>First, thanks to all who have offered suggestions on the "low output on
>10m" question. I have several very plausible things to try and thankfully
>it's a three-day weekend.
>
>Now for another kinda question...fired up this same amp this AM on 80m, got
>mostly full output, but what I mostly got was smoke out of the grid current
>meter. Meter continued to indicate normally entire time, showing 120mA Ic
>(meter is 500mA movement, amp is 2 3-500Z GG in a textbook configuration).
>Typical setup with meter running from filament CT to ground and bypassed
>with 0.01u disk. But no diode protection...yet!
>
groan
>Plate idle current then went from 120mA @ 2500V to 240mA, telling me at the
>least that the zener (7.5V, 50W) went short. Oh well.
>
or a grid shorted to a filament
>Investigation of the meter movement (thankfully still operational, but
>really smelly) revealed that the built-in shunt (in the form of a small
>coil of resistance wire) had evidently heated to the extent to melt some
>insulation on the inner wirings from shunt to movement.
>
>Now why would this be?
Even though they appear normal, I would measure the resistance of the vhf
suppressor resistors by unsoldering one end.
>My guess (and that's exactly what it is) is that
>enough 80m energy got into the filament CT/B- circuit loop to toast the
>zener and heat up the meter. Even though it's supposedly bypassed.
seems unlikely for a bypassed 50w zener
>
>Two other salient items: with amp idle (keyed to idle plate current but no
>excitation) something (the zener probably) generated quite the hash in my
>receiver. I seem to recall seeing this mentioned somewhere...should one RF
>bypass the zener (across itself or to ground?).
Zeners are normally bypassed across the junction. A 0.1uF disc cer.
should suffice. If the zener is shorted, it is less costly to replace it
with a string of 10 or so 1A fwd biased rectifiers mtd on a rectangle of
perfboard - which is more rugged and more adjustable.
> Second, the grid current
>meter isn't grounded at the meter panel; rather, a shielded two-wire cable
>runs from the bottom of the amp up to the meter (bypass is applied directly
>at the meter terminals). So it's 20" wire up to the meter, cross the
>meter/bypass cap, and then 20" wire down to the place where it's actually
>grounded.
>
>One other thing is that in the bias tee, the 1st bypass cap after the
>B&W800 choke is one of those 500pF, 20kV red TV-style jobbers that Rich
>specifically recommends *against* on his web page. Could this seemingly
>poor bypass cap be the true culprit (allowing too much RF voltage back into
>the PS circuit?)
The hv electrolytics could be damaged by RF but zener and meter shunt
damage seems unlikely.
Rich...
R. L. Measures, 805-386-3734, AG6K, www.vcnet.com/measures
--
FAQ on WWW: http://www.contesting.com/ampfaq.html
Submissions: amps@contesting.com
Administrative requests: amps-REQUEST@contesting.com
Problems: owner-amps@contesting.com
Search: http://www.contesting.com/km9p/search.htm
|