[Premium-Rx] Racal RA6790/GM problems

[Phil R]

 Hello Heinz and the whole list,
Gerry Wineguard is quite active on the Racal RA6790/GM yahoo group and is very, very helpful.  If you need assistance finding his email address, contact me directly and I'll get his email addy for you.  I believe he still has some parts etc.  I think he might have displays, including the ones with backlit LEDs.  I'm not sure if he is a member of premium-rx but he is a great guy who would add a lot, especially on things Racal.  He used to work for Racal USA.
I have a Racal RA6790/GM [12 and 5 volt rails IIRC] and a Rockwell Collins HF-2050 [15 and 5 volt rails].  I have some familiarity with the trials and tribulations of tantalum capacitors.
I agree with the previous poster's comments on the tantalum capacitors.  In my opinion, tantalums should be derated 2 to 3 times working voltage, e.g. a 5 volt rail should have caps rated at 15 volts or better, a 15 volt rail 35 volts or better.  Back in the 80s etc. this did not seem to be know by Racal or Rockwell-Collins.
When tantalums go, they often short, thus the previous poster's advice of checking supply rail voltage is very good advice.  In my experience, tantalum capacitors will fairly often discolour from heat when they short out.  The yellow ones are a little easier to tell this way than the red or blue for obvious reasons.  With good light and a modest magnifying glass, you may be able to identify suspect tantalums and replace.  Look for an overall darkening compared to others of the same color and especially look for darkened spots, or brownish or black spots.  Check adjacent components and circuit board traces as well since a shorted tantalum could cause other damage.  I've had fairly good luck with the 'replace toasted looking tantalums method'.  Do the voltage check as well.
Remember static safety - a BIG issue especially in winter but it is a year round issue as well.  Wear a wrist strap, always touch the metal case before touching components etc.  
And, remember personal safety when taking voltage measurements etc. and always unplug the receiver when you are replacing components or doing other work that does not require it to be energized.
You'll likely need to replace the battery as well - another topic.  I replaced it with a AAA holder using premium quality charged up NiMhs with great results.  Only trick is how to secure the holder physically.
Keep in touch with us on premium-rx as to your progress!  
Phil Rafuse, VY2PRStratford PEI Canada