HH Rev C PTH project...

[Nicko]

Hi - My PTH version of a RevC HH is nearly complete.

I want to re-align the differential integrator balance now the board is complete and I've made a few changes.

Every DIP IC is socketed, so I was going to:

1. Remove the coil
2. Unsocket IC11 & IC12
3. Short IC11 Pin 12 to Pin 8 and do same for IC12 - This should turn off Q8 & Q9 & thus turn on both IC7a & IC7b.
4. Power up & leave system for 5 minutes to warm up
5. Using a 'scope & leaving IC8 in place, adjust R18 for decent voltage on TP4
6. Adjust R23 for minimum output on TP5
7. Adjust R18 for 0V on TP4
8. Check TP5 is still very very small voltage (very low mV)
9. Remove shorts on IC11 & IC12 sockets, and replace IC11 & IC12
10. Put dab of Locktite on R18 & R23.

Have I missed anything - it seems that I can leave IC8 in situ during this, and it would actually be better as any imbalance in the operation of IC8 would also be nulled.

Cheers

[habitbraker]

I dont know. I think adjusting that way you tell ic8 that i should generate 0v at output when in input is applied that "decent voltage".That could reduce sensivity.

only Purpose of R23 is to mach those resistors (r24-r27) and by applying decent voltage into these resistors you should do it easier. You could do it without rised tp4 voltage but it will be more difficult.

[Nicko]

More on this - when I made the coil as per the design notes with 26 turns of 26AWG wire (actually approx 25AWG) on a 10.5" former with Al shielding, I measured coil characteristics of 540uH and 2.4 ohms - this with 2mtrs of RG62A/U cable (47.6pF/mtr).

The 510uH figure seems a bit high - this was measured with a Peak Electronics LCR meter @ 15kHz, so I suspect its pretty accurate. The PE kit is generally very reliable.

Should I be lowering this to around the 350uH mark with fewer turns?

My calculated value for 26 turns on 10.5" with an bunch diameter of about 0.25" is approx 375uH, which seems more reasonable... The measured value seems high...

Cheers

[habitbraker]

well, It depends on what you want to detect.

Lower inductance (L) means lower interwinding C and faster decay of flyback pulse. That means that you can sample erlier and increase sensivity on small and resistive targets (gold for example)
It also means that B field will be lower so that transmit power will be smaller. So you could loose depth.

Higher L means more powerful pulse and grater depth but coil is slower and tou could miss small targets.

Thats my (beginner) answer. You should also wait for gurus

[Nicko]

Hi - My PTH version of a RevC HH is nearly complete.

I want to re-align the differential integrator balance now the board is complete and I've made a few changes.

Every DIP IC is socketed, so I was going to:

1. Remove the coil
2. Unsocket IC11 & IC12
3. Short IC11 Pin 12 to Pin 8 and do same for IC12 - This should turn off Q8 & Q9 & thus turn on both IC7a & IC7b.
4. Power up & leave system for 5 minutes to warm up
5. Using a 'scope & leaving IC8 in place, adjust R18 for decent voltage on TP4
6. Adjust R23 for minimum output on TP5
7. Adjust R18 for 0V on TP4
8. Check TP5 is still very very small voltage (very low mV)
9. Remove shorts on IC11 & IC12 sockets, and replace IC11 & IC12
10. Put dab of Locktite on R18 & R23.

Have I missed anything - it seems that I can leave IC8 in situ during this, and it would actually be better as any imbalance in the operation of IC8 would also be nulled.

This seems to have worked really well - only change is that you can see the transmit pulse on TP4 when adjusting R18, so a corrected list would be:

1. Remove the coil
2. Unsocket IC11 & IC12
3. Short IC11 Pin 12 to Pin 8 and do same for IC12 - This should turn off Q8 & Q9 & thus turn on both IC7a & IC7b.
4. Power up & leave system for 5 minutes to warm up
5. Using a 'scope & leaving IC8 in place, make sure you have nice pulses on TP4 - these pulses are what you will "null out" in the next step.
6. Adjust R23 for minimum output on TP5. You will see about 10 - 20mV of noise - just try and minimise any signal that you see on that noise - it's pretty obvious - if your 'scope has a low-pass filter (b/w limit) on its input, use that to reduce the noise signal.
7. Adjust R18 so that the bottom of the pulse on TP4 (the "space", not the "mark") is at 0V.
8. Check TP5 is still very very small voltage (very low mV)
9. Remove shorts on IC11 & IC12 sockets, and replace IC11 & IC12
10. Put dab of Locktite on R18 & R23.

Happily detecting small cuprous coins (UK 1 pence piece) at about 10"+ now and my wedding ring (22ct) at about 8". HH is drawing about 70mA.

Cheers