IDX-PRO+VDI

[Koala]

If you remove the coil wires totally - it should be silent, No Tx, no Rx - does it still oscillate like this?

My build is not quite without the coil. It chirps away. There were a few miner evolutions of the project. So might very from build to build

[golfnut]

sure,

but if I unplug my coil - the Tx oscillator stops, so the Rx channels have no gating, plus the preamp has no input and it sounds dead.

In this state, Id of thought all of them should be as good as silent, or at least not oscillating/motorboating/ repetitively chattering.

[Koala]

see what your saying and I have an original Classic and that's silent with no coil. With mine I get a rapid tut tut sound that is allways there when I switch it on regardless of settings. However a quick flick of the ground balance pot will shut it up but if I switch it of and back on again the tut tut comes back.

[golfnut]

Yes Im with you, that is the filer/amps or differentiators hooting, sounds a bit like russian woodpecker, as the amps have masses of gain over a narrow BW and can become oscillatory as about 70dB of gain there - in band (around 8Hz peak) - I scoped it once and was 125ms = 8Hz

I applied a resistive load to terminate these amps one time and it helped. Also the mid point between the two amps to gnd a few kohms. minor gain reduction but enhanced stabiity.


I modelled the gain and phase of the pair of amps one time in spice and the phase response from in to out, passes thru 180 degrees while there is still positive gain - in anybodies books thats the basis for an oscillator..

try a 22k to gnd from U3 pin8 see if you can tame it

[ampirate]

Thanks for all the advice guys.
I am trying them out.

And it is a Silverdog kit.

[der_fisherman]

Right.
What happens if ONLY the receiver coil is unplugged?
regards
Andy

[ampirate]

If only RX Coil and Cap is disconnected then it continuously beeps: buzzz buzzz buzzz ...
If RX Cap and/or Coil is attached then it is random noise,

[Gwil]

Just thinking aloud...

Low frequency instability is very often related to inadequate regulation or decoupling of the power supply rail(s). This allows a wobbling voltage to be fed back from one stage to an earlier one, with unfortunate results. Can be caused by an open circuit decoupling capacitor, among other things. Not sure if I'm looking at the right schematic, but the LM2930 low dropout regulator is a lot more fussy about what's connected to it than for example the 78xx series. The datasheet goes into some detail about what sort of capacitors to use, etc.

As I said, just thinking. May not be relevant.

[ampirate]

I tested the VReg and its output is 7.89V probably just Multimeter off slightly.
All Parts seem tight.

Range Tests give me 6in-10in on Coins depending Coin type/size.
Range for large 6in Can is 24in.

With some popup Chatters.

[Koala]

It's hard to say if the problem is with the coil or detector. What is the peak to peak on pin 6 U1

[golfnut]

The multimeter could be ok, and the regulator may be struglling to keep 8v,,,,, - try a 12v to rule it out.

Nobody else uses 9v cell on 8V reg


as aside, Can you reject ferrite?

where do you live - uk?

[ampirate]

I am using (2) 9v bats to give me a measured in of 16v - should not be a problem.
The 8V Reg warms up a little bit.

I am from nj, usa

[der_fisherman]

The multimeter could be ok, and the regulator may be struglling to keep 8v,,,,, - try a 12v to rule it out.

Nobody else uses 9v cell on 8V reg


as aside, Can you reject ferrite?

where do you live - uk?

Beautifully put and fully correct.....
Andy

[golfnut]

thx andy, bit of a struggle this one..

Can u post some piscs,

does it reject ferrite

[Gwil]

Just for the record, the output of the LM2930-8 is specified in the datasheet as 8.5V max, 7.5V min, so 7.89V is well within spec.