surf pi 1.2 coil problem

[Luuk]

i have finally got my surf pi to work, not like i would have liked but nevertheless is works. i have built several coils from spiral to bunched ranging from 170uH to 560uH. they all seem to work relatively well but the problem is that any sudden sideways movement or wrist twist movement causes a lot of screeching and this is driving me around the bend. what would be the cause of this problem?? help would be appreciated here before i land up in a looney bin

[Reg]

Do all the coils display the same thing with the delay advanced?

Now, one logical answer would be the circuitry isn't working correctly. Look for problems with the later sample circuitry using a scope if you have one. Actually, look at both samples to be sure they are there and working right. I suspect one of them isn't quite right or not there at all.

If you put in sockets, then try replacing the related IC's, but only do this after a thorough investigation of the related parts involved with the sampling. More importantly, first look closely for missing soldering or bad soldering connections on U3, U4, U7 and related parts. Finally, make sure you have the correct values of parts around U3b or the circuitry directly before it.

I suspect the problem will be obvious once you go over things again.

Sudden stops of the coil causing loud signals is usually caused by an unbalance or lack of low frequency cancellation which is needed to eliminate the earth field effect. The low frequency cancellation is done by taking a later sample and subtracting it from the main sample. This is done in U3b.

If the delay is too short, there can be an unbalance simply because the early sample is distorted. So, advancing the delay will usually eliminate that problem, thus if the problem still occurs, then more than likely a component problem or circuitry failure as the main cause.

Reg

[golfnut]

Ive made two simple coils for the surf.

Both were un shielded and were ok .

I make a better version, with a sheild, and potted to a shell and it does screech and false when I roll my wrist fast.

Did you make yours with a shield?

S

[Luuk]

Reg, thanks a lot for your reply. I do not have a scope but have a couple of friends who do so i will have to take the board over and have a look see as soon as i can. i have gone through the entire board very carefully under magnification and all the soldering is fine. as soon as i can i will test and check all the components to make sure everything is up to spec.
many thanks
Louis

[Luuk]

Hi Golfnut
the coils that i am presently using are all shielded and potted. maybe i should just make something simple and stop trying to be clever

[Reg]

luuk,

Please advance the delay quite a bit and then double check to see if all coils still make strange noises when you twist your wrist. Also, check some some small low conductor target like foil to see if the signal reduces in signal strength as the delay is advanced. This checks to see if the delay circuitry is working.

As I mentioned before, if the delay is too short for the coil used, you might get a screech. I have experienced this myself and found that if I advanced the delay a little, it went away on the coil I had do this on which was one coil that was shielded like others. So, I don't think it is the shielding. However, what did you use for shielding?

Double check to see if when moving the coil you get a strange noise when you swing the coil like you are hunting and then stop the coil very quickly. This also checks the earth field effect circuitry.

One other thing to check is the coil cable. Make sure it isn't moving by tying it down so it can't move near the coil. If you didn't use a coil cable relief connector at the coil, you might try stiffening the cable as it comes out of the coil. Any slight wiggle of the coil cable too close to the coil might also cause this problem. What are you using for a coil cable?

Reg