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GO / GOI / MLS COSMOS Tool Notes

Introduction

Almost certainly, there are more Gearhart-Owen (GO / GOI) / Gearhart Industries / Mineral Logging Systems (MLS) COSMOS nuclear logging tools in daily use than any other design.  COSMOS was the original RCA trademark for the first CMOS integrated circuits; the name was apparently borrowed by GO for their tool using said RCA chips.  The tool was made in a variety of configurations including gamma ray, gamma ray-neutron, unfocused density, dual detector gamma ray, and others; the circuitry was even adapted for use in some open hole tools.  1-7/16, 1-11/16 and 3-1/2 inch pressure housing sizes were offered.  The tool was produced for decades, and a few clones were even produced over the years by other manufacturers.  Like most electronic designs, there are a few weaknesses, the worst being a downhole CCL amplifier with lackluster performance.  Remarkably, the design goes back to almost the time the 4000 series CMOS logic chips were first introduced, and the combined anti-coincidence and pulse shaping circuit is a marvel of ingenious design, consisting of a mere two chips.  The following constitutes abbreviated application notes on some of the possible fixes and modifications.

CCL Amplifier

The COSMOS downhole CCL amplifier is truly pathetic.  There are some fixes, but none improve performance significantly.  Replacing the stock 36 volt, 5 watt zener diode with a 39 volt, 5 watt component improves CCL performance slightly.  When it fries, the U2T105 can be replaced with a S2T105 or a 2N6351 transistor.  Remove the stock U2T105 Darlington transistor if installing a replacement downhole CCL amplifier.  A replacement upgrade printed circuit board (pcb) is available from AnaLog Services, Inc. that will work at the stock voltage, or in tools converted to 120 volt operation.  See Application Note CCL-2 for more information about the various uses of this cute little upgrade pcb.  The SIE single transistor 2N2102 / 2N3439 style circuit can be installed as a replacement only when also converting to 120 volt operation.

AnaLog's Super CCL

 for GO/GOI/MLS COSMOS Tools (PS-2a / CCL-2a)

Polarity Protection

A 1N5061 polarity protection diode is a good idea, essential if the tool will be used in conjunction with negative voltage powered tools.  A pulse coupling capacitor parallel to the diode may be necessary if a polarity protection diode is installed in a positive pulse tool.  The stock tool has a diode that shorts negative voltage to ground; hence, stock tools cannot be used with other tools powered by negative voltage.

120 Volt Operation

A series resistor can be added to obtain 120 volt operation.  Try a 1.3 kilohm, at least 10 watt resistor for 120 volt, 60 milliamp operation for a tool with a stock CCL circuit.  Try a 1.5 kilohm, at least 10 watt resistor if an SIE or TIP120 style downhole amplifier has been substituted.  The stock 50 ohm series resistor should be retained because it regulates the line driver circuit, setting the 6-8 volt pulse height.  An additional 2f, at least 150 volt, pulse coupling capacitor is required (non-polar is preferred; if a polar capacitor is used , the positive lead must face the cable head), a 2f, 200 volt film capacitor works nicely.  If positive pulses are involved, the pulse coupling capacitor must be connected to the line on the cable head side of any polarity protection diode (to the anode of the diode), but if only negative pulses are involved, it may be connected to the point where the polarity protection diode connects to the new series resistor (to the cathode of the diode).

Pulse Polarity

If only one pulse polarity is utilized, the unused input of the 4013 anti-coincidence chip should be grounded.  If the tool is being converted to 120 volt operation for simultaneous gamma ray-cement bond logging, the gamma ray pulses should be switched to positive by simply changing to the other input of the 4013 (ground the one that was previously used).

Bond Kill

The line connection going to the bond kill circuit input must come from the cable head side of any added polarity protection diode (to the anode of the diode).  Use the bond kill circuit that has a 2N2222A (2N2895) that provides a grounding output during the kill interval and connect it to the input of the 4007 gamma ray preamp.  See our improved bond kill circuit and Cement Bond Log Related Printed Circuit Boards (PCBs).

Line Noise

High voltage oscillator noise gets on the line through the +36 volt supply.  Matters are made much worse in tools converted to 120 volt operation, especially positive pulse tools, because of the added pulse coupling capacitor.  The best fix found to date is to split R28, the 200 ohm series resistor feeding the +36 volt supply to the high voltage oscillator board, and to bypass the junction of the two 100 ohm resistors to ground with a 10 or 20f capacitor.  This procedure is an especially good idea when bond kill is added to a COSMOS gamma ray tool for use with SIE style bond tools.

CCL Coil Shorted Winding

GO/GOI/MLS inadvertently created a shorted winding in the CCL coils used in COSMOS tools.  The green Teflon lead wire was stripped far enough back to make a full loop around the windings on top of a wrap of high temperature tape in a slight depression created during the winding process, and was soldered to itself.  While this solution is mechanically strong and gives a stable point for connection of the magnet wire end, it does create an undesirable shorted winding.  While it is unlikely that this practice significantly reduces coil sensitivity, it does have some effect and should be avoided.  If the CCL coil in a COSMOS tool must be serviced for any reason, the shorted winding should be removed.

Wiring Issues

The wire running to the bottom feed-thru should be shielded cable to minimize pulse noise creeping into the circuit of whichever preamp is at the bottom of the tool (neutron or density usually).  More than one technician has contemplated suicide over pulse cross-talk, only to finally discover that shielding the feed-thru solved the problem.  The feed-thru line should hook directly to the cable head line and not to the cathode of any added polarity protection diode.

Presumably to save money, GO/GOI/MLS omitted the ground wires to the individual circuit boards in most tools.  As the tools have aged, there have been reported grounding failures due to corroded mounting hardware.  Ground wires should be run throughout the tool; said action sometimes cures intermittent operational problems.

Thermistors

The thermistors used to compensate gain in the neutron and gamma ray scintillation preamps have been known to cause problems in aged tools.  It is now standard practice to remove them.

Neutron Detector

In COSMOS neutron tools, noisy operation is sometimes caused by loose or dirty connections to the Helium-3 (He3) detector.  The HN connector adapter threads in the top of the He3 detector and was apparently never tightened at the factory.  Clean all surfaces carefully (including the finger stock in the detector holder) and tighten the HN adapter.


Also see GO / GOI / MLS COSMOS Tool Schematics (pdf files).


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