Hello :) ....and HELP!

[Satindas]

Greetings to all

My first post having just completed the MMP build and yep, "Houston we have a problem".
I do have some electronics experience (analogue synthesizers) but I'm certainly no expert so be gentle ....
Before I waste time trying to describe in detail the nature of the problem I'd like to ask a simple, component related question;
The build doc BOM clearly states 2 x NE555's. I received 1 x 7555 and 1 x TLC555 with my kit (both CMOS versions which differ not only in themselves but also from the stock 555 ) : should this present any problems?
I need to remove them from the sockets and test them independently yet as CMOS is easily damaged without anti-static protection and it could obviously be a dud bug.

I followed the build procedure to the dot (every resistor and capacitor was tested before fitting) and have built a fast mono coil according to the usual source (21 turns, 7 strand, PTFE, AWG26)
For testing, I'm using a piece of 500mm Van Damme 75ohm Plasma grade Coax (67pF/m)

I'll try to get some scope shots today, get these timers tested and report back.

(I'm actually chomping at the bit here cos I already got my first permission from a local farmer and my bloody wand isn't working properly

[Satindas]

Ok, before I state my findings I guess I should describe the problem(s); first of all I can hear an annoying, constant, steady, high pitch tone in addition to the expected audio. This whine does not change when adjusting the Audio Threshold and I suspect it is bleeding through from somewhere. Secondly, the response to a target seems highly erratic - sensitivity seems to drop off very close to the coil (there seems to be a dead spot at less than 4cm from the center) and is generally quite poor although I have managed to get a response at around 250mm with a 10cm square of copper clad board.

Results of poking around with the scope; all test points read pretty much as spec except for TX pulse width of 47uS, and TP3 where I expect to see a DC voltage but see a waveform. ( I know I had a DC voltage during the build but now I see this ... ... and I don't see any change in the DC level when bringing an object close to the coil.
Next, Audio is pretty distorted with the supplied speaker so swapped for a 30mm mylar speaker and now its clearer but barely audible.
Lastly, the SAT seems to take a long time to settle every time there is a strong signal... it shoots of the scale and then takes 3 or 4 seconds or so to settle down again

Any Ideas ?

Many thanks in advance
Simon

[Satindas]

Anybody ?

[Qiaozhi]

What results do you get with the coil disconnected?
Is there still a constant audio tone?

[Satindas]

Hi Qiaozhi,

the constant tone is not there with the coil disconnected. It is 230mm Dia, 22 turns of 26AWG 7 strand PTFE, spiral wrapped and Graphite/acrylic base shield.
I haven't yet tried to adjust the damping resistor but it looks fine on the scope so I guess if anything it could be overdamping. Could this be the cause?
It's a bit late in the day to start testing but I'll try to get some scope shots of the audio output tomorrow, with and without the coil attached.

cheers
Simon

[Qiaozhi]

Hi Qiaozhi,

the constant tone is not there with the coil disconnected. It is 230mm Dia, 22 turns of 26AWG 7 strand PTFE, spiral wrapped and Graphite/acrylic base shield.
I haven't yet tried to adjust the damping resistor but it looks fine on the scope so I guess if anything it could be overdamping. Could this be the cause?
It's a bit late in the day to start testing but I'll try to get some scope shots of the audio output tomorrow, with and without the coil attached.

cheers
Simon

Have you shielded the coil with aluminium foil by any chance? And if so, did you leave a gap?
Without a gap the shield will act like a shorted turn.

[Satindas]

The coil is shielded with graphite powder/acrylic base. 2cm gap.

cheers