72-100 Electron Multiplier Supply Notes

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The 72-100 electron multiplier supply is used on older Physical Electronics (PHI) Auger electron spectroscopy systems to provide voltage to the cylindrical mirror analyzer (CMA) and secondary electron detector (SED) electron multipliers.

The 72-100 comprises a digitally controlled 0 to +4000V variable switching supply, a 0 to 300V fixed supply, and a voltage to frequency monitoring circuit. The digital control from the PC software is converted to an analog output which drives a switching supply.  

The AES and SED 72-100 board addresses are set by changing the segments on SW2 as follows;

72-100 switch settings
Switches set up for AES SW2 segment 3 closed

Changing the switch settings allows you to swap the 72-100 boards to confirm whether or not there is a problem. For example, if you think that your SED 72-100 board has a problem, you can swap it out with the CMA 72-100 after first changing the CMA 72-100 address to SED (SW2 segment 4 closed). If the SED image works after swapping the boards, then you know that your SED 72-100 board does have a problem.

Once you confirm that your 72-100 is not functioning properly there are a some simple things that you can try.   ***CAUTION – high voltages (up to 6kV) are present on this board, refer servicing to qualified personnel who have been trained to work safely with high voltage.***

Note that the CMA 72-100 is controlled by AugerScan software and the SED 72-100 is controlled by AugerMap software.   You can also use the AugerScan diagnostic menu to send commands to the board per the 72-100 manual calibration procedure. 

Make sure that the card rack power is OFF and then extend the 72-100 board.   Use the high voltage extender (the one with red wires on the bottom section).

72-100 board mounted on extender card

For testing the AES 72-100 board, you can first check the analog output from the DAC to see if you are getting the proper control voltage. Note that the card rack power needs to be ON when measuring voltages.  For the high voltage supply, measure from the left side of R 32 with respect to the left side of CR 3.   

Meter ground left side of CR 3
Meter POS left side of R 32

With the electron gun supply off, set up and acquire a survey in AugerScan. The DAC output voltage should ramp up from 0 to about 5 volts as the multiplier voltage is being set by AugerScan.  Since there is no electron beam current, the AES electron multiplier voltage will default to the PC (pulse count) voltage which is typically 1800 to 2200V. 5 volts on the DAC output corresponds to 2000V on the multiplier supply POS output.

If the DAC voltage output is correct, then the problem is probably related to the switching supply transistors.  With the 72-100 board on the bench (card rack power OFF when removing or inserting the 72-100), test the diodes and transistors for shorts.   Note that two legs on all of the power transistors will show as shorted because those points are connected to the switching transformers.

One common problem that causes the HV output to be zero is that Q7 and Q8 are defective, even though they may check out OK with the diode test.  If the DAC voltage is correct but you do not have any high voltage output, then try replacing Q7 and Q8 with the correct 2N5337 transistor.  Some 72-100 boards have 2N3725 transistors instead of 2N5337 transistors installed. The 2N5337 can dissipate 6 watts of power but the 2N3725 can only dissipate 3 watts. The lower power rating of the 2N3725 can result in poor switching characteristics.

Q7 and Q8 are located under the black high voltage cover. It is held in place with 3 screws. Remove the cover and you will see Q7 and Q8.

Remove the high voltage cover
Q7 and Q8

Assuming that replacing the transistors solved the issue, you can adjust the high voltage by adjusting R7 to match the software.  For example, if the AES PC voltage is 2000 V then adjust R7 for 2000V between the POS and NEG cables. Note that you need to use a high voltage probe when measuring the POS output voltage as most DVMs are limited to 1000 V or less.

R2 is the 300V adjustment and you measure that voltage from the NEG cable center pin to the outer cable or vacuum chamber.

The easiest place to connect your high voltage probe to measure the AES high voltage is between the center pin of the J6 HV POS IN cable to the 96A/B / VF4 preamp and the center pin of the NEG cable on the CMA electron multiplier.

When testing the SED 72-100 then you need to control the SED voltage in the Auger Map TV dialog box.  As you change the SED voltage in AugerMap you should see the DAC output voltage change.  2000V would correspond to about 5V on the DAC output. For the SED high voltage, measure between the center pins on the 97 SED preamp POS and NEG cables. Set the SED voltage in Auger Map to 2000V and then adjust R7 on the SED 72-100 board for 2000V. Remember that you need to use a high voltage probe when measuring the SED and CMA POS output voltages.

Regarding the V/F circuitry on the 72-100 board, those components are not used and so can be removed if your board has them installed in sockets. Some 72-100 boards do not have those ICs installed at all, but others do and they may be soldered in.  If they are soldered in just leave them as is; it is not worth the effort to pull them off the board if they are soldered in place.  The ICs which can be removed are:

U1, U2, U3, U4, U5, U6 and U9.

72-100 board with no V/F ICs installed
72-100 with V/F ICs removed from sockets

Finally, the 1K ohm filter resistors on the 72-100 mother board can fail (burn and become open).  In that case the 72-100 board may be working properly, but the voltage will not get out to the cables.  You can ohm out the resistors on the motherboard to see if that is an issue.

72-100 R1 R2
72-100 R1 R2

72-100 R1 R2

And here is the schematic that shows both resistors:

72-100 mother board schematic
72-100 mother board schematic

If you can’t repair the 72-100 board yourself, RBD Instruments provides a 72-100 repair service and loaner 72-100 boards. 

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