[R-390] CV-591 Questions

[Roy Morgan]

At 08:01 AM 1/22/03 -0500, Jim Brannigan wrote:
>There is a 10Mfd. bathtub capacitor in the audio driver B+ circuit.  This
>has failed open, is this the usual failure mode of a bathtub capacitor?

Jim,
Usually they don't fail. hehe... leakage is more common than open I think, 
but whatever, yours failed open. You may be able to unsolder the case and 
fix an open connection.. though at that point a new cap inside is a good 
idea.  (If you use an elecrolytic, mark the polatity!)  puttng a new cap on 
the terminals of the old one is a fine solution though.


>The variable oscillator is VERY touchy to align.
>Any ideas?


Is the thing stable on crystal control (assuming you have the crystals 
installed)?

Apply TINY amounts of De-Oxit or Pro Gold to the cap rotor wiping 
contacts.  This thing is operating around 400 kc or so and should not be 
touchy in any way.

Check ground lugs and capacitor mounting frame studs for corrosion.  Also 
pay attention to shields in the same way.   Replace all paper caps, if you 
have a flaky one in a bypass function, it may be wavering in value enough 
to affect the oscillator frequency.

You may have a flaky trimmer cap.  Be careful if you are tempted to force a 
ceramic trimmer that is stuck.  I broke one and discovered that it is a 
temperature compensated one (N750 perhaps) and was not easy to 
replace.  For our use the temp compensation may simply not be 
needed.  Summing fixed and a variable cap is perfectly fine if you can't 
locate the right value.. These circuits should essentially stay tuned up 
forever once set.

To save a bit of heat and unnecessary tube aging, removing the reactance 
tube as you did is a good idea.

Roy

- Roy Morgan, K1LKY since 1959 - Keep 'em Glowing!
7130 Panorama Drive, Derwood MD 20855
Home: 301-330-8828 Work: Voice: 301-975-3254,  Fax: 301-948-6213
roy.morgan@nist.gov --