Inside Those Bird Slugs
In addition, I was always curious as to what was on the insides of those little beasts. After all, what could be inside to make that little thing worth so much money? Since mine was dead, and I had little to lose, I opened it up to see. I will share with you the inside story, and I now know why they are so dear to the pocketbook.
First, to test a Bird slug (assuming that you are going to a hamfest), take along a small VOM that has, at least, a resistance scale. If the seller (assuming you are also buying at a hamfest) will allow you to test it, place the VOM on a higher resistance scale (I used the 20K scale on the little yellow $9 marvel I had). Hold the slug in one hand and place one probe on the round shell of the slug while carefully touching one of the small connection which is exposed on one side. Then reverse the VOM leads. Test it just like you would test a diode -- you want continuity in one direction and an open connection the other way. Then test the connection on the other side of the slug the same way, expecting the same results. If you find continuity, or a very low resistance, from the shell to a connection in both directions, something is shorted in the slug. If you find no continuity in either direction, something is open in the slug. Either way will cause it not to work. On the one I had, I had no continuity.
Over a period of months I would pick that slug up and try to twist, poke, pull and generally try to see how to get inside it to see if anything was obviously gone. It resisted my every attempt. My wife would have said, "throw it away... it isn't any good. Just forget the money that you wasted on it..........It's gone!"
I don't think so. 'Course I really didn't tell her that I bought a dud so she didn't actually say that. Finally, over a year past the original purchase, I decided to take drastic steps. After all, it's dead anyway ---- what can I hurt?
With my Dremel tool and several different bits and stones, I was able to grind down past the aluminum disk which is the label, and pop the label disk off. I regret that when this activity took place, I did not own a digital camera so I have no actual pictures of the following steps. Even though I now have a camera, I cannot bring myself to re-enter the slug, just to get pictures. I will attempt to be, in words, as descriptive as a camera would have been in pictures.
Once the label disk was off, it revealed a cavity in the center with a single screw which was covered with a sealer to show whether it had been "messed with." Again, nothing to lose so I unscrewed that screw which released the larger circular part of the slug. This is the widest part of the slug which has the grooves and is gripped to rotate the slug in the wattmeter housing.
Once the finger piece was off, I could see a 1/2 watt resistor with a piece of spaghetti over one lead. It was soldered on one side to a small terminal which was one of the two connection pieces which are always visible on the side. The other lead of the resistor connected to another terminal and on to the top of a diode coming through the remaining part of the slug.
Two more screws were also revealed and once they were removed, the white nylon cover which shielded the primary RF detection pieces was removable. This round white piece of nylon is the piece which you see on the lower section of all Bird slugs. The slug I had was a 50C which was calibrated for 50 watts in the range of 100-250 MHz.
The RF detection parts were a 1 1/2 turn coil of about #18 silver plated wire connected to the diode on one end and a flat feed-thru capacitor on the other. The white nylon cover was actually a machined-out piece with room to go around the coil and feed-thru. I had originally thought it to be just an inverted bowl-type cover but it is much more rugged than that.
I did not bother the coil or other parts inside the slug once I realized that my problem was an open resistor. This resistor was marked 14K 1% and had, no doubt, gotten too hot from excessive power so it opened up. As Fortune had smiled upon me, I had an exact replacement resistor which helped to speed the repair. I carefully bent the leads on the replacement resistor to look exactly like the one it was replacing. The defective resistor was easily and quickly removed and the replacement fit back perfectly. I tried to use the least amount of heat on the connections so as not to risk changing the value of the resistor.
Putting things back together, everything fit fine and there were no left-over pieces (unlike some of my previous projects). The slug was tried and worked as it was supposed to. At least, it read upscale when power was applied through the wattmeter connectors.
Bird slugs are expensive because they are manufactured to close tolerances and I imagine it takes quite a bit of time (Time = $$$$) to assure the accuracy to which users have come to expect of Bird products. My repaired slug certainly did read something but there was no assurance that it would anywhere near the accuracy that was demanded of that unit. I compared the repaired slug to another I had which covered the same range and they read to same values. Yes, since that one had been defective and I still needed that value, I had purchased another. The repaired slug read the same as the untouched, other one which had become its replacement. Lucky? Probably. Happy? Absolutely!!
Had I been able to verify the contents and inner make-up of the Bird slug earlier, that is, if I could have read what you just read, I would have started the repairs much sooner. It may be that I was just lucky and attempting this project again on a different dud slug would have resulted in a much different outcome. I was, however, very pleased with the final result on this particular attempt. If you attempt the same, I hope you have similar success because, as we all know, "A Bird in the hand is............ oh, never mind....." ----- K5LAD
Updated 09/25/07 11:44 PM
Note: Slug pictures were borrowed from several peoples' ebay sale items. Thanks!