Even More Info on Modifying Bird Slugs
In Sept. of 2006 I was emailing back and forth with a ham in Australia, Steve Darveniza - VK4XRA. I've reproduced several of the messages from Steve as he discovered more about his Bird slug. Actually it was part of the built-in device in the Bird Termaline 694. He was able to fix the problem and he gave permission to share his results.
I have found an old bird 694 termaline that is underreading a lot.The load reads 53 ohms on a DMM and the meter reads full deflection 3k ohm. I suspect the diode is broken. Any chance of sourcing a replacement?
Here is what I have learned about the Bird 694 termaline A germanium diode is indeed
the way to go,it has to have a forward resistance of 10K OHM or 2.4volt forward voltage
this is best, as you will not have to change out the 9.1K OHM series resistor that feeds
the 100ua meter,its resistance is 3.0K OHM. So you have a 12.1 K OHM resistance from the
meter through the 9.1 K resistor through the germanium diode to earth all is working now
but I have to open up the "do not open" movement as the front bearing has let
I THINK that a shotky diode is the go, also the meter is now fixed. It is quite easy to
get into all you do is get a thin and sharp blade screwdriver and run it around where the
black aluminum surround has been folded and it comes off very nicely,then lift off the
heavy bezel underneath and then with the widest flat blade you can find, gently apply
pressure under the glass whilst watching the black rubber seal moving away from the glass.
This particular meter had the actual pointer loose on top of the coil gymble so I just
dabbed slow curing super glue under it and repositioned into place and there you have it.
I have finished the last bird 694. The 1N5819 shotky diodes are ideal, also replace the
9.1k ohm carbon resistor with 2 x 18k 1% resistors in parallel and then use your hex key
to screw the nylon sleeved RCA jack (capacitive coupling) in or out from the central
conductor pin to calibrate the meter. Ensure you do not wind it in so far as for the RCA
jack outer to contact the bottom of the post as this will destroy the diode. The
calibration is perfect across the whole one kilowatt scale. Very satisfying. I hope this
data will help others.
> OK Steve --
There is a variable capacitor that couples the rf to the detector diode. This is in the form of what appears to be a RCA jack on a thread that runs into the rf input feed line. This RCA jack can be moved in and out along a centre pin that feeds the diode. This RCA jack has an insulated centre,there is no conductivity. The back centre section of this "RCA jack" has a hexagonal hole for a normal allen key.
Update: The shotky diode tracks perfectly with the dial scale. I tested the meter on every 5 watt gradation from 5 watts to 1 kilowatt and the design is a credit to the Bird company. It appears the original germanium diodes do deteriorate over the years.
Modifying Bird Slugs
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