HH question for Carl

[KingJL]

Carl,
Is there an alternative acceptable procedure for adjusting R23 when using the SMT board. Shorting R44 and R48 pull the +5 down to about 2.5 V. Removing Q8, Q8, and IC8 is downright dangerous (for me).

Thanks,
J. L. King

[Reg]

Hi J. L.

First, the easiest way to adjust R 23 is to center it and forget it. You can measure from the center pin to both outer legs of R 23 to make sure it is centered and the resistance is the same from the center pin to either end. If everything else is right and you used 1% resistors it should be a non issue. I suspect if you built the smt version you did use the correct resistors.

Now, I am not sure just what you are shorting, but you don't short the two resistors you mentioned with the associated transistors in place.

The idea or the use of R 23 is to assure there is a balance of signals going into the differential integrator to compensate for any imbalance because of component values in the differential integrator. If there is an imbalance, it can be one of the causes of strange signals as the coil is swept in the air or over the ground. It is easier to check in the air because the ground minerals will have no influence.

The idea or concept is to balance the signals at the differential integrator such that all slow responses are canceled. If the gains are the same and the signals the same at the input to the amp, then the output will be the 0 or canceled.

Taking a main sample to determine a target and then taking a much later sample and subtracting this later sample is how the low frequencies are canceled. The later sample is simply sampling slower signals, such as the earth field effect and typical 60 hz signals. The subtraction is done in the differential integrator.

This imbalance because of resistance values becomes a non issue if you try to sample at less than 15 usec anyways. There will be an imbalance created at the shorter delays as the result of the preamp stage amplification characteristics. Because of the high gain of the preamp, the signals created by a shorter delay sample are not amplified the same as those from a longer delay. In other words, the preamp will not have the same gain for slow signals as fast signals and, as such, will create an imbalance R 23 can't do anything for. Here is a link to a discussion by Eric Foster on this issue.

http://thunting.com/geotech/forums/s...t=11040&page=2

This is why I have recommended reducing the gain of the preamp stage down to something like a gain of 100 instead of a gain of 1000. This signal loss will then be made up in a later stage by increasing the gains of the later stages.

Reg

[KingJL]

Done!!

Reg,
Thanks for the reply. I normally wouldn't have even tried to short R44 & R48, except for HH Article, pg 13, bottom left:
"For this procedure R44 & R48 should be installed, but Q8 & Q9 should NOT be installed. If *8 & Q9 have been installed already, simply short R44 & R48."

Like I said in my original post, shorting these two resistorspulls down the +5V line.

I was just wondering if there was another way to balance R23. I took your suggestion and balanced the resistance at the wiper to pins 1 & 4 of IC11 (I think I ended up with 50.4 ohms on each leg).

Thanks again.

BTW: Your treatise on a DD coil for the HH on another thread was invaluable. I implemented my project with a DD coils as you described and so far I am extremely pleased with the outcome.

J. L. King