[R-390] 390-390A PTOs

[Roger Ruszkowski]

wb3fau55  R390 VFO is a permeably tuned oscillator.There is only one model of R390 VFO and it is the better Collins design for linier operationsThe R390 VFO output cable has  a regular  BNC connector. The mounting frame of the R390 VFO is different in shape and an R390/A VFO will not bolt into an R390 receiver.The tube circuit passive components have different design and operating values than the R390/A VFO   The R390/A VFO  is a permeably tuned oscillator.There are two stock numbers that will get you one of the two models of R390/A VFO.One model is a Collins design that follows the R390 design.The second model is the Cosmos VFO. The Cosmos VFO's obviously work and are in use.The part was a reduced cost item injected in latter production contractsthat were open bid build contracts and not won by Collins. Other than the maintenance Fellow adjusting end point spread of the oscillator the VFO part difference is transparent. If you need to diddle the linier curve of a VFOthen you will prefer a Cosmos VFO to work on. The stack in the original R390 and R390/A Collins VFOs were assembled withslow drying sticky varnish. The assembly was jigged in a fixture.The shaft was spun.The little slide plates were ozzed into their needed places to lay the curve needed intothe tracking alignment stack. Now the varnish is dry and the stack is glued into shape.You just do not try and improve the tuning curve of a VFO if you do not have to.The factory build, jig setup, tighten the bolts, and let the varnish dry got the job done and it had been goodevery since. Parts do come loose and some times things need to be repaired. An impact can cause a crack in the stack and small misalignment of linier tracking to result.Some detectives have found this problem and repairs were conducted.Aggravation exceeded the return on the investment.As you own the critter you can do it yourself for the education and experience.While in service you did not seek this education and you exchanged your suspect VFO for another packageyou expected to perform better than the VFO in hand. The Cosmos VFO was introduced to meet this diddling need of those who can notlive with some one else's prior work. The just checking needed had to be meet. An alignment jig in every tool box was not the solution Collins proposed for this end play. The Cosmos VFO never made any of the maintenance manuals the alternate part number for the Cosmos VFO was published as a change page note to the parts manual. If not in stock then use this alternate number. Your first VFO was a Cosmos and you think that is the majority common part in the Field. To the casual procuring officer these things all look alike and please sign here. Respectfully,  Roger AI4NI