So I took another look at the diagram supplied with the kit. It shows a jumper from the yellow sense wire shorting out a resistor and a potmeter which are connected in series. Thinking that this is always the same whatever the lay out I started to measure several junction to find out were to solder the wire. The picture at the right shows the results. The conclusion is that when I solder a wire between point 3 and point 1 I short out the potmeter and the resistor below the potmeter which are obviously connected in series.
The result including the short out wire can be seen on the picture. Ready to assemble without any difficulties this time...
Below the finished project finally working:
Testing: I hoped for a more accurate FSM but so far it is just a indicator just like a analog FSM. The potmeter is not sensitive enough to set it to a accurate value. So, if you finally set it, for instance to 10 as a start value, next time you switch the meter on it is on 15. When you only touch the potmeter the value will change. I might look around for a 10 turn potmeter to increase the sensitivity in the future.
The push button, by the way, is a momentary on switch in parallel with the normal on switch. I had to read the manual several times before I discovered why it was there. Just for information ;-)
4 comments:
thank you for the info on the QRP Guys FSM. I have not started building mine yet, but when I saw your blog post a while back, I wondered if I should bother at all and use it for parts or something. I'm a bit confused. Is the FSM accurate enough for your use... or is it worthless if built without your mods? Please clarify. Thank you and 73.
Interesting, but against what do QRPGuys expect the meter to be calibrated against? Seems to me that it has no advantage at all over a pocket money analogue unit which, with two telescopic whip antennas, would almost certainly be more sensitive than that very small antenna. You can even bring about considerably more sensitivity by touching one of the whips!
Tnx for your comment. It is a interesting device that is not accurate enough yet. It senses RF, but a analog FSM does the same. However, on a analog meter you can see no difference when comparing two antennas. Most of the time the difference is only very small, I hoped the digital readout would give me something I can write down and copare later on. The problem is only that the device is not stable enough. I can't set it for instance on a start value of 10 and rely that it is the same within a few hours or minutes. I think a better potmeter will solve the problem. 73, Bas
Hello John, it is not calibrated. The manual is clear about that. It is just a RF sensitive indicator. By the way it has a separate BNC connector to connect a antenna. And I found it is very sensitive, more as my analog meter. You can adjust the sensitivity with potmeter but it isn't really stable as mention in the article and in the answer above. 73, Bas
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