ami.net/amradiofaq.html List Home: http://mailman.qth.net/mailman/listinfo/amradio Post: [email protected] To unsubscribe, send an email to [email protected] with the word unsubscribe in the message body. 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