[R-390] replacement R390A gear clamp source
Don Heywood
donwc4g at gmail.com
Sat Jul 25 11:33:58 EDT 2026
Here is what you're looking for.:
Regards,
Don WC4G
On Sat, Jul 25, 2026 at 11:24 AM Jordan Arndt via R-390 <
r-390 at mailman.qth.net> wrote:
> Paul....What you're seeeing is the cover for the adjustment....the real
> adjustemnt is hidden behind that cover screw which you need to remove.
> Behind it is a small slotted brass rod that you adjust...I wouldn't try to
> adjust it until you're sure you have everything properly synchronized.
>
> What type of PTO is it...? Collins type or Cosmos type...?
>
> YMMV....73...Jordan VE6ZT
>
> ----- Original Message -----
> From: "Barry via R-390" <r-390 at mailman.qth.net>
> To: "PAUL ROCHE" <rochep at comcast.net>
> Cc: "R-390 Mailing List" <r-390 at mailman.qth.net>
> Sent: Saturday, July 25, 2026 9:16 AM
> Subject: Re: [R-390] replacement R390A gear clamp source
>
>
> > Paul,
> >
> > I wish I could give you good advice on that one but I think you have a
> > slightly newer(?) PTO than mine. Yours has an adjustment for the
> > transformer and mine doesn't. Either way, with mine insutu, I can't
> quite
> > see that adjuster so not sure.
> >
> > HOWEVER, if you've adjusted that screw, check both ends of the PTO as it
> > may not be changing the end you're checking.
> >
> > Looks like that clamp will get there on time so I hope things go back
> > together good for you.
> >
> > Barry - N4BUQ
> >
> >> In anticipation of my new clamp, I thought I would set the end point on
> >> my PTO.
> >> It goes from2455 to 3450, it’s off by 4 KC. I thought I might try and
> >> adjust
> >> it. But I am not sure I’m looking at the correct endpoint adjusting
> >> screw. Is
> >> it the one just above and to the right looking at the PTO? It doesn’t
> >> look like
> >> it does in the manual. I indicated it with an arrow in my picture. I
> >> turned it
> >> 1/2 turn while looking the PTO on 3450 when I was at 2450 and nothing
> >> changed
> >> on my frequency counter.
> >>
> >> [image/jpeg:image0.jpeg]
> >>
> >> .
> >>> On Jul 24, 2026, at 10:36 AM, Barry via R-390 <r-390 at mailman.qth.net>
> >>> wrote:
> >>>
> >>> John,
> >>>
> >>> Paul is the one who is needing to do this. Once he gets the clamp
> >>> replaced, this
> >>> should help as it's a bit more complete than the description I sent.
> >>>
> >>> Thanks,
> >>> Barry - N4BUQ
> >>>
> >>>> Barry,
> >>>> Here is the specific process I think you were looking for, from the
> >>>> R390A
> >>>> document collection
> >>>> Variable Frequency Oscillator (VFO) Odham Alignment per R390A
> >>>> maintenance School
> >>>> Class
> >>>
> >>>> VFO Alignment Procedure
> >>>> (1) Deenergize the receiver.
> >>>> (2) Check V-701 in a tube tester for good GM or transconductance.
> >>>> (Think of Gm as the ratio between the plate current change and the
> grid
> >>>> to
> >>>> cathode voltage change with plate to cathode voltage held constant.
> If
> >>>> we call
> >>>> Ip the plate current, and Vg the grid voltage,
> >>>> then Gm = dIp / dVg.
> >>>> Reinsert the tube.
> >>>> (3) Remove VFO connector Plug-717 from Jack-217.
> >>>> (4) Disconnect P-116 from J-116 (IF OUTPUT jack).
> >>>> (5) Connect P-717 to J-116 then connect this output to a frequency
> >>>> counter.
> >>>> (6) Set the receiver at 7+000 MHz. (There is a red separator that
> >>>> appears
> >>>> between the 7 and the KHz reading when reaching this upper end of the
> >>>> Kc
> >>>> frequency band).
> >>>> (7) Tighten the ZERO ADJ knob, and center the KILOCYCLE CHANGE
> control
> >>>> at the
> >>>> midpoint between the two extremes of its movement.
> >>>> (8) Turn the receiver FUNCTION switch to Manual Gain Control (MGC)
> and
> >>>> check the
> >>>> VFO frequency. It should read 2.455 MHz ±1 KHz. If it does, proceed
> to
> >>>> step
> >>>> (12). If it does not, proceed to step (9).
> >>>> (9) Locate the PTO Oldham coupler (located underneath the receiver on
> >>>> the VFO
> >>>> tuning shaft), loosen the Oldham clamp nearest the front panel.
> >>>> (10) Turn the section of the Oldham coupler still connected to the
> VFO
> >>>> until the
> >>>> frequency counter indicates 2.455 MHz.
> >>>> (11) Once the frequency is correct, retighten the front clamp on the
> >>>> Oldham
> >>>> coupler. Do not change the VFO setting while tightening this clamp!
> >>>> (12) Loosen the ZERO ADJ knob and tune the receiver to 7.000 MHz. The
> >>>> VFO output
> >>>> frequency should now read 3.455 MHz ~ ±3 KHz.
> >>>> (13) Turn the receiver OFF.
> >>>> (14) Reconnect P-717 to J-217 and P-116 to J-116.
> >>>
> >>>> NOTE: The limits of error on the VFO frequencies will decrease is in
> >>>> relation to
> >>>> the additional time you expend improving the key receiver subsystems.
> >>>> 500 Hz is
> >>>> a very good and reasonable target for overall accuracy.
> >>>
> >>>> Hope this helps with your new clamp
> >>>
> >>>> John
> >>>
> >>>>> On Monday, July 20, 2026 at 09:18:04 AM EDT, Barry via R-390
> >>>>> <r-390 at mailman.qth.net> wrote:
> >>>>
> >>>> I notice that on 29 OCT 69, the aluminum alloy used for those clamps
> in
> >>>> the
> >>>> design was changed from 5454 to 7074 (copy attached). Wikipedia tells
> >>>> me 7074
> >>>> "is more susceptible to embrittlement than many other aluminium
> >>>> alloys".
> >>>> Perhaps that contributes to this type of failure and the earlier
> clamps
> >>>> were
> >>>> more durable with in this respect.
> >>>
> >>>> 73,
> >>>> Barry - N4BUQ
> >>>
> >>>>> My feeling is that there is a tendency to over tighten screws. The
> >>>>> result of
> >>>>> over tightening is that either the screw or the split clamp will
> fail.
> >>>>> It must
> >>>>> be remembered that the following paragraphs were written for NEW
> >>>>> equipment
> >>>>> assembly. But we are working on the very same equipment some 50
> years
> >>>>> later.
> >>>>> So all of the hardware has been stretched, compressed and stressed
> so
> >>>>> the
> >>>>> torque needed to make a tight mechanical connection is much less
> then
> >>>>> what was
> >>>>> needed for initial assembly. Perhaps if the screw threads are lightly
> >>>>> lubricated much less torque will be needed to achieve a snug fit
> that
> >>>>> will not
> >>>>> slip?
> >>>
> >>>>> From the Collins Quality manual:
> >>>
> >>>>> 3.1.6 Tightness
> >>>>> All threaded fastenings must be tight. Torque wrenches or torque
> >>>>> screwdriversoften aid in assuring proper tightness of threaded parts
> >>>>> and should
> >>>>> be used wherescrew torque is specified. An adequate concept of
> >>>>> "tightness"
> >>>>> cannot be basedon torque alone, however. It must be considered that
> >>>>> torque is
> >>>>> just the meansby which tension is developed in the shank of the
> >>>>> tightened
> >>>>> screw. This tensionmust compress the parts being fastened
> sufficiently
> >>>>> to
> >>>>> withstand separationforces such as vibration or shock that may occur
> >>>>> during the
> >>>>> useful life of the equipment. Over-tightening may result in broken
> >>>>> screws,
> >>>>> either at initialassembly or at some later date.Many factors affect
> >>>>> the optimum
> >>>>> torque required for driving small machine screwsto a point where the
> >>>>> proper
> >>>>> tension is reached but not exceeded. These factorsinclude friction
> at
> >>>>> the
> >>>>> bearing surfaces, friction of the mating threads, lengthof thread
> >>>>> engagement,
> >>>>> presence or absence of lubrication, type of lock washer orlock nut,
> >>>>> and
> >>>>> hardness and surface finish of the mating parts.Most materials
> >>>>> commonly
> >>>>> fastened by machine screws (including metal) are compressible, and
> all
> >>>>> screws
> >>>>> are elastic within narrow limits. A few hours or daysafter initial
> >>>>> tightening,
> >>>>> a properly tightened screw is sometimes pronouncedslightly loose
> >>>>> because of
> >>>>> metal fatigue in the stressed screw and/or creep ofthe compressed
> >>>>> material.
> >>>>> Repeated tightening in such instances can be moredetrimental than
> >>>>> beneficial.
> >>>>> Reliable locking devices will compensate adequatelyfor this apparent
> >>>>> relaxation.Because of the many variables affecting hand-tightening,
> it
> >>>>> must be
> >>>>> stated thatno simple chart of tightening torque or easily applied
> >>>>> "rule of
> >>>>> thumb" has yetbeen devised that will replace a good assembly
> >>>>> operator's
> >>>>> judgment and experience. However, when fasteners loosen and cause
> >>>>> equipment
> >>>>> failure, they usuallydo so, not because of failure of the locking
> >>>>> device, but
> >>>>> because of insufficientinitial tightening.
> >>>
> >>>>> 4.2.6 Clamping Adjustable Gears
> >>>>> On gears utilizing split hubs for the purpose of adjustment in a
> gear
> >>>>> train,
> >>>>> thereare two common methods employed to tighten them to shafts: (1)
> a
> >>>>> collar
> >>>>> with twoor more set screws, or (2) a split clamp with one screw. In
> >>>>> all cases
> >>>>> where anappreciable amount of torque is involved, method (2) should
> be
> >>>>> employed. Thescrew through the clamp must be lubricated to prevent a
> >>>>> false
> >>>>> indication of tightness. It is preferable to specify a tightening
> >>>>> torque on all
> >>>>> such clamps. Aftertightening, the "lips" of the clamp must not be
> >>>>> touching. The
> >>>>> set screws orclamp screws should be accessible at the point of normal
> >>>>> adjustment. Wheremethod (1) is employed, care should be used that
> the
> >>>>> screws do
> >>>>> not engage theslot of the split hub.
> >>>
> >>>>> This said, I have a limited number of split clamps from Collins
> >>>>> equipment that I
> >>>>> have scrapped over the years. Most are from KWT-6 mechanical
> >>>>> assemblies but a
> >>>>> few are from R-390A receivers. You can't save everything so I
> >>>>> disassemble as
> >>>>> much hardware as I can and scrap the rest.
> >>>>> Regards,
> >>>>> Jim
> >>>
> >>>>> Logic is a systematic method of coming to the wrong conclusion with
> >>>>> confidence.
> >>>>> Murphy
> >>>
> >>>>> On Sunday, July 19, 2026 at 06:51:24 PM CDT, Paul Roche via R-390
> >>>>> < [ mailto:r-390 at mailman.qth.net | r-390 at mailman.qth.net ] > wrote:
> >>>
> >>>>> This must have been discussed 1000 times but I can’t find it. Does
> >>>>> McMaster Carr
> >>>>> carry replacement clamps that would work for the 3 or 4 sizes
> needed?
> >>>>> I have a
> >>>>> broken 0.312” (5/16”) clamp on my VFO. This is my third broken clamp
> >>>>> in this
> >>>>> unit; I am expecting more.
> >>>
> >>>>> Paul
> >>>>> Sent from my iPhone
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