[ARC5] BC-459 Drift (Oxidation makes high "Z" circuit).

[Leslie Smith]

  Hello all,
  The focus of this posting is to add another observation about
  "oxidation".

  Some time ago I had a faulty command receiver.
  The problem appeared to be in an  I.F. can.
  Put simply, they were way off frequency, and when I coupled a signal
  into them (GDO + loop) they were WAY off frequency.
  The solution?  As suggested below.  I "exercised" the trimmer caps and
  the socket pins.  The problem vanished.
  I'm posting this to add emphasis to the of invisible high "Z"
  circuits,  see the posting below.

  I really HATE trouble-shooting that depends on intuition.
  Sometimes it's the only method that solves a problem.

  73 de Les Smith
  vk2bcu at operamail.com


On Wed, Jul 1, 2015, at 04:30, J Mcvey via ARC5 wrote:
> Not sure which variable caps you have cleaned already, but there is one
> inside the oscillator "can" that might be needing attention as well as
> the PA tank caps.. It might sound silly, but unplugging the oscillator
> tube and re-inserting it a few times will help break up the socket
> oxidation.The old fashioned "tap lightly with a screwdriver handle" might
> help locate the loose or oxidized component.
> Otherwise there really isn't much else to cause the instability.
>  
> 
> 
>      On Sunday, June 28, 2015 6:51 PM, Mark K3MSB <mark.k3msb at gmail.com>
>      wrote:
>    
> 
>  Well, I guess the problem is not licked.
> 
> I drove my SCR-274N over the weekend to pick off some remaining states
> for
> my ARC-5 WAS effort.
> 
> I was getting drift (on key down) and a  rough note, but not as bad as
> before I did the cleaning as mentioned in my earlier post.  Based on one
> of
> the other threads,  I adjusted my variac to bring the plate voltage down
> from 550V to 440V, and the situation improved dramatically.
> 
> 80M Is rock solid as usual at 550V Ep.  The Oscillator tube was replaced
> in the 40M TX with a tested good tube, and from restoring the TX a few
> years ago, all the resistors are within tolerance,  the bathtub cap was
> replaced etc.
> 
> I suppose it's time to go in a replace components in the Oscillator
> section?
> 
> 73 Mark K3MSB
> 
> 
> 
> On Mon, Jun 22, 2015 at 1:56 PM, Mark K3MSB <mark.k3msb at gmail.com> wrote:
> 
> > Well, I think I have the problem licked.
> >
> > I used 600 grit wet/dry sandpaper to clean behind the wiper arms of each
> > of the 3 air variable caps, then cleaned with a bit of DeOxit.
> >
> > I put the transmitter back in the rack and the tone sounded good with no
> > drift,  but then I noticed the power output was fluxuating a great
> > deal.      I pulled my phones off at that point, and then I heard the sound
> > of arcing.  With my vast level of expereince in ARC-5's I deeded this to
> > be not good.  So, I played with the tuning and that didn't seem to have
> > much effect.  At that point I realized that the arcing was not coming from
> > the transmitter, but the the small matching unit I built to connect to my
> > 50 ohms antennas (4:1 UNUN and an old HT-37 tuning capacitor).    I removed
> > the side panel from the Bud Box that houses my matching unit and my oh my
> > was that arcing pretty.    For some reason the rotators are closer to the
> > stators than I remember, so I bent the offending rotator plate a bit, and
> > the arcing stopped and power output was nominal (about 50W).
> >
> > I did not notice the drift problem like I originally reported but the tone
> > still has more chrip in it than I'd like.  I want to play with it for a
> > while and see how it performs.
> >
> > 73 Mark K3MSB
> >
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