[R-390] CV-591 Questions

[Jim Brannigan]

Roy,
Thanks for your input.


> 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.

Actually the cap. is somewhat redundant.  I'm not sure it adds much to the
circuit, a replacement is on order.
>
> >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.
>
Crystal control is OK and the trimmer is in circuit in only one position.  I
put some Deoxit on the variable shaft...

Your suggestion to re-check all ground and contact points is a good one.

Jim