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/amradio@mailman.qth.net/ List Rules (must read!): http://w5ami.net/amradiofaq.html List Home: http://mailman.qth.net/mailman/listinfo/amradio Post: AMRadio@mailman.qth.net To unsubscribe, send an email to amradio-requ...@mailman.qth.net 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
