>I bought a new AL-82 about 2 weeks ago. ...snip...
>
>A couple of nights ago, I was on 40m CW (I think, though I may have been on
>ssb trying to work the ZK1). A loud "SNAP!" came from inside the case. The
>amp continued to work OK, but now my idling current is up to 325 ma. (from
>the normal 75ma CW/150ma SSB). The plate voltage is about 3500vdc unloaded,
>and about 3300vdc loaded, exactly as it was when I first powered it up. The
>SSB/CW switch has absolutely no effect on the idling current, it stays at
>325ma while the amp is keyed with no drive.
>
>The rig I use with this amp only drives it with about 80 watts. After the
>"SNAP!", the power output of the amp seems to have increased a little. What
>could have failed?
325mA of ZSAC counds like 0v of bias -- i.e., a shorted zener.
>Wow...The amp is only 2 weeks old.
>
Apparently, Forrest Gump was right.
>I studied the schematic a bit. I don't think the problem is in the metering
>board, especially since the current doesn't change when the CW/SSB switch is
>toggled. I guess it's possible that the zeners (ZD1 and ZD2) could have
>failed, but isn't it unlikely that they would both fail at the same time?
What if a large burst of grid current occured? Ordinary zeners are
intolerant of current transients. If the zeners are in series, one burst
of current could have shorted both zeners.
>If
>the Zeners failed, wouldn't that indicate some sort of momentary arc-over in
>one of the tubes??
Perhaps. Perhaps not. To find out if there is enough gas to cause such
an arc, check the vacuums with a high-pot. If there is under 10uA of
leakage between the anode and the grid at 8kV, gas was not the problem.
If you don't have access to a high-pot., apply the normal 3500v. In a
darkened room, if you can see blue gas inside the envelope, the tube is
gassy.
- I would also be interested in measuring the grid/filament breakdown V.
Normal is at least 6 kV at a few uA. My next move would be to look for
signs of parasitic arcing on the bandswitch. [see Figure 17 at
http://www.vcnet.com/measures]. Then I would measure the resistance of
the VHF parasitic-oscillation suppressor resistors. If the resistance is
substantively higher than the specification, an intermittent VHF
oscillation may have taken place.
- Is the AL-82 supposedly "neutralized"?
good luck, Dave
Rich---
R. L. Measures, 805-386-3734, AG6K
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