Barracuda Legend Kit Problems

[Old cart]

Hi Old cart,

I added a 1nF across the 4.7nF (C9) associated with the Delay preset and pot. This seems to have improved the first sample pulse shape some, but across the adjustable Delay range the pulse width varied from about 44us to something over 50us. This did allow me the ajust the pulse width through the matching point, so I tried the ceramic magnet. I found a minimum sensitivity point around where the pulse widths matched. Interestingly the first stage of the LF412ACN was offset at around +1.3V from earth. One would expeRay zero offset.

I've yet to make it portable enough to test outside again.

Ray

Ray, I think all this would do is to increase the delay. Caps in parallel just add so 1nF plus 4.7 nF = 5.7nF. So did you match the main and EF pulse widths? That mod I mentioned really just adjusts the signal going to the samplers (main and EF) by adding
Resistance to make the channels match.

[Michaelo]

I tried the instruction too... set pulses width to matched but I'm still not getting 12" for a UK pound... more like 6" ;(

@Ray, did you notice my last post (2 up) re caps on regulators?
Turns out the -5 volts supply has both the 220uF caps on it for rev 2 board... one on input and one on output... the +5 has no smoothing caps...
I think I may have disconnected the output cap (-5) which would be an issue... tomorrow I'll correct that and try again...

[raygdunn]

Did you adjust both the main and delay sample pulses to the same width before tweaking with a magnet? There is another way to adjust the 412 sample integrator for better balance. It involves adding a 100ohm trimmer with the two outer pins going to the two fet sources and the wiper going to the junction or r20 and c11 ( KT315 schematic). This involves cutting the trace between the two sources. By the way the schematic mentioned above does not agree with this. The layout has the sources and drains exchanged. This should allow very fine balance of the FETS wither the magnet test and the sample pulse widths equal.

Hi Old cart,

I added a 1nF across the 4.7nF (C9) associated with the Delay preset and pot. This seems to have improved the first sample pulse shape some, but across the adjustable Delay range the pulse width varied from about 44us to something over 50us. This did allow me the ajust the pulse width through the matching point, so I tried the ceramic magnet. I found a minimum sensitivity point around where the pulse widths matched. Interestingly the first stage of the LF412ACN was offset at around +1.3V from earth. One would expeRay zero offset.

I've yet to make it portable enough to test outside again.

Ray

[Michaelo]

One more inconsistency between bara PCB (rev2) and schematic...

On the schematic, both the 78L05 & 79L05 have 220uF caps on inputs (even though both are connected incorrectly)... however, the 79L05 actually has one 220uF connected to its input and another 220uF connected to its output... The 78L05 has no 220uF cap on it input as indicated by the schematic... And neither regulator have the 22uF caps on their outputs as indicated on the schematic...

The input to the 78L05 comes from a battery so chances are it really doesn't require a 220uF cap on its input (good practice is to include one) but more importantly voltage regulators almost always require an electrolytic or tantalum cap on their outputs...

So... in a nut shell, the board arrangement is close with the following exceptions... 1) the fact that the 79L05 has both cap's positive terminal are connected to Bat+ when they should be connected to Ground (Ground is positive with respect to -5) and 2) the 78L05 has no output cap (~220uF)...

Just thought I'd add this for those making changes to the caps and as a reference for next build...

Mike

[Old cart]

Hi Qiaozhi,

My new 40106 is working well with all the mods in place. ☺ Audio send is more stable. Many thanks for the capacitor mods. Haven't tried it for movement sensitivity yet. First stage of 412 shows an offset of something like 1.25V. I guess this is likely to show some earth field sensitivity when off balanced from zero. This varies somewhat with the Delay setting. I take it that the ceramic magnet test is better than plain zeroing. Both by pulse width adjustment.

ICL7660S is a different story. Still overheats at 11.8V without 40106 in circuit. But runs cool on 9V (so useable). Other chips the NE5534A and LF412ACN seem to be running normally. I've checked all pins connect through the DIL socket OK. I still have the option to change the 79L05.

I finally figured out the basic workings of the 412 stages. Not bad, once I focused on the op. amps. I now see why the control is called Threshold. I had wondered.

Ray

Did you adjust both the main and delay sample pulses to the same width before tweaking with a magnet? There is another way to adjust the 412 sample integrator for better balance. It involves adding a 100ohm trimmer with the two outer pins going to the two fet sources and the wiper going to the junction or r20 and c11 ( KT315 schematic). This involves cutting the trace between the two sources. By the way the schematic mentioned above does not agree with this. The layout has the sources and drains exchanged. This should allow very fine balance of the FETS wither the magnet test and the sample pulse widths equal.

[Old cart]

I've added your request to the REV-3 change notes.

Would there be any value in moving the battery input terminals directly to the main storage cap? Maybe even putting a 1 ohm resistor in series with the plus battery feed. This, along with referencing all the power supply caps to ground should help supply noise a lot.

[raygdunn]

Hi Qiaozhi,

My new 40106 is working well with all the mods in place. ☺ Audio send is more stable. Many thanks for the capacitor mods. Haven't tried it for movement sensitivity yet. First stage of 412 shows an offset of something like 1.25V. I guess this is likely to show some earth field sensitivity when off balanced from zero. This varies somewhat with the Delay setting. I take it that the ceramic magnet test is better than plain zeroing. Both by pulse width adjustment.

ICL7660S is a different story. Still overheats at 11.8V without 40106 in circuit. But runs cool on 9V (so useable). Other chips the NE5534A and LF412ACN seem to be running normally. I've checked all pins connect through the DIL socket OK. I still have the option to change the 79L05.

I finally figured out the basic workings of the 412 stages. Not bad, once I focused on the op. amps. I now see why the control is called Threshold. I had wondered.

Ray

[Koala]

Just put mine back together. New coil probably need dialling in. Getting faint signal to 11 inches and diggable at around 9 inches. This is with a speaker expect a bit more with headphones. From memory everything tested between 7" and 12" last time I tried it.

Been doing a bit more tweaking. Changed for matched FETs and readjusted the POTs now getting nice beep to just over 12" on a pound coin but the iron disc is not so good. Still there. More of a rounded tone compared the coin which is nice and sharp.

Pretty good for a 8.5" coil and as far as I would like to dig in sand.

Offset I adjusted until distance started to reduce the turned the other way until it started to reduce again then split the difference between the two points.

Maximum depth on a small gold ring appears to at the point where the threshold stops chirping

[Michaelo]

In my last post is should be uF not Uh...
Koala, it's 6.8 alright.... I think it's only 16 volts though...

[Koala]

Can't read the value but the red capacitor to the left of the delay trimmer looks a little large for a 6.8nF then again is could be 250V or something ?

[Michaelo]

3300uH is the minimum, anything more is perfectly fine... it might even be worth using two smaller caps in parallel so we don't have to lay the cap down on its side

[Koala]

Just noticed that its a 3300uF and I am using a 4700uF doubt it will make any difference ?

[Michaelo]

Based on what we've seen so far, I would expect the answer to be "yes". However, someone should first check that the changes work.

I got some parts coming in from silverdog (any day now), when they arrive I will build it and slap the scope of it...
I've already included it in my schematic (other post) as datasheets are almost always correct
Mike

[Koala]

https://m.youtube.com/watch?feature=youtube_gdata_player&v=Gs-AScbBGsg

Standard build rev 2

450uH coil measured

8.5"

[Qiaozhi]

Just noticed a few difference with two version of the schematic relating to the main amplifier circuitry...
Strangely enough they both differ from the recommenced "Frequency Compensation and Offset-Voltage Nulling Circuit" from the datasheet...

Would there be anything to gain from making these changes?

Based on what we've seen so far, I would expect the answer to be "yes". However, someone should first check that the changes work.