mjj@stda.jhuapl.edu (Marshall Jose) (12/07/90)
I hope this isn't old news. I recently picked up a DX-440-on-sale, as well as a service manual for same, and soon set about trying to figure out how to make it receive one sideband or the other. After getting past a delusional fixation I had about needing additional IF filters, I tried someone's advice: use the narrow (4 kHz) filter, and shift the BFO up or down 2 kHz as is needed to receive the proper sideband. It works! And rather well, indeed. Great, but to shift the BFO you will have to get inside the unit. The "BFO" control on the front of the DX-440 only shifts the 1st LO -- it doesn't affect the BFO frequency at all. The one to twiddle is VT103 on the A-1 assembly, which is a 100 pF trimmer capacitor in the BFO xtal oscillator circuit. Of course, doing this means that you're stuck with one or the other sideband mode after you reassemble the radio case. I'll be puttering around in the next week or two with ways to make the mode switchable, preferably with a minumum of holes cut in the case. I'll most likely use diodes to switch in another trimmer or two. In any event, if you've been putting up with QRM while listening to the shuttle, the above adjustment might be worth it to you. Later, Marshall Jose WA3VPZ mjj%stda@aplcen.apl.jhu.edu || ...mimsy!aplcen!aplvax!mjj
ken@swbatl.sbc.com (Ken Gianino 5-9081) (12/07/90)
In article <1990Dec7.144911.174@aplcen.apl.jhu.edu> @aplvax.jhuapl.edu:mjj@stda.jhuapl.edu (Marshall Jose) writes: >I hope this isn't old news. I recently picked up a DX-440-on-sale, as well >as a service manual for same, and soon set about trying to figure out >how to make it receive one sideband or the other. > >After getting past a delusional fixation I had about needing additional >IF filters, I tried someone's advice: use the narrow (4 kHz) filter, >and shift the BFO up or down 2 kHz as is needed to receive the proper >sideband. It works! And rather well, indeed. > >Great, but to shift the BFO you will have to get inside the unit. The >"BFO" control on the front of the DX-440 only shifts the 1st LO -- it >doesn't affect the BFO frequency at all. > >The one to twiddle is VT103 on the A-1 assembly, which is a 100 pF trimmer >capacitor in the BFO xtal oscillator circuit. Of course, doing this means >that you're stuck with one or the other sideband mode after you reassemble >the radio case. > >I'll be puttering around in the next week or two with ways to make the >mode switchable, preferably with a minumum of holes cut in the case. >I'll most likely use diodes to switch in another trimmer or two. > >In any event, if you've been putting up with QRM while listening to >the shuttle, the above adjustment might be worth it to you. > >Later, >Marshall Jose WA3VPZ >mjj%stda@aplcen.apl.jhu.edu || ...mimsy!aplcen!aplvax!mjj I love to modify radios but I thought I might add my 2 cents worth here. Everything you say here is correct but this mod will not buy you anything. With a 4KHz filter your radio will still receive 4KHz worth of spectrum, only about half of which will ever be the intended SSB signal. Shifting the BFO to the low side of the IF <will> enable USB only reception, but it will be just as noisy. As you suspected, you need a more narrow filter (about 2.4 KHz for SSB). I think I would leave the BFO centered and still be able to copy both sidebands. If you want to know which sideband you are listening to, remember this: If you shift the frequency <up>, a USB signal will go <down> in pitch and a LSB signal will go <up> in pitch. Ken - WB0QNA