[R-390] replacement R390A gear clamp source

Barry n4buq at knology.net
Sat Jul 25 12:54:36 EDT 2026 

If you do, you will likely find it best/easiest to adjust the 1,000kc spread with the PTO removed from the radio. YMMV 

Barry - N4BUQ 

> Thanks to all!! I would never have figured that out. I will take Jacques advice
> and just put her back together, make sure everything is OK with only 4kc off,
> and then maybe adjust it later.
> Sent from my iPhone

>> On Jul 25, 2026, at 11:34 AM, Don Heywood <donwc4g at gmail.com> wrote:

>> Here is what you're looking for.:

>> Regards,
>> Don WC4G

>> On Sat, Jul 25, 2026 at 11:24 AM Jordan Arndt via R-390 < [
>> mailto:r-390 at mailman.qth.net | 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" < [ mailto:r-390 at mailman.qth.net | r-390 at mailman.qth.net
>>> ] >
>>> To: "PAUL ROCHE" < [ mailto:rochep at comcast.net | rochep at comcast.net ] >
>>> Cc: "R-390 Mailing List" < [ mailto:r-390 at mailman.qth.net |
>>> 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 < [ mailto:r-390 at mailman.qth.net |
>>> >>> 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
>>> >>>>> < [ mailto:r-390 at mailman.qth.net | 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: [ mailto:r-390 at mailman.qth.net | r-390 at mailman.qth.net ] | [
>>> >>>>> 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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>> <PTO ENDPOINT PLUG.JPG>

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