[R-390] Megacycle Change Stuck R-390

Barry n4buq at knology.net
Thu May 1 22:26:49 EDT 2025 

I checked and I can rotate the MC knob CW to 32.xxx.  I checked the 10 1/2 turn stop mechanism and it has 11 winged "washers" and I think that's correct as it is labeled a "10 1/2 TURN STOP" in the manual and I have 11 washers on both the MC and KC shafts.  I think, however, that I may need to make an adjustment to my MC stop mechanism.

When I turn the MC knob CCW to 00.xxx, I only have 2 or 3 degrees overtravel before I hit the stop point due to the 10 1/2 stop mechanism and I think that may be why I have sufficient overtravel at CW end of travel to go to that extra MC.  I'm thinking that when I reach 00.xxx in the CCW direction, I should be able to rotate the knob about 90-degrees before hitting the stop and if that's set, then I should run out of travel at the CW end before getting to 32.xxx.

Is ~90 degrees overtravel appropriate to have at the end of the CCW and CW rotation?  If so, I'll make that adjustment.  At this point, I don't think I've damaged the Geneva mechanism - at least the part that I can see from the underside of the chassis.

Thanks,
Barry - N4BUQ

> It is imperative that the clockwise rotation of the megacycle shaft stops at 31.
> This is where the ten-turn stop should lock. Any further rotation could damage
> the Geneva mechanism.
> 73
> Don WC4G
> Sent from my iPhone
> 
>> On May 1, 2025, at 20:13, Tom via R-390 <r-390 at mailman.qth.net> wrote:
>> 
>> The radio only tunes to 32 mhz. So the mhz dial will stop at 31 and the top end
>> of that band is 32 mhz
>> 
>> Tom
>> W3TA
>> 
>> -----Original Message-----
>> From: r-390-bounces at mailman.qth.net <r-390-bounces at mailman.qth.net> On Behalf Of
>> Barry
>> Sent: Thursday, May 1, 2025 7:34 PM
>> To: Alex Hopper <ahopper0216 at gmail.com>
>> Cc: R-390 Mailing List <r-390 at mailman.qth.net>
>> Subject: Re: [R-390] Megacycle Change Stuck R-390
>> 
>> Alex,
>> 
>> I just realized that I've been telling you to rotate the MC knob CW to "31.xxx".
>> That should be "32.xxx". Apologies. I had it in my head that the radio only
>> tuned to 32 MHz but it will turn to 33 MHz (or, perhaps, more formally
>> "31.999.9")! Sorry for the misinformation. I hope I didn't cause you to do
>> something incorrect.
>> 
>> Barry - N4BUQ
>> 
>>> From: "Barry" <n4buq at knology.net>
>>> To: "Alex Hopper" <ahopper0216 at gmail.com>
>>> Cc: "R-390 Mailing List" <r-390 at mailman.qth.net>
>>> Sent: Thursday, May 1, 2025 4:24:55 PM
>>> Subject: Re: [R-390] Megacycle Change Stuck R-390
>> 
>>> Yes. The 10-turn stop is pinned to the shaft so the idea is to rotate
>>> the knob which will still rotate the 10-turn stop.
>> 
>>> In your case, since it's stopping you before it should in the CW
>>> direction, I would rotate the knob CCW to the "00.xxx" position,
>>> loosen the clamp, rotate shaft farther CCW until the stop hits, rotate
>>> slightly back CW, and tighten the clamp. After that ensure there is
>>> about the same amount of overtravel on the fully CW direction (from "31.xxx" as
>>> there is in the fully CCW direction.
>> 
>>> Again, I think there's a correct procedure to do this in the manual
>>> but this should work as well.
>> 
>>> Barry - N4BUQ
>> 
>>>> All - is the adjustment point for the 10-turn stop the
>>>> yellow-highlighted “collar” on the shaft of the megacyle change as
>>>> pictured? There is a small bristol head screw on it.
>>>> -Alex
>> 
>>>> Barry < [ mailto:n4buq at knology.net | n4buq at knology.net ] > schrieb am
>>>> Do. 1. Mai
>>>> 2025 um 12:04:
>> 
>>>>> Alex,
>> 
>>>>> There should be very little CCW overtravel past the lower end and
>>>>> about the same amount of CW overtravel at the upper end. Sounds like
>>>>> you've found the problem and, provided you have the correct Bristol
>>>>> wrench, it should be fairly easy to correct.
>> 
>>>>> Barry - N4BUQ
>> 
>>>>>> Barry,
>>>>>> with the MC dial set to "00" I am able to rotate the MC knob CCW
>>>>>> and nearly get the dial to read "99" but it doesn't 'click' into
>>>>>> place. I'll have a look at it (power OFF, of course).
>>>>>> -Alex
>> 
>>>>>> On Thu, May 1, 2025 at 11:17 AM Barry < [ mailto:n4buq at knology.net
>>>>>> | n4buq at knology.net ] > wrote:
>> 
>>>>>>> Alex,
>> 
>>>>>>> I think there's a procedure to do that in the manual and that
>>>>>>> might be best to follow; however, are you able to rotate the MC
>>>>>>> knob to where the two left-hand digits on the counter are at "00"
>>>>>>> and then continue CCW rotation one more click? If so, then that's another
>>>>>>> indication the ten-turn stops are off.
>> 
>>>>>>> If everything else is correct, then when the left two digits are
>>>>>>> at "00", you should be able to rotate the knob just a few degrees
>>>>>>> CCW beyond that but no more and when the left two digits are at
>>>>>>> "31", you should be able to rotate the knob just a few degrees CW
>>>>>>> but no more than that. If those conditions are not met, then
>>>>>>> loosening the clamp on the ten-turn stop and moving the stop
>>>>>>> mechanism to get those two stop points correct should be all that you need to
>>>>>>> do.
>> 
>>>>>>> Good luck with it! And, at the risk of sounding overly-concerned,
>>>>>>> please make sure the radio isn't powered up when you do that (or,
>>>>>>> at least, take the proper precautions).
>> 
>>>>>>> Barry - N4BUQ
>> 
>>>>>>>> Barry - it is a R-390 not a 390A. How does one adjust those
>>>>>>>> points on the MC shaft?
>>>>>>>> -Alex
>> 
>>>>>>>> On Thu, May 1, 2025 at 11:01 AM Barry < [
>>>>>>>> mailto:n4buq at knology.net | n4buq at knology.net ] > wrote:
>> 
>>>>>>>>> If you turn the radio over, those stops are quite visible (there
>>>>>>>>> is a set of them on both the KC and MC shafts).
>> 
>>>>>>>>> BTW, is this an R-390 or R-390A? It shouldn't make much
>>>>>>>>> difference for this but would be good to know.
>> 
>>>>>>>>> Barry - N4BUQ
>> 
>>>>>>>>>> Thank you both! Is adjusting the 10-turn stop something that
>>>>>>>>>> would require removal of the crystal oscillator or RF modules?
>>>>>>>>>> -Alex
>> 
>>>>>>>>>> On Thu, May 1, 2025 at 10:53 AM Barry < [
>>>>>>>>>> mailto:n4buq at knology.net | n4buq at knology.net ] > wrote:
>> 
>>>>>>>>>>> Hi Alex,
>> 
>>>>>>>>>>> There are a series of "wings" on the shaft that determine
>>>>>>>>>>> where the stop point should be at both ends of rotation (I
>>>>>>>>>>> think those limit the shaft to just a tad over 10 turns
>>>>>>>>>>> end-to-end). If all of those are completely stopped against
>>>>>>>>>>> each other, then that would cause this and the solution would
>>>>>>>>>>> be to loosen the clamp and adjust those stopping points. If
>>>>>>>>>>> the stop point is reached but those "wings" can still be
>>>>>>>>>>> rotated slightly with your fingers (be aware there is power near that point so
>>>>>>>>>>> best to unplug the radio to try this), then something else might be a problem
>>>>>>>>>>> but I think the 10-turn stop is the problem.
>> 
>>>>>>>>>>> Good luck,
>>>>>>>>>>> Barry - N4BUQ
>> 
>>>>>>>>>>>> Hi All,
>> 
>>>>>>>>>>>> I have an R-390 that has recently been restored. It is
>>>>>>>>>>>> performing very well, with one exception...
>> 
>>>>>>>>>>>> Last night I attempted to tune up through all of the
>>>>>>>>>>>> megahertz bands all the way to 32 MgHz for the first time. I
>>>>>>>>>>>> got to '30' on the megacycle change knob, and the knob
>>>>>>>>>>>> wouldn't rotate any further to 31. I could get it to go
>>>>>>>>>>>> slightly towards 31, but it was obviously stuck and I didn't want to attempt to
>>>>>>>>>>>> jam it.
>> 
>>>>>>>>>>>> What would potentially be the cause for this being stuck?
>> 
>>>>>>>>>>>> Thanks,
>>>>>>>>>>>> -Alex
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