[R-390] R390/URR Gear train

[David Wise]

I second what Don Heywood says.  Be suspicious of the molded mica cap that tunes the transformer secondary.  Today they are on the rising end of the bathtub curve.  Moisture contaminates the dielectric and the crimp contacts, and they go intermittent when the stress on the leads changes slightly.  Transients, such as applying a test probe, can temporarily break through the corrosion and make the part appear healed - for a while.  They also go leaky, especially if there's voltage across them.  There's been plenty of time for electromigration of ionic contaminants.  Sometimes this is intermittent too.

HTH,
Dave Wise 
________________________________________
From: r-390-bounces at mailman.qth.net <r-390-bounces at mailman.qth.net> on behalf of jgedde at optonline.net <jgedde at optonline.net>
Sent: Monday, June 10, 2019 4:10 AM
To: 'Don Heywood'; 'Francesco Ledda'; r-390 at mailman.qth.net
Subject: Re: [R-390] R390/URR Gear train

Thanks Don.  I am an engineer/engineering manager/project engineer by day.  I design ultra-high reliability, radiation hardened, motion controllers and motor drivers for space use.  My company's main product line is motors, gearboxes and actuators for space applications.  For example, the Mars Curiosity rover and the upcoming Mars 2020 rover - we designed and built those actuators.  Being exposed to that sort of thing, I work with very, very smart people who are the top experts in their fields.  People from JPL, NASA, Northrop Grumman, Lockheed, BAE, etc.  Needless to say, I pick up a thing or two about mechanical engineering...

Right now, my controller designs are in orbit around the earth deploying mechanisms and controlling the movement of solar arrays.  It's very rewarding to see a launch go well and having something one conceived come to life and operate.

Getting back to R390 talk...  I too suspect something with one of the transformers, but as you point out, testing by substitution is a good way to test.  Alas, I don't have substitutes, so I have to debug the problem using other methods.  I suspect a problem with the first can as I can see junk on the grid of the first IF tube.  My mental block is reasoning out why.  The grid is biased by the AGC line and the problem still occurs with the radio in MGC with the AGC line pulled to ground.  That point makes me wonder...  My crackle sounds very much like what you hear when lightning is around.  But, there's no voltage on the first can when in MGC mode.  So, what could be breaking down?  Or, is anything breaking down?  I find myself wanting an old school signal tracer...

The radio has only two issues left to resolve before tackling cosmetic restoration.  The crackling and the PTO endpoint adjustment.  It's come a long way since I got it.  Back then it had a bum power supply/regulator, dead bands, poor sensitivity, a wacky carrier meter (tracked down to loose resistance wire coils in the carrier meter adjust pot which were moving around during adjustment), an inoperative calibrator, RF geartrain issues, etc. etc.

John

-----Original Message-----
From: Don Heywood <wc4g at knology.net>
Sent: Sunday, June 9, 2019 10:35 PM
To: jgedde at optonline.net; 'Francesco Ledda' <frledda at att.net>
Subject: Re: [R-390] R390/URR Gear train

I am really impressed with the way you lubricated that RF deck, you sure know your lubricants. Maybe I can help with the crackle on the 1-2 mc band.
Inside those RF transformers are mica caps across the coils, I have found thru substitution of these RF transformers a few instances where these mica caps are breaking down. It has gotten to the point that they are failing, especially the ones with B+ on one side. To bad you don't have a second source of these RF transformers, I have a couple of junker RF decks which come in handy for substituting these transformers to find the offender and replace its caps. A cap checker may be of some help, but you need a higher voltage to make these caps fail.
Regards,
Don WC4G