On Mon, 21 Sep 1998 09:50:03 -0230 Gus Samuelson <gussam@newcomm.net>
writes:
>
>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.
That can occur if the socket filament connectors have developed a
resistance to the tube pin(s). The heat will be enough to melt the solder
out of the pin and cracked glass is possible.
Before replacing the tube be certain that the socket connections are not
discolored, or have lost tension.
>
>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
> the fracture of the tube glass 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 ?
I would suspect that that tube had an internal gas arc and was possibly
weakened over a period of time. The 220pf is a bit high for that circuit,
switching all to 120pf might be a good idea. Also those grid chokes are
way too robust, replace them with anywhere from 470-1000 microhenries at
200ma maximum rating. Let them act as fuses the next time and help
protect the tube. By the time the original one opens the damage is done.
>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.
>
>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.
Remove the VHF choke L2 and replace with a 15-20 Ohm 25W wirewound
resistor to act as a surge suppressor for future arcs. Gas arcs are a
normal occurance and several amp manufacturers fail to provide the
necessary protection.
>
> 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.
That is always a sign of trouble. The TL-922 is also prone to bias supply
problems. C3, C26 and D2 should all be checked.
While you are at it check the resistance of the parasitic suppressor
resistors, they should be within 10% of original.
>
>Any thing else I should look for while I have this unit's covers off
>?
>Any body else encounter a similar problem ?
All common problems.
73 Carl KM1H
Amplifier Repairs and 6M Conversions Our Speciality
>Any recommendations from the Group ?
>
>BTW this was an old tube that I scrounged up to get the unit
> back onthe air and had been in service for about 2 years.
>
>
>Any help / recommendations would be appreciated.
>
>73 Gus VO1MP
>
>
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>
>
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