On Wed, 24 Mar 1999 22:55:36 +0000 "Joseph J. Duerbusch"
<jduerbus@marz.com> writes:
>
>I have a very nice Drake L-7. It has been in storages for years and I
>just got it going this past fall. Stock as a rock, low serial number
>of
>134. Original tubes.
>
>Last weekend I was operating, everthing normal, full output 1200+ but
>I
>started to smell something burning. Not bad, like maybe something has
>fell against one of the tubes. I finally got time to look inside.
>Using my nose, I quickly found the smell coming from the filament
>transformer. Oh no!, but wait, it wasn't from the tranformer but a
>the
>junction of the filament transformer and filament choke. At this
>intersection, Drake has a PC Board which the connection is made.
>There
>is also a .05 Cap to ground and a connection for the meter lamps. It
>is
>the PC Board that is burning and making the smell.
I have repaired several L-7's that had partially shorted bypass caps C-29
and C-30. I use .01 or .02 1KV disc ceramics for replacements. Not sure
if the .05 is too high a value or just that the L-7 had a bad lot of
caps.
>
>Something on one side of the of the filament transformer is drawing
>too
>much current. I suspect a filament-to-grid short in one of the
>tubes(?).
Possible but then one or both grid chokes would be open.
>
>Drake cautions as not to defeat the interlock but on the L-7 the
>cabinet is closed so there is not way to see the tubes without the
>cover
>on.
Thats a real pain in the butt! If you are comfortable around HV then
remove all of that stuff. Always treat HV with extreme caution. There is
a cabinet cover and a HV shorting interlock. To test with the cover off
both must be removed or defeated.
GL Carl KM1H
>
>So before I go any farther, I thought I would asked the "Experts".
>Any
>ideas, or comments? BTW this junction is the same as on a SB-220 if
>you
>want to look at the SB220 Diagram on Measures WebPage.
>
>Thanks Joe KØBX
>
>
>
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>
>
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