[R-390] replacement R390A gear clamp source

Jordan Arndt Outposter30 at shaw.ca
Sat Jul 25 11:23:56 EDT 2026 

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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