[R-390] replacement R390A gear clamp source

Tue Jul 28 19:12:21 EDT 2026

 Great news
When you get some time and feel the need, attached is a graphics file I made back in 1998, when I worked on my first R390A receiver.
My goal was a linear dial readout for checking linearity of the PTO.
Print it out, use double stick tape and apply the dial to a circular stiff 1/4 inch backing (Plexiglass), epoxy the backing to a wooden dowel concentric center drilled for PTO shaft with a partial split for small hose compression clamp to secure the split shaft to the PTO shaft.
Rotate the mounted dial against a fixed 0.040 inch pointer  mounted in front of the scale.

As you turn the dial, you can count the turns, partial turns and assess the offset as you move thru the frequency.
Your mileage may vary
Enjoy

John

    On Tuesday, July 28, 2026 at 03:29:15 PM EDT, Paul Roche via R-390 <r-390 at mailman.qth.net> wrote:   

 Got the clamp, installed it, and everything is great again!!! Thank you SO much!!
Sent from my iPhone

> On Jul 25, 2026, at 9:22 AM, Paul Roche via R-390 <r-390 at mailman.qth.net> wrote:
> 
> 
> Sent from my iPhone
> 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.
> <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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