The DC input contains:

1) The yellow power input cord
2) The DC power connector and cap
3) The AC/DC power switch
4) The DC 2 Amp fuse
5) 3 Amp. 50 Vdc Diode, protects unit from connecting battery backwards.

PHOTO MULTIPLIER PLATEAU SETTING

The plateau setting was obtained by placing a cell with a high count rate, (250 - 500 CPM), in the counting chamber and taking counts for 1 - 5 minute intervals at increasing voltage levels. To do this place the high count rate cell in the chamber. Start the Counter/Timer and gradually increase the HV by turning the multi-turn dial from 0 to the point at which the counter just starts to count. At that point, begin a plot of dial setting verses total counts for 1 - 5 minute intervals, increasing the dial setting (HV) by .10 units on the dial until you can ascertain the flat portion of the plateau, (For ETI-9266KB-19, or Hamamatsu R2154-02, 1250 Vdc max) and between 3 to 5.5 and seldom ever past 6.00 on the HV dial.). Be careful you do not allow the photo multiplier to go into discharge. NOTE: (You can tell that you are nearing discharge when the count rate rises quite rapidly (200 or more counts) with each .10 increase of the dial setting----Stop increasing HV, reducing the HV dial setting). The plateau operating point will be just past the knee of the curve and just past the beginning of the flat portion of the plateau. Do not allow light to enter the photomultiplier, especially when the unit is operating, be careful!

This positive pulse is fed into the amplifier input stage of the counter, which is a high precision differential comparator. This comparator has a threshold control, factory adjusted, to eliminate noise and spurious pulses below 250 millivolts. This comparators output is a 5 volt logic pulse, shaped by nand gates, and fed into one input of another nand gate. The other input of this nand gate is connected to a JK flipflop, that acts as the (start/stop) switch. The pulse is then cleaned up and sent to a pulse stretcher to increase the pulse width, which is a requirement of the counting module. This pulse is sent to the LCD pulse Counter/Timer display module. The timer and pulse counter differ only because pulses are from an EPSON Xtal controlled clock chip, instead of the PM tube preamp.

The timer is crystal controlled for accuracy and is not affected by incoming line frequency variations. Timers time in hundredths of minutes to a maximum of 999999.99 minutes. The counters are set to reject pulses less than 250 mv and resolve single pulses up to counting rates of 5000 pulse/sec.

A fan is provided for system thermal stability, when operating on 120-240Vac. The fan and AC power supply are out of the circuit, when operating on 12-24 VDC. The fan filter should be cleaned every few months. The Filter holder is removable and lifts off the front panel. You may clean the filter using a small vacuum to remove dust from the insert, or replace it with a new foam or fiber filter, found in stores for a window air conditioner,(cut to size). Input power is transient protected by a 70 joule 6000amp metal oxide varistor and an EMI/RFI Filter.

MAINTENANCE

CAUTION : Remember that the High Voltage Circuit Cards operate up to 2000Vdc, and the HV capacitors can retain that charge for a considerable time. Any maintenance should be proceeded by removing the power cord from the AC or DC line. With the unit turned off let 5 minutes elapse until any disassembly occurs. This allows the Preamp's bleeder string to discharge the HV Circuit Card HV Supplies, and the associated capacitors, after which the HV circuit cards may be handled without fear of shock. No maintenance should be done with any input power (AC or DC) connected.

Remove the (4 white nylon acorn nuts) or the (4 black nylon slotted nuts) and washers on the Plexiglas front panel. Lift off the panel. Circuit cards may be removed by inserting the card puller tool provided. Insert tool in center hole in board, from bottom up. Place one hand on the front panel, across the circuit card opening, forming a bridge above the board, with the puller on the index finger of the other hand, extend the thumb across the index finger as you close the hand. Placing the thumb on the back of the bridged hand as a fulcrum pull up on the board and rock side to side until the board pulls loose. All of the integrated circuits are socketed and are replaced with ease. The entire electronics package may be removed from the case by removing the (12) outermost panel screws and then, using the center handle, lift out the electronics package. Readjustment of the threshold settings on the counters require special equipment and careful technique and should be done only at the factory.

Two adjustable high voltage supplies provide PM tube alpha plateau voltage operation. The supplies are identical and have a range of 600 to 2000 volts DC positive. The range trim pot, (R5) (2kW) on the HV schematic) is factory set to provide a range of 600-1800 volts using a 10 turn potentiometer(R3)(5kW). Refer to the plateau graphs to set up the counter for operation or to readjust the Plateaus and HV supplies, if necessary, and usually only when the Photo multiplier tubes age. In Alpha counting the average life of an operational Photo multiplier tube is approximately four (4) years. As the Photo multiplier tube ages the knee of the plateau moves upward towards a higher voltage, as you increase this counting voltage to remain on the plateau, you also increase electronic noise. You then have to play the "game" of signal to noise ratio (High Electronic background noise, no cell in chamber) and when it becomes too noisy the Photo multiplier tubes have to be replaced. Be reminded that no two Photo multiplier tubes are identical, indeed they all have different plateaus, different dark currents (backgrounds), and different efficiencies. The power used to operate the HV LED pilot lights is taken from the programmed side of the HV power supply system. A quick check that the HV is operating properly is to look at the pilot lights when increasing the HV adjust upwards from its lowest setting. You should be able to see discernible differences in LED brightness when increasing the HV ten turn dial from 0.00 to 5.00 etc. Several different PM tubes such as the (ETI #9266KB-19, Hamamatsu #R2154-02, or BURLE #S83019F) are available, and all are direct replacements. However, since all tubes operate at different plateau settings, a new plateau should be run and a change in the adjustment of the 10 turn dial will be necessary.

On the front of each HV board is a HV test connector and a switch for disabling the HV power supply. To check the stability of the HV setting use a DVM with a range of 1500 VDC, and using the supplied cable attach the DVM to the HV test jacks. Be careful not to short the output of the HV Supply.

CAUTION : If you attach the DVM to the test jack with the HV on, you may trigger the OVP (over voltage protector) on the HV supply. The pilot may go out, however, you can reset the HV by turning the HV power switch to OFF and then back to ON again. It is possible to damage your test meter by attaching it with the High-Voltage turned on. As you can see it is best to connect the DVM prior to turning on the HV supply.

A Clients Question: - One of our detectors shows a decrease in efficiency with time when counting standards. Before opening the counter, I wondered if you might have suggestions on what to look for. Switching the PMT's would tell me if it is the PMT that has gone bad but I was unable to find instructions on how to do this. Is this something we could do here and if so would it need to be done under specific lighting conditions.

Caution: In no case should you let either daylight or roomlight into the counting chambers with the HV turned ON.

There are several troubleshooting possibilities:

1) Re-Plateau the PMT
There is a chance that you need to re-plateau the PMT. Those instructions are in this manual. When you re-plateau, if you notice that your HV setting is on, or below the knee of the curve, then that may be your problem and increasing your HV slightly will correct this. Be reminded that as your tubes age, you have to set your voltage higher (just above the knee) to keep on the plateau, and your electronic noise increases with increasing HV. When this situation becomes too noisy you have to replace the PMT tube.

2) Switch the PMT's, etc.
Turn the unit off and let it stand for 5 minutes. This provides safe handling of the components and enables the Hi-Voltage to decay through the bleeder resistors in the pre-amps. You may then unplug the chambers from the unit, using a steady upward pressure pull the chamber up from the unit rotating it like you would use a Mortar & Pestle, with a grinding motion. Of course instead of pushing down, pull up. When the tube comes free, look at the tube base and notice that there is a key-way that needs to match the socket on the pre-amp. Now exchange them, being careful to align the key-way. See if the problem switches with them...Be reminded that you have to switch the voltage dial settings on the HV power supplies, because the HV is adjusted to the plateau value for each tube. Remember you have two identical channels that have a common Low Voltage AC input Power Supply, and this is the first step in switching components. This procedure checks out the PMT...however there may also be a problem in the pre-amp. In the most recent units, I have slightly lengthened the cables connecting the pre-amps, so that you may swap the pre-amp cables without physically exchanging the pre-amps. If you notice that the "good tube" counts OK in the switched position, after correctly adjusting the HV to that tube, then your whole system is OK and the problem is the PMT that was in question. If you find you have one channel that checks out OK, then it makes sense that the Low Voltage AC input Power Supply, which is common to both channels, is also OK. With two identical channels, using a little common sense and by switching elements, you can figure out just what part, board, tube, pre-amp or HV supply is the culprit. I would suggest that you mark the tubes, boards, pre-amps etc., so that you know what's where, and nothing gets mixed up while switching components, otherwise your whole troubleshooting job crumbles, and you will never know for sure where you started.