Hammerhead Help

[Molzar]

Hi,

Recently finished assembling the hammerhead. I turned it on with all pots in the mid range and a high pitched tone was present. I then tried to adjust volume and threshold but was unable to get the unit to dead silence. I can make the volume higher/lower but not silent, the threshold seems to have no effect. It does detect metal, but at very short range (2 inches).

When I built the unit I did the non VCO option thinking that I could switch between VCO and /Non VCO using a DPDT switch. When I set the switch for VCO, metal detection makes the tone higher. When I use Non VCO the tone goes silent when metal is detected.

Any help would be appriciated, Thanks

[Carl-NC]

Is your autotrack switch (SW2) closed or shorted?

[Molzar]

The switch (SW2) is closed. The autotrack feature works in both VCO and Non VCO as the unit retunes after a second or two.

[Reg]

Hi Molzar,

Double check all resistor values around the threshold pot to make sure they are correct. I suspect that may be the problem.

If all resistors are correct, measure the output voltage at TP 9 with the threshold at both ends of the pot and let us know that these voltages are. Quite some time back the correct voltages were written down in an earlier post.

Double check the voltage at pin 5 of IC8b to make sure it is 0V. This can be done with a simple digital voltmeter or a scope. You might want to disconnect the coil to reduce the noise when making the measurements. If it is not 0V then C 19 may be bad or have a solder connection across it.

Now, with the coil connected, measure TP 9 when you move a test object near the coil to see if TP 9 increases in voltage. If it does then the polarity is correct to that point.

Did you purchase a pc board or make your own? If you made your own, double check all solder points to make sure there are no solder bridges across areas. Actually, you should do this even with a factory board. It is just harder to make solder bridges on them.

Double check to make sure you have the right transistor in for Q6 and it is in correctly.

Let us know what you find.

Reg

[Molzar]

Hi Reg,

Checked R28 thru R33, all values are correct.

TP9 voltage w/o coil both ends of threshold pot = +1.27V and -0.39V

IC8 pin 5 = 0V

TP9 increases with metal target (2-4 volts depending on target size). No target, the voltage jumps up and down a few hundred millivolts.

I bought the rev D PCB from Carl.

I've checked the board again for shorts or bridges. I do Quality in the electronics industry so I 'm familar with soldering/pcb errors. (Of course our own mistakes are the hardest to see)

Checked Q6 is correct (2N3906).

I've built the ETI561 BFO from this site and Gary's PI. So I have some idea what the VCO should sould like. Varing the frequency pot varies the pitch, as I think Carl described in the artical but the threshold has no effect.

Made two coils using BBSailor methods: 10.3" Dia, 26 turns, 26 ga magnet wire, 3.7 ohms and 10.3" Dia, 19 turns, 30 ga teflon wire, 5.5 ohms. No difference using either coil.

Thanks again for your assitance.

Molzar

[Carl-NC]

Everything you describe sounds right so far. I don't think you can switch between VCO and non-VCO with a DPDT. For the VCO option you want R50 installed, but not Q10. R51 is not needed, but can be left in. For the non-VCO option you would not want Q7 left in. Choose one of the audio methods and make the necessary changes.

- Carl

[Molzar]

Hi,

Removed Q10 and R51 for the VCO option. Threshold (r31) now has an audible effect, but I still can't bring it to a low growl (or total silence). Didn't spend too much time on it last night as it was late. Will do some more tests tonight.

Molzar

[1843]

Hi,

Removed Q10 and R51 for the VCO option. Threshold (r31) now has an audible effect, but I still can't bring it to a low growl (or total silence). Didn't spend too much time on it last night as it was late. Will do some more tests tonight.

Molzar

Hi Molzar,
Try to replace TL072 with TL082 and see if you can silence your HH or not.

[Molzar]

Hi,

Thanks 1843, but I don't have that chip, will put it on my shopping list. I did get a cmos opamp for IC8, but I wanted to get the stock HH running first before I try any upgrades.

Looked over the schematics and carl's build picture. R55 looked like it was only needed for the non-VCO, so I yanked it out. Fired the unit up and heard the sound I was expecting. That Motorboat growl. Was able to adjust R31 to dead silence.

However, this was with no coil attached. When I put the coil on I thought I was listening to and old "Flipper" rerun. Chirping patterns and such. R31 had some effect on this but varing R2 and R3 would really change the patterns in a very chaotic way.

Tried some of the other adjustments but couldn't get it to sound anything like when the coil is disconnected. I tried both coils (one is sheilded, one is not) similar results.

One other thing I noticed, in Carl's build pic R15 (10K) is installed, but the "passive pulldown option" in the article says to short R15 as I have done. None of the other "active pulldown" components are in the picture.

Not sure where to go from here, I don't own a scope but I have access to them at work. Any sugestions?

Thanks,

Molzar

[Carl-NC]

For VCO, omit R55. For passive pull-down, short R15, though it probably makes no difference.

Chirping is probably interference... get the detector outside, away from the house. Or, try rotating the coil around to find a relatively "quiet" spot. Or, use a Figure-8 coil for inside testing.

- Carl