>I recently acquired an older AL-800H, which uses a pair of 3CX800s. When I
>first checked it out, I found that diode D117 was shorted - this is the
>diode that protects the meters and overload circuit. According to the
>manual this diode will usually short "if there is a large
>high-voltage-to-chassis current fault". Upon close inspection, I could find
>no obvious problems in the high-voltage area, and suspected a case of
>"flash-over" of one of the tubes, as I've read about in this forum. I
>replaced the diode and ran the amp for several weeks, with full output on
>all bands. At one point, I heard a muffled bang (more like a "ting"), but
>never hear it again and the amp continued to work OK.
>
? I was able to make two 3cx800A7s with loose gold inside the envelope
stop flashing over by tapping on the top of the "plate" cap with the flat
side of a 4oz ballpeen hammar. This procedure moves loose gold into the
base of the tube, out of harm's way. However, if the tube is inverted,
the gold gets back on the anode insulator and another flashover is likely
and the tapping process needs to be repeated. // Such tapping must be
rather firm to dislodge gold meltballs. The tube should not be in the
socket. Measuring leakage during the tapping process tells one how hard
and how much tapping is needed. (Eimac told me that gold evaporates from
the grid in this type of tube due to an "oscillation condition".
http://www.vcnet.com/measures/Foote,L.GIF
>I recently installed a new pair of 3CPX800s and again found the D117 diode
>shorted prior to firing up the new tubes. Replaced that again and all is
>working just fine. When inspecting the old tubes, I hear a faint noise
>inside one of the tubes when I rotate it about its axis, as if something
>were loose inside. I seem to recall that this was also a symptom of
>internal flash-over damage. So, the question is - where can I get these
>tubes tested to see if one or both are flashing over?
>
? with a high-potential tester that can provide 5kV. See my Web site
for the Funderberg (W6IHA) gold-sputtering test.
http://www.vcnet.com/measures/Gold.meltballs.html
>Floyd - K8AC in Angier, NC
- R. L. Measures, a.k.a. Rich..., 805.386.3734, AG6K,
www.vcnet.com/measures.
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