>
>I have just pulled the covers off my Sick TL-922 and have found
> the following problems.
>
>1: one of the 3-500's had lost it's vacuum seal due to a fracture of the
> glass, everything inside the tube was covered with a white film,
>which I under stand is normal when this occurs.
>
... when the filament is lit
>2: The only other component that is visibily damaged is a 220pf silver
>mica cap at the base of this tube.
>
>No problem, of course to replace the tube or the Capacitor, however am
wondering
>which event is likely to have occured first. Did the CAP go out and
>cause the fracture of the tube glass
not likely
>or did the tube blow and cause the
>cap to explode ...... or.....and more importantly, is there something
else that I have not yet
>discovered that
>might have caused this catastrophic failure ?
>
Failed grid caps are typ. caused by parasites at c. 130MHz. Failed
glass/metal seals are typ. caused by rocking the tube horizontally during
removal from the socket.
>I have had this unit for about 14 years now . am the original owner
>and have had a few other failures over the years of a different sort..
>LOst tube ..... fried filament xformer.
When a tube grid/fil. shorts from the aforementioned parasite, the +120v
power supply is shorted on standby. The unfused fil. trans. burns out in
short order due being shorted. The fix is to remove the +120v connection
to the bias relay contact so it can't happen again, replace the int.
shorting tube, and replace the filament trans. . [see Circuit
Improvements for the tl-922 article on my Web site for details] .
SB-220s have the same prob.
>
>I am a bit apprehensive about just replacing the CAP and tube before
>being certain that it won't do a repeat when I fire it up.
RF Parts allegedly sells a Chinese 3-500Z that has a similar Mu (130) to
the Eimac variety that is no longer made. .
>
> There were no loud bangs before this thing went south I switched it off
>and found that it did not fire up when I next tried to use it.
>The only unusual occurance prior to the failure was an intermittant GRID
> current reading of about 50-75 mils while the unit was only idling.
>
It is costly to ignore warnings of impending disaster from the Amplifier
Fairy Godmother.
>Any thing else I should look for while I have this unit's covers off ?
>Any body else encounter a similar problem ?
>........
- later, Gus
Rich. Measures. Web site: www.vcnet.com/measures
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