[R-390] Another "Close to Perfect" R-390a

[Mark Huss]

According to a conversation I had many years ago with the son of one of 
the Engineers that worked on the R-390 and R-390A design, there was 
indeed something 'Special' about the Original 'Collins' PTO.  From what 
I remember, we had an R-390A with a PTO that was out of adjustment 
range.  It was a low serial number PTO made by Collins, the first time I 
had seen one.  I had decided to try repairing it as the School was not 
exactly high on the priority list for spare parts.  That is when this 
fellow told me about his history, and the solution to the problem.  
I.E., take a half turn off the small coil in the PTO.  The cause, 
according to him, was the material used for the large coil form.  It 
changed dimensions  with age and heat, causing the inductance to 
change.  He said his dad told him about it when he joined.  Said that 
Collins discovered the problem in the 75A series, but it did not show up 
until about the middle of the first year Collins R-390A run since it 
took years to show up.  They fixed the problem in the latter half of the 
Collins run, and all subsequent PTO's made by other manufacturers.  So 
yea, it is 'Special' in that it is one of the worst PTO's you can have.
       As for the bonifidies of this issue, I never met his father, just 
what he told me.  However, the solution did work.  And later I heard 
that PTO's on the 75A series Collins do have a problem with being out of 
adjustment range.  And I never had the problem with any other PTO by 
other manufacturers.  If the tale is true, then Motorola also made PTO's 
before 1957, so they may also have the same problem. 

Bonddaleena at aol.com wrote:

>In a message dated 2/27/2006 12:02:44 AM Eastern  Standard Time, 
>fwbray at mminternet.com writes:
>Just saw another R-390A -- item  5872325416 -- from you know who.
>
>This is an interesting radio, as it is  described as an "improved Motorola
>version " but bears a Capehart  plate.
>
>Now my question.  Is there anything unique about the PTO in  this radio
>which is described as "the exceptional Collins PTO which is very  rare to
>find as they were only usually put in to special one off receivers  built
>to a higher spec receiver for a special purpose" ??  It looks like  the
>regular R-390A PTO, but maybe I am missing something as I only have  one
>radio?  I think I know the answer, but I am always willing to  learn
>something new.
>
>73,
>
>Fred  Bray
>W6WAW
>
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>The way to tell Martyn is lying, is to see if his lips are moving!
> 
> 
>ron (I've been to his 'shack')
> 
>N4UE
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