Josh wrote: > Ok so here's what I've got (I think) > > http://www.n2ckh.com/FORSALE/REPEATERS/DUPLEXERS/DB4076/DSC02678.JPG > > Hamvention special, 4 cavities, appears to be a DB Products 4076 family unit. > My bench tools: HP 8924c w/ Spec Analyzer and Tracking Generator. > > My problem - existing set of cans is tuned for 460~mhz. I need to bring them > down to 443/448. It was my understanding that they would have dual > adjustments - one for setting the band pass frequency, one for setting the > reject frequency. Sounds simple. Except under the 'covers' there is > nothing else to adjust... So are they just single frequency pass or notch > filters? > > So thats the first question - The second question is - ok , so if they're > just single frequency filters, why can't I spin them 'into the right range' > as easily as I thought I'd be able to do.... I've tuned a couple of motorola > micor filters on the spec analyzer in the last couple of days, mostly made > sense and was smooth as silk... However, after messing with the first can, > I'm confused. Moving the adjustment certainly changes the properties of the > notching - but it didnt really move the bandpass around... It mostly changed > the shape and depth of the notching - not the frequency. > > What knowledge am I missing - I'm an amateur :) > > Yes I've seen the 'how to tune db products duplexor' doc - but it talks about > dual controls, which apparently I don't have. > > Did I buy a piece of junkola? Teach me obie-wan. > > j > >
Josh, The large nut tipped rods in the center of each cavity are the pass adjustments. Loosen the locking nut at the bottom of each tuning rod and turn the shafts clockwise to set the pass responses where you want. Be sure to keep them set high and low as they are labeled. Once the pass is where you want it, tighten the clamp nuts back down. Then remove the small round covers between the N connectors on each cavity and use a small screwdriver or metal tipped tuning tool to _carefully_ put the notches where they belong. Put the covers back on and enjoy. Be sure to put a termination on the side you are not tuning. Ed Yoho W6YJ
