[R-390] replacement R390A gear clamp source

Fri Jul 24 10:35:42 EDT 2026

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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