Hi The gotcah is that the internal adjust coil may not have enough range to get the PTO back to where it needs to be. You may need to get a bit creative. Better to do this *before* any of the other work.
This is not a new issue. Back in the day, the military spent a lot of time and money swapping out PTO’s. It was cheaper / easier to do that than doing a full rebuild. There are stories of “small mountains” of PTO’s building up behind repair depots as a result. Like any story, the size of those mountains likely got bigger and bigger with each telling of the story :) :) :). Bob > On Sep 18, 2024, at 8:28 AM, Barry <[email protected]> wrote: > > Hi Bob, > > I see where the PTOs for the R-390/URR and R-390A/URR both have end point > adjustments. L701 performs that in both. The schematic I see for the > R-390A/URR conveniently labels that as such on the schematic. RM > 11-5820-357-35 calls it out on page 12, Paragraph 12-b defines the function > of that coil. I just wasn't seeing that last night. I sure wish I had a > searchable PDF for the R-390/URR's service manual. > > Thanks, > Barry - N4BUQ > >> Hi >> >> Motorola most certainly made their own PTO’s for the radios they supplied. >> Long >> ago I talked to the folks who did the linearity adjustments on them. They >> still >> had (not so) fond memories of doing those adjustments. >> >> Bob >> >>> On Sep 17, 2024, at 11:29 PM, Barry <[email protected]> wrote: >>> >>> Hi Larry, >>> >>> Regarding the tutorials, I'm wondering which might apply to the PTO in the >>> R-390A/URR. I see most(all?) of them are regarding the COSMOS units but am >>> wondering which PTO is in the R-390/URR and whether any of the >>> rebuild/linearity documents apply to that one. Were all the R-390/URR PTOs >>> made by Collins and no COSMOS in that version? >>> >>> Mine is running just a bit fat - maybe 1.5 kc end-to-end. I have the actual >>> numbers written down and can post that when I get back to the workbench. >>> From >>> what I remember, removing a turn shortens the end-to-end but perhaps >>> additional >>> C would work as well. I don't know what effect that might have on linearity >>> but I don't think it should. >>> >>> The R-390A/URR PTO has an endpoint adjustment. Does this exist for the PTO >>> in >>> an R-390/URR? >>> >>> Thanks, >>> Barry - N4BUQ >>> >>>> Hi Barry, I thought what you wrote makes sense and is correct. It's good >>>> that you are understanding how it all works. It makes diagnosys so much >>>> easier. Good going. >>>> >>>> If you're interested in more reading on the 390A PTO's, there are 3 docs on >>>> our website in the repair tutorials section by Tom Marcotte, Jim Miller and >>>> myself. >>>> >>>> Regards, Larry >>>> >>>> On Mon, Sep 16, 2024 at 5:51 AM Barry Scott <[email protected]> wrote: >>>> >>>>> Okay, Larry. Thanks for the reply. >>>>> >>>>> On further reading, the VFO is a Hartley design and, given that the >>>>> frequency formula for a Hartley is an inverse function of the LC values, >>>>> presuming that at xx 000, the iron core is "out" of the coil and a >>>>> clockwise turn of the KC knob causes the core to be pushed further inside >>>>> the coil increasing the L, then the frequency would indeed drop with CW >>>>> motion of the knob. Sorry for the awkward way of stating that but I think >>>>> it makes sense to me. >>>>> >>>>> Thanks again, >>>>> Barry - N4BUQ >>>>> Thanks, >>>>> Barry - N4BUQ >>>>> >>>>> >>>>> On Sun, Sep 15, 2024 at 11:10 PM Larry Haney <[email protected]> wrote: >>>>> >>>>>> Hi Barry, You are absolutely correct in your deductions. When the KC is >>>>>> at its lowest of 000, the vfo is at its highest (3.455 mh). >>>>>> >>>>>> Regards, Larry >>>>>> >>>>>> On Sun, Sep 15, 2024 at 7:23 PM Barry Scott <[email protected]> >>>>>> wrote: >>>>>> >>>>>>> Looking at the simplified schematic for the 3rd mixer (V205), the output >>>>>>> from the 2nd mixer (V204) tunes from 3 to 2 MC and the VFO tunes from >>>>>>> 3.455 >>>>>>> MC to 2.455 MC yielding a constant 455 kc mixer product. Is it correct >>>>> to >>>>>>> interpret those numbers to mean that if the counter starts at XX 000 >>>>> and >>>>>>> the KC control is rotated CW to XX +000 the VFO's output frequency >>>>> starts >>>>>>> at 3.455kc and falls 1000 kc for 10 turns CW on the KC knob? >>>>>>> >>>>>>> I'm asking because I want to know what the frequency of the VFO is >>>>>>> supposed >>>>>>> to be when the dial is at XX 000 and I presume it's 3.455 kc but >>>>> wanting >>>>>>> to make sure. >>>>>>> >>>>>>> Thanks, >>>>>>> Barry - N4BUQ >>>>>>> ______________________________________________________________ >>>>>>> R-390 mailing list >>>>>>> Home: http://mailman.qth.net/mailman/listinfo/r-390 >>>>>>> Help: http://mailman.qth.net/mmfaq.htm >>>>>>> Post: mailto:[email protected] >>>>>>> >>>>>>> This list hosted by: http://www.qsl.net >>>>>>> Please help support this email list: http://www.qsl.net/donate.html >>>>>>> >>>>>> >>>>> ______________________________________________________________ >>>>> R-390 mailing list >>>>> Home: http://mailman.qth.net/mailman/listinfo/r-390 >>>>> Help: http://mailman.qth.net/mmfaq.htm >>>>> Post: mailto:[email protected] >>>>> >>>>> This list hosted by: http://www.qsl.net >>>>> Please help support this email list: http://www.qsl.net/donate.html >>>> ______________________________________________________________ >>>> R-390 mailing list >>>> Home: http://mailman.qth.net/mailman/listinfo/r-390 >>>> Help: http://mailman.qth.net/mmfaq.htm >>>> Post: mailto:[email protected] >>>> >>>> This list hosted by: http://www.qsl.net >>>> Please help support this email list: http://www.qsl.net/donate.html >>> ______________________________________________________________ >>> R-390 mailing list >>> Home: http://mailman.qth.net/mailman/listinfo/r-390 >>> Help: http://mailman.qth.net/mmfaq.htm >>> Post: mailto:[email protected] >>> >>> This list hosted by: http://www.qsl.net >>> Please help support this email list: http://www.qsl.net/donate.html ______________________________________________________________ R-390 mailing list Home: http://mailman.qth.net/mailman/listinfo/r-390 Help: http://mailman.qth.net/mmfaq.htm Post: mailto:[email protected] This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html
