HammerHead Problem

[Qiaozhi]

I guess I'll have to go back to active turn on and one gain stage to get it running....

I think that would be a good plan.

[Silver Dollar]

I changed the FET today. Here is the pulse on the gate of the FET;


But the Preamp output is still not correct. I don't know why...


The signal looks good but it's on the wrong edge of the TX pulse.
So that seems to say the TX is upside down somehow.....

The pictures in the HH article do not really show much. I'd like to see
each TP and maybe the TX pulse vs sample 1 and sample 2.

I guess I'll have to go back to active turn on and one gain stage to get it running....

[Qiaozhi]

Have you compared your board with the scope images at the end of the HH article (page 15)?

[Silver Dollar]

Thanks for the pictures. Mine is a little different so maybe they won't help much.
I'm thinking the FET is bad. If I cannot get it to work I'll change it back to the
standard mode.

This picture show what I think is the problem. The wave should look like the red
lines;


I'm not sure what could be wrong with the FET to cause this but it looks like a
cap in series with the gate... The FET came out of a junk PCB. It is a nice one
600v .9 ohms and low capacitance. I'll try a different one today.

Here's the Datasheet; STB8NM60

I found one of these to try too. 500V 1.3 ohm and output capacitance is lower...
IRF830

[godigit1]

Hopefully this works.
Here are the photos of my board, I hope it helps. You can see we have different resistors installed and jumpers. Maybe your double front end has caused you to change these I dont know but hope you get it going!

[godigit1]

I recently got my HH up and running as well. I built to the VCO option 1 schematic and I notice we have some different resistors and jumpers. Im a total newbe so I cant offer much more insite other than to provide some pics of my working board.
Sorry I cannot get my pics to post I will try again tomorrow.

[Silver Dollar]

Here's the schematic I am working to;

[Silver Dollar]

I am using a 300 uh coil with a 470 ohm resistor. I haven't tried to set the damping yet.
Underdamped means I need a smaller resistor?

It looks like the signal is gone before I sample. I did as you say and checked the 2 sample
pulses to the TX pulse and they were way off! I was trying to use the active turn off so made
pin 2 of U11 high and sent n1 to pin 1.

Here's some more pictures;
This is TX to first sample now after I adjusted it to ~20 us out


Second Sample


This is TX to TP4;


I seem to get a good waveform on the rising edge of the TX but bad on the falling edge.
I did add 15 ohms in series with the coil to limit current...

This shows the problem, the sample cannot get close enough to see the short recieve signal
(sample pulse in red)

[Qiaozhi]

I think you need to display both pin 1 of the 4066 and pin 13 of the 4066 together on the scope to see where the sample is taking place.
Also what are we looking at on TP4? Is this the no-target signal? It appears to be severely underdamped.

[Silver Dollar]

Well it looks like the 4066 is not working...
Pictures

Pin 3 Of 555 - TX


TP4


TP5


4066 Pin 1


4066 Pin 2


4066 pin 13

[Qiaozhi]

No increase at TP9, I think there's something wrong with the threshold.

I'm using VCO.

How about TP5?

[Silver Dollar]

No increase at TP9, I think there's something wrong with the threshold.

I'm using VCO.

[Qiaozhi]

I'm using the SilverDog PCB I just stacked up a couple of 5534's to spread the gain out.

Yes I have a scope. I was able to make the adjustments per Carl's instructions then
swapped out the 4016 as I had a signal going in but not coming out. But still no audio.

Two questions:

1) Can you see an increase in amplitude on the scope at TP9 when a metal target crosses the coil?
2) Which audio option are you using; vco or non-vco?

[Silver Dollar]

I'm using the SilverDog PCB I just stacked up a couple of 5534's to spread the gain out.

Yes I have a scope. I was able to make the adjustments per Carl's instructions then
swapped out the 4016 as I had a signal going in but not coming out. But still no audio.

[Qiaozhi]

Well I did the 2 stage with less gain per stage mod...

I found out the sync pulse from the 555 was only 627 hz (as I had also done the
active pulldown mod) so the 7660 could not make enough voltage.

I removed C11 and now I have good voltage! The only problem now is my
CD4016 seems to have died and I'm not too sure about that old TL082 either.

Well I'll try a couple of new chips and see how it goes!

OK ... I didn't realise you'd made your own PCB.
Do you have a scope?